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FM Tuner Tweaks and Repairs

Intro

After nigh on 2 decades, I recently rediscovered my passion for tweaking FM tuners (tuner tuning, if you will). My old tuner page in German, UKWellensalat, was last touched in 2007 and I don't really dare disturbing the dead, so I initially started writing about this in English-language forums and then added entries to my music and audio "blogalike", but it all got a bit much, hence this page.

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New Mods for the Kenwood KT-80

Motivation

You may not be aware that yours truly has a history as a bit of an FM tuner enthusiast from back in the 2000s. (My hobby trajectory basically went shortwave → FM → music.) From those days, I kept a Kenwood KT-80 in the bedroom – while not a world-beater, it's a neat little analog tuner of above average build quality that is kinda technically interesting (due to sporting a pulse count discriminator), performs decently and sounds good (well, after bridging or replacing the 10µ/16V output coupling caps, which have never seen any bias voltage in decades and would be degraded and leaky), all the while being of manageable complexity. I'd call it "Pulse Count Detector, Essential Edition". These were sold from about 1979 to 1982 and then as the black KT-80B in 1983/84, which by the standards of the time was an eternity, so they are fairly common on the used market.

Mods for a better set of matched filters (of limited interest nowadays, as the parts have sadly long gone EOL) and moving the muting onto another button have previously been documented.

I wanted to address the following issues in my unit:

  1. The sole incandescent lightbulb, an 8 V / 50 mA affair lighting up the power button, had long since been burned out when I got my unit – I strongly suspect it was run out of spec from the factory now. Meanwhile, the series resistor in the power supply was frying away close to 2 W while supplying about 6.6 mA to a single green dial pointer LED, with pronounced mains (50 Hz) flicker at that. (I now think that a leaky zener diode was making things even worse.) When the whole tuner only draws 10-12 W from the mains, that's quite a lot.
  2. Out of curiosity I was poking around with a multimeter and noticed that regulated voltage was quite variable depending on load (i.e. number of LEDs lit). A variation of about 92 mV was observed despite the implementation of load-side voltage sensing; besides, the voltage varied between about 15 V idle and 14.9 V under load, a fair bit above the nominal 14 V noted in the schematic.
  3. The AFC always struck me as a bit more "sticky" than I like.

So I fired up LTspice, made a decently accurate model of the power supply and lighting and regulator circuitry and developed some mods. Here is the result:

[LTspice schematic of lighting and regulator circuitry with mods]
KT-80 lighting and regulator circuitry with mods.

The mods in detail:

KT-80 Voltage Regulator Mod

[LTspice schematic of regulator circuitry with mods]
KT-80 regulator circuitry with mods.

The mediocre voltage stability turned out to be largely caused by mediocre suppression of input voltage fluctuations near DC. The culprit is zener reference RC filtering via resistor R117 (see above schematic). The current through this part approximately equals that through R115 + R116, so the voltage drop over it is approximately

ΔVR117 = R117/(R115 + R116) × (Vunreg - Vreg - 2Vbe)

which as you can clearly see is decidedly not constant when unregulated input voltage changes! While the variation is only in the double-digit mV, times a gain of 1 + R119/R120 (about 2.2) it's enough to account for the bulk of load variation seen. The zener itself proved rock stable in comparison.

In the spirit of KISS, I added some feedforward resistance to pull up the feedback node voltage along with input voltage. RC filtering had to be used to avoid worsening AC PSRR performance. In sum, a total of 3 inexpensive passives plus some wire will do the job, which can be inconspicuously installed under the PCB.

Since ground is readily available at the wire bridge next to R119/120, Rff1 and Cff can be installed right there. Then install Rff2 at D16/17 junction and run wire between them, avoiding the mounting tab at the side. Fix wire with a dab of hot glue or somesuch.

With the values of 68k/10k/68k that I settled on, load variation went from -92 mV to +1 or 2 mV – going with 75k for Rff1 would arguably have been preferable, but eh, good enough. (I hadn't bought any of these, just 56k, 62k and 68k.) It's probably a bit overcompensated for pure input voltage changes, but definitely way better than stock. Perhaps not coincidentally, frequency stability is about an order of magnitude better than for either my KT-1100 or KT-900 (but unfortunately I didn't check it pre-mod).

Voltage came down to about 14.23 V with a bit of negative tempco, much closer to what they had originally intended (I am pretty sure someone messed up the calculation when designing the regulator).

The same voltage regulator architecture also finds use in:

although those with many blinkenlights, notably the KT-900, would be most obviously affected. (Now that guy is a whole different story in and out of itself… *foreshadowing*)

KT-80 No-Flicker LED Lighting Mod

I had already tried to simulate and improve upon the lighting circuitry back in 2007, but didn't have any good ideas at the time how to tackle the high AC voltages. Well, now I did – what about a capacitive dropper?

[LTspice schematic of lighting circuitry with mods]
KT-80 lighting circuitry with mods.

I settled on a slightly oversized capacitor and zener regulator, and in hindsight am glad that I did because voltage regulation turned out to be excellent, about 7.95 V with only 10 mV of load variation once 5 mA worth of warm white LED was added for power button illumination. That's actually better than the main voltage regulator stock… precision lighting. ;) OK, there is about 15 mV of initial warmup drift, but still.

The following had to be done:

Note, it's a clear LED and in hindsight I could have tried diffusing it, although the result is not bad as-is. Also, 5 mA is plenty bright (actually too bright when it's staring you right in the face) and half of that would arguably still be plenty sufficient.

Since finagling the power supply board in and out of there is a bit tricky, I would very much consider replacing the main filter cap while you're in there. I didn't and later noticed signs of incontinence. Post-mod, rectified voltage rose by 2 V, reaching a tad over 24 under light load, which is a bit hard on a 45-year-old 25V part. The original is a 16x25 mm size (d x h), though you could install something a bit bigger still in both diameter and height… I found some 2200µ/35V caps in 16x32 locally, and after installing one of those I was kind of glad that I didn't go with a 3300µ/50V in 18.5x35.5, as things turned out to be fairly tight as-is. Definitely whip out the insulating tape for the top when going with one of those. The original glue is a bit of a bear to get off, scrape off what you can. I didn't actually find any major leakage, only the glue on the cap struck me as softer than I'd have expected. Electrically the part still appears to be in perfect working order.

Mains power consumption with all mods has been reduced to 5.8…8.5 W. You could arguably drop this by another third or so with a different transformer of lower output (you really don't need 22-24 V unregulated for a 14.2 V supply), but since this one doesn't appreciably hum I'm not in a particular hurry to replace it. Plus, a soft, slow transformer protects my dropper capacitors and the small stock rectifier diodes (only a 0.6 A type). (The secondary measures about 10.3 ohms across, the primary about 450.)

Should you merely want to replace the original lightbulb (you can apparently get kits), I would strongly recommend the following additional mods:

This, inspired by the KT-900, should keep both the bulb and zener running in spec. You would still have the flickery dial pointer LED, of course. That could still be taken care of e.g. by running it from regulated +14 V with higher series resistance, but things would no longer be as minimally invasive then.

AFC Mod

The main AFC time constant is determined by (R74 + R75) × C72. So I tacked on an additional 47µ/25V below the board at C72, with similarly long legs, pointing away from the frontend just in case. That's literally it.

Listening to the LO signal with AFC turned on now reveals a fair bit less low-frequency content. There is also a pronounced difference in hum levels between this and the KT-900 now, which was already somewhat there before (I think the '900 has a fair few hours on the clock) but has seemingly widened with the new, bigger filter cap installed. Update: A new filter cap for the KT-900 has brought hum levels roughly back in line again, although for the newer model it's still more of a hum whereas for the KT-80 it's more of a buzz; maybe the KT-900's 100µ/25V could stand being changed as well.

Tips and Tricks

The stereo decoder VCO alignment procedure outlined in the service docs is quite convoluted. If you happen to have a fancy multimeter with a frequency counter function, odds are you'll be able to follow the procedure outlined in the KT-900 service manual, i.e. measure from the VR1-R28 junction to ground. If you do not have an RF generator, turn on REC CAL for a much more steady oscillator frequency. Incidentally, according to the HA12016 datasheet, separation should be maximum for a free-running frequency of about 75.1 to 75.6 kHz, but note that the VCO also drifts down when warming up.

Of course, on a tuner like this that'll enable stereo pretty much as soon as possible, you can easily do VCO alignment by ear and not be very far off at all (aim for the middle of the range of working stereo, then quickest locking on a weak signal after momentarily enabling REC CAL).

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Entry last modified: 2025-07-09 – Entry created: 2025-05-24

The Kenwood KT-900 AM/FM Tuner Project

Overview

The Kenwood KT-900 is a neat slimline analog tuner with digital frequency counter made in 1981. It is kind of a cross between the older KT-80 or KT-615 and its bigger brother KT-1000 and thus also features a pulse count discriminator.

Now that I have a physical unit to go with the service docs (it even came with the ever elusive AM loop and a similarly elusive user manual), I want to bring it back up to spec and turn it into the awesome band cruiser it could be.

[KT-900 in muted lighting]

Here are some pros and cons I noticed:

Service Doc Inaccuracies and Hardware Bugs

While the KT-80's schematic only had one major issue (the incorrect output connections at the stereo decoder IC), the KT-900's is downright blooper city, indicating a bit of a rush job. I have found the following inaccuracies, plus even a straight up hardware bug:

Work Log

My unit seems to have a fair few hours on it. The plan is to make it an awesome band cruiser in my local, fairly high-RF environment.

Work already carried out

TO DO

Tips and Tricks

If you happen to have a fancy multimeter with a frequency counter function, odds are you'll be able to easily realign the stereo decoder VCO, as your multimeter with its high-impedance input will be VTVM/SSVM and counter rolled into one. (The service instructions assume a lab counter with a 50 ohm input, which would completely throw off the frequency if you were to attach it to the circuit directly, hence the voltmeter as a buffer.) With this assumption, follow the procedure outlined in the service manual, i.e. measure from the VR1-R28 junction to ground.

If you do not have an RF generator, turn on REC CAL for a much more steady free-running oscillator frequency. For some reason, this function does not defeat the VCO, which in turn is completely unaffected by the ca. 430 Hz cal tone.

Incidentally, according to the HA12016 datasheet, separation should be maximum for a free-running frequency of about 75.1 to 75.6 kHz, but note that the VCO also drifts down when warming up, so given how warm the KT-900 gets, aligning to 75.8-76.0 kHz may be just about right.

Of course the VCO alignment is one of those that can generally be dialed in pretty well by ear already, even with no special tools at all. Even on a tuner that won't enable stereo below a certain signal level you can still determine where the range of working stereo ends (preferably on a station as weak as the unit will permit) and aim for the middle of that range, and odds are you won't be very far off. You can also momentarily enable REC CAL and observe how quickly the stereo indicator lights back up afterwards – obviously the closer the oscillator is to 76 kHz to begin with, the quicker the PLL will lock.

AM RF input circuit tracking alignment points may be worth playing with; I peaked it up at 1458 instead of the usual 1400 and have been quite happy with evenness of sensitivity towards the top end.

Frankly, AM alignment instructions are a bit rubbish in general and could trip you up if you don't have a general idea of how this normally works. First, align the LO to the dial scale via L12 (600 kHz) and TC2 (1400 kHz). Peak up RF tracking via L11 and TC1, as outlined above. Repeat each step as needed. AM IFT L13 should be adjusted where RF tracking is as broad as possible, i.e. towards the top end of the band rather than the 600 kHz suggested. You will have a much better chance at getting a nice and symmetrical IF response this way. Otherwise you are running the risk of optimizing L11 and L13 for a nice combined response at 600 kHz, with a rude awakening waiting elsewhere.

Mods!

WIDE IF switching diode bias

You know I couldn't leave R9 alone (originally a 100 ohm fusible btw, heaven knows why). While the 470 ohm resistor I had on hand isn't quite the 560-750 simulation reckons it should be, 3.7/6.0 mA is a fair bit closer to the 3.2ish Kenwood usually seems to have been going for than the original 4.8/8.4 mA. It'll never be quite right for both diodes anyway, and 1k would have landed me sub-3 mA.

S-meter

Looking at potential reasons for the optimistic S-meter on FM, I found the LED driver input amp to have more gain than it should. According to its datasheet, amplified voltage needs to reach 3.5 V to light the 7th segment, but that already happens at 0.23 V, indicating a gain of about 15.2 when according to resistor values it should be 9.333… instead, which normally would require 0.375 V. (In a way that is quite lucky, as I haven't seen more than 0.36 V yet, which is 2.81 V out of IC2, so the 7th segment would actually never get lit if the circuit worked entirely as intended!) Amplified voltage can reach 5.5 V. I suspect 150k resistor R135 has drifted quite thoroughly (must be a good 260k if so); at least 16.2k looks close enough for an in-circuit measurement of 18k R138.

Now I actually like S-meter action on AM just fine, it certainly doesn't need to be any less sensitive, plus our suspected culprit R135 is a bit buried next to L19 and wouldn't be the easiest to get ahold of (component legs were bent over quite thoroughly during production, so things won't come out without a bit of a tug). So I decided to change FM S-meter series resistor R31 (68k) instead, which forms a voltage divider with R136 (10k). A calculation reckons the ideal value to be about 109.5k, I went with a 100k I had on hand. A lot of my previous 7s are still 7s so range still doesn't set any records, but at least the previous 7 threshold should now be down to about 5 instead.

I just read a 1980 midrange tuner review, and insufficient S-meter range seems to have been shockingly common at the time, with the KT-80 of all tuners (max at ca. 300 µV, consistent with DD0OT's 51 dBµV) still being among the very best, and I don't exactly consider that a world-beater. Standards must have improved substantially in the following years, much like they did for tuner performance in general.

Stereo threshold mod

The previous mod can backfire on you as stereo operation is tied to the 4th S-meter segment being lit and the stereo threshold is higher than necessary to these ears to begin with.

If you want stereo at (almost) all times, you can unsolder and pull out the end of wire bridge J31 facing R142/IC6 pin 12, and run a short wire or spare component leg from the other end of J31 over to the R141/IC6 pin 9 junction instead, which should drop the threshold to 1 S-meter bar (plenty low enough for anyone, I would reckon). Even just unsoldering J31 already seems to work as well (while a connection to J29 weirdly doesn't).

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Entry last modified: 2025-08-30 – Entry created: 2025-05-30

Kenwood KT-1100 No AM Repair

Here's another from my stash of tuners from the olden days. I had done a bit of tweaking on it back in the 2000s (mostly S-meter and center-tuning discriminator alignment), but it had been living in my parents' basement for almost a decade before I brought it back recently. Most of the musty smell thankfully went away after a good wipedown with some vinegar, and contact cleaner plus an old toothbrush made quick work of the oxidation on the varicap wiper contacts.

Borrowing the KT-900's AM loop to try out the fairly fancy AM section, I noticed that it did not actually work any more. After a brief blip of activity, the frequency counter settled on 550 kHz (which I assume is some sort of default, since 550 kHz = 1 MHz - 450 kHz IF). I also noticed that the FM LO remained on.

In order to get down to this problem, I finally had to understand how AM power works in this model. You see, the highlighting in the schematic only leads you back to a 10k resistor on the base of a switching transistor that provides power to the FM frontend when AM B+ is off. So that led me to suspect that the AM section didn't have any power, but where was it supposed to be coming from then? Well, there is an innocuous-looking branch going to the cathode of diode D15, from which it turns out there's a connection all the way back to the FM/AM mode switch. Bingo!

So then I proceeded to locate D15 inside the actual unit, which seemed a convenient place to measure and bisect if nothing else. On AM, the multimeter indicated 13.5 V on the anode and 1.7 V on the cathode. So the regulated supply clearly made it to the diode, but there was nothing going through it as 11.8 V is far from your normal silicon diode forward voltage drop. I quickly verified with the thermal camera that it wasn't getting hot either (which may have indicated a short in the AM section), but nope, nothing warm at all. Should it really be this easy?

I only had some massive hulking 1N5406s on hand, so a trip to the local components shop was in order, from which I returned with several types of diodes. The old D15 is a type that can sustain 0.3 A and 30 V of reverse voltage (1N4148 for comparison: 0.2 A / 75 V), so playing it safe I settled on a 1N4936 quick recovery silicon rectifier (1 A / 400 V), the intermediate option between a 1N4148 and 1N4007.

Now Kenwood always bent over their component legs super well, so D15 put up a bit of a fight, but nothing that a desoldering pump and needlenose pliers couldn't overcome. The new diode soldered in without issue, and a quick test revealed 12.8 V after D15 and a working AM section! Phew. (The 1.7 V seem to be a result of current from the aforementioned 10k resistor to switching transistor base affair.)

I then proceeded to realign the AM section as per service docs (note, they leave you to figure out which trimmers are TC3/5/7 on the varicap, but it isn't too hard to figure out as touching the LO part will detune it), which proved quite necessary as alignment actually turned out to be a fair bit off even on the bottom end.

Unfortunately, receiver performance turns out to be a bit of a mixed bag. It is as hot as the presence of a tuned RF stage would imply, however I have a hard time getting rid of 9 kHz hets entirely even when switching to NARROW mode, which activates additional audio filtering. The base IF filtering – a 4-element SFP450F 12 kHz filter plus IFT – is best described as "barn door". Good for audio fidelity on strong local stations in 10 kHz territory, I assume (and it does sound good), but not very useful to me with a band that only comes alive in the evening. I actually prefer the more basic KT-900 with its much narrower filter. Anyone got an SFP450H or SFP450G to spare? (UPDATE - SFP450Hs have been obtained. Meanwhile, realignment has yielded a somewhat improved filter curve. I also bought an original AM loop to avoid having to borrow the KT-900's… well, an AM loop that came with a dead tuner, see next entry…)

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Entry last modified: 2025-08-30 – Entry created: 2025-06-06

Kenwood KT-880L Repair and Tweaks

Preamble

Since I had been unsuccessful in obtaining a spare AM loop antenna for my KT-1100 with its just-repaired AM section, I went looking on the used market and found one – well, one with a tuner "for spares or repair" attached to be precise. The antenna part number appears to be shared with the KT-770(L), KT-770(L)B and KT-880(L) models from the 1984-1987 time frame, and I thought that the latter's black antenna looked better than yellowed light grey. (Why they shipped these slimline tuners with this comically large loop is anyone's guess. Possibly they had to order a lot more of these antennas than they had KT-1100s to sell.) Figuring that I'd get my money's worth with the antenna alone and reviving the tuner would make a nice bonus challenge (even if I needed yet another black-faced tuner like a hole in the head, not to mention a rather ho-hum model), I lowballed the seller and the offer was accepted.

This is a fairly basic PLL synthesizer tuner sporting a 4-gang frontend sans AGC. Filters in this Euro model (I don't think longwave coverage was a thing anywhere else) are 2x SFE10.7MS3G (180 kHz), slightly deviating from the MJG + MS3G (150 + 180 kHz) listed in the service manual. Arguably the most interesting part is the FM demodulation, with a PLL discriminator plus a distortion cancellation circuit (only part of which is installed here, I guess the rest wasn't worth bothering with when using those IF filters).

Symptoms

The tuner arrived well-packaged and with the loop antenna and user manual (another one for the scanner, by the looks of it), which leads me to believe that it must have been cared for to some degree despite being slightly cosmetically challenged (one memory key seemingly glued back on many moons ago which had not held up as it was now missing, sticky case top).

Functionally, the unit turned on and could be operated normally but there was dead silence on all bands, no S-meter activity on FM, no stereo indication.

Diagnosis

Figuring that complete silence was pointing to something common in the signal path, I started in the stereo decoder vicinity. Now Kenwood was stingy when putting together the schematics for this model and no voltages are indicated at the IC, however the KT-770(L) is very similar and can be used for guidance. Sure enough, I found 11.7 V on the output pins 8 and 10 and a measly 0.01 V on the voltage reference pin 5, all of which should show closer to 6.5 V. Moreover, pin 5 showed a short to ground.

That short to ground could only be either of two things: A bad IC (tough luck if so) or a shorted electrolytic capacitor C110, 470µ/10V. Luckily, it turned out to be the latter, and a quick test with it removed showed pin 5 voltage back at 6.5 V.

The lack of FM reception had to be something else. I noticed some test points for tuning voltage – it would vary with frequency on mediumwave but was stuck at 28 V on FM. No heat in the FM frontend either. I then proceeded to poke around R29 and R27, and found no supply voltage of note after R29, although there was over 9 V coming in at R29 and the resistor measured OK. And what do you know, there was a short to ground. In this spot this could only mean either C29 (10n) or C30 (47µ/16V) being shorted. Either would only be reachable after removing the bottom LO shield – that figures. Just my kind of luck.

Repair

As basic as this tuner is, it is quite service-friendly still, with a removable bottom cover. (The newer 880D and 990D no longer had that, to the chagrin of those attempting to work on them, particularly the more complex 990D.) Only getting the top case back on is a bit finicky.

Removing the shield proved less dramatic than I had feared even with my trusty 20W ERSA iron, with some help from a large slotted screwdriver for prying. I could even reach the solder joint very close to chassis, even if just barely. Figuring that a 10n leaded ceramic would be unlikely to fail in this spot, I took out the electrolytic C30 first, and what do you know, dead short. Put in a new 47µ/25V – reception, hooray!

I replaced the bad C110 at the stereo decoder. Frustratingly, the voltages, while different, still weren't back where they should be after putting in a new 470µ/16V – about 3.8 V on the output pins and 0.75 V on input pin 2. No stereo either. Turns out the voltage reference amplifier's output was also shorted to ground. Have a guess what the culprit was. Yep, another shorted capacitor, C95 this time, the second 470µ/10V.

After all of that, you'd think that everything would be working, right? Nope, still no stereo. I got a bit lost in the weeds of the stereo inhibition logic converging at MPX pin 13, but ultimately it turned out that the LA1231N IF IC still wasn't happy. A classic candidate would be quadrature discriminator alignment then, and boy was that ever off (voltage across test points being +4 V at nominal center instead of ~0 mV, easily half a turn of T2). I suspect that the capacitor in the can may have started to corrode as these seemingly can do in yet older Kenwoods. Thankfully this is only relevant to center tuning and not sound, as the actual demodulation is performed by a PLL discriminator.

Finally, a working tuner!

Alignment

Unlike its quadrature cousin, the PLL discriminator only needed a minuscule tweak. My multimeter's frequency counter let me down when it came to MPX VCO adjustment this time, so I went oldschool – ballpark center of adjustment range with working stereo (at least I had a DC reading) and observe how quickly the stereo light comes on. (I did eventually get lucky when connecting the meter between the non-grounded trimpot leg and chassis ground as I had done on my older Kenwoods. Input impedance may be low enough for series 220k R137 to be thwarting my attempts at using the prescribed test points as the UT-71C wants at least 200 mV worth of signal, but one can realistically hope that it's not so low as to be having much of any influence on a pot of <5k.)

FM IM3 performance has substantially improved with RF realignment, so that must have been somewhat off (in addition to the usual minuscule tweak of the LO trimmer). I would say sensitivity is about on par with an Onkyo T-4650 (aka T-4500) in LOCAL, so the spec seems a tad optimistic. (Mind you, the Onkyo is a spectacular tuner with RF AGC.) Still better than my Grundig FineArts T904 though, which mind you never struck me as the hottest tuner on the block, so it may be in even greater need of realignment. (And in fact, it turns out realigning the LO on that one brought back several dB of sensitivity. It remains more prone to IM3 than the Kenwood, not a real surprise given its BJT mixer, but sports a second attenuated input for cable level attacks.)

This tuner is one of those that doesn't have trimmer capacitors for the tuned FM RF circuits. Should you find that tracking is increasingly off towards the upper end of the band, you may need to tweak LO alignment for a tuning voltage range slightly deviating from specified values (which one would hope would normally have been picked such that good tracking is achieved).

You are really going to struggle peaking up the RF circuits with the instructions provided. Instead of probing audio output level (which doesn't make a ton of sense on an FM receiver), go for the signal strength indication voltage out of the LA1231N instead, available e.g. on R141 or R47. It can be quite jumpy, so an oldschool analog instrument may be superior to a fancy modern digital multimeter. The coils do need a small nonconductive (plastic) hex, with the IFT a tool with a small brass blade fit.

If you find yourself in need of tweaking FM IFT T1 with an FM stereo RF generator at hand (you will need one with hi-fi grade distortion and channel separation specs), feed in a 440 Hz or 1 kHz sinewave either L only or R only and listen for distortion on the opposite channel after crosstalk has been minimized. Said distortion is obvious enough that you can do this by ear, at least on this model.

Stereo separation did need a small adjustment. This basically eliminated a previously perceived dullness in sound.

MW and LW alignment proved to be more or less off in just about all respects, as previously on the KT-1100. Seems to be a thing for 4-decade-old Kenwood tuners. I would very much advise to slightly deviate from the instructions again: T9 is the 450 kHz IFT and should be aligned towards the high end of the band, i.e. step (4) 1440 kHz (rather than (3) 630 kHz) to minimize the effect of RF filtering on the filter passband. Aside from that, it's all quite doable by ear.

Notes

With so many shorted electrolytics, I suspect that the unit may have sat for an extended period of time (and I mean years and years) before being powered back up. (There's also indications for a fair bit of use though.) Capacitor voltage ratings in this model tend to be on the tight side in general. Funnily enough it is not the parts with the highest ratio of sutained to rated voltage that failed here. Both 470µ/10V ones being dead almost can't be a coincidence though. C30 may just be a case of the far end of the bathtub curve.

I have yet to track down a popping noise when the unit is switching between mono and stereo. I suspect another bad cap but can only speculate which one it might be – C122 perhaps, or one of C102/103/104?

Both a 62Kxxxxx unit serial number and lots of 1985 IC datecodes point towards a (possibly early) 1986 production date. The manual shows some codes indicating 86 and 87. I've seen serial numbers from 5xxxxxxx to 7xxxxxxx. (The later 880DL then stretches from 7xxxxxxx to 9xxxxxxx.)

The stickiness was tackled with turpentine substitute (white spirit) and alcohol. It's not perfect (could probably use a respray) but better.

Of the handful of KT-880Ls currently listed on the 'bay around here, there is one that's being sold as defective with "no audio, tuning and saving stations works". Since you've been reading this, you should be able to tell what's bad… ;)

Performance wise, the frontend seems to perform similarly to other basic 4-gangers without AGC, with some mixing products from the local flamethrowers. (Realignment has improved matters substantially in this respect and largely eliminated those, and sensitivity seems about in line with other tuners.) The two 180 kHz filters provide decent 200 kHz separation but can struggle in more severe cases of weak stations next to strong ones as skirt selectivity just isn't quite there, so not exactly pure canned DX. Center frequency appears to be roughly 25 kHz off (results are very similar at 0/+50, -50/+100, -100/+150 and -150/+200 kHz from fnom), so the red dot filters seem to be on the limit of their rating. There are no accommodations for such issues in the frequency synthesizer.

Sound quality strikes me as good, as one would hope (that's the one thing they arguably didn't skimp out on, with a PLL discriminator plus a distortion cancellation circuit). It doesn't seem to have as brilliant a top end as the KT-1100 though. The difference to the KT-80 isn't as big, but aside from being a bit quieter in general (I reckon output impedance has to be about 3.5k, a quirk shared with the 880D and 980F), the top end still seems a bit duller, and there isn't quite as much bass either. (The perceived dullness basically went away after realigning stereo separation, which does seem to have been a bit off. Now KT-900 and KT-880L sound pretty much identical after having been aligned to the same standard, level difference notwithstanding. Initially they would have been quite different as their separation adjustments were off in opposite directions. I do still think that there is an ever so slight mid-treble depression in this one.)

This model is quite similar to the preceding KT-770(L) but I would rather consider it a slight step down in several respects, e.g. somewhat worse selectivity and dropping a gang for FM LO tracking or the pilot leakage adjustment. Stereo/mono in the older model being tied to auto/manual tuning must have been super annoying so they dropped that link in the KT-880L, but as a result it doesn't have any explicit stereo/mono switching at all, which is a bit naff (I don't think I've ever had a tuner without this before).

Using this tuner for band scanning isn't exactly a joy – while the up/down tuning buttons are nice and large at least, manual tuning is quite sluggish (auto tuning seems a bit faster). You will be gravitating towards the two banks of 6 presets each (which can be assigned at will to stations on any band, something that wasn't the case with the preceding 770 model as far as I can tell) for getting from A to B in a timely manner. At least saving and recalling those is pretty much intuitive.

The S-meter is faking more resolution than it actually has – behind the 6 dots there's only 3 LEDs plus varying intensity of the respective topmost segment. (The circuit is literally less complex than the 5-year-older KT-50's.) At least it seems to cover a decent amount of dynamic range.

All the indicator LEDs on this model are red and quite dim. The frequency / band display is a greenish-white VFD of good brightness. The band indicator appears to be visibly flickering while the frequency appears static.

The AM section is bog standard fare in the selectivity department (one SFU450E presumably 1-element ceramic filter plus IFT is nothing to write home about) but at least retains tuned RF stages so should potentially be quite sensitive. (Only the Euro models get the luxury of RF preamps.) Indeed, that's what I get post-alignment – it's a super hot receiver but the 9 kHz whine drives you batty. (And by super hot I do mean that. It's literally more sensitive than a "barefoot" Sony ICF-SW7600.) The KT-900 sports about the same audio bandwidth but its tighter filter skirts are much appreciated, not to mention being able to off-tune instead of being stuck to the 9 kHz grid. The KT-880L's longwave band goes to the other extreme by offering 1 kHz steps and only those, so tuning is not exactly rapid. 3 kHz would arguably have done just fine. I'm not sure why the band had to extend all the way to 360 kHz either (I cannot recall a single broadcast station past 279 kHz in my lifetime, which had been the official upper limit since 1975), but apparently it was a thing at the time (e.g. up to 353 kHz in the Grundig T7500). Perhaps a Swiss carrier current broadcasting system with channels on the longwave band including one on 340 kHz (and only turned off in 1998) is the solution to this mystery. One would have to ask the folks at Toshiba who they designed those particular quirks into their digital tuning IC.

The KT-880L is kind of a power miser when on, about 6.2 W on AM and around 7.5 W on FM (at least once fixed… it was several watts more initially). The same can't be said about the off state though, the power switch is secondary-side so things don't go below 1.6 W and change. (At least it's not as bad as the '90s standby sins.)

I was wondering how this model has any sensible memory backup times at all with only a single 2200µ/16V capacitor (not even of the double-layer variety, although I guess choosing the voltage rating generously was aiming to bring leakage current down) when specified TC9147BP sleep current is between 15 µA at 5 V and 5 µA at 2 V, equivalent to 333 to 400 kOhms. That's an RC time constant of 880 seconds (less than 15 minutes) at best, and I know mine has kept its memory for many hours so far. As it turns out, the standby power draw is not entirely in vain, as the 5V supply is being kept on and will supply part of the digital circuitry. That would explain why its filter capacitor C171 is generously rated, which hardly ever happens otherwise. Still, I am quite certain that I've had no issues with memory backup even with my switched power strip being turned off for hours, even days. The Toshiba IC may be a lot better than specified.

The PROGRAM function that will alternate between the last station received and preset A6 at each power cycle seems awfully niche. It had to have been geared towards timed recording, but making good use of it does not seen easy to me.

I ended up liking this model a fair bit more than I anticipated – sometimes there's just something to be said for a simple, good-sounding tuner. (Post-alignment, stereo distortion is much more like my KT-900's in wide than in narrow. The AM section sounds better, too, even if selectivity is not great. At least that was the case until I fitted a new AM coupling cap to the KT-900, which means that one is ahead again.)

Mods and Modding Potential

Looking at the layout, coupling cap C36 is virtually begging to be replaced by a 3rd FM IF filter. (Maybe move CF2 there and place something wider like a MA8, MA5 or MS2 in the CF2 position?) You've got some leeway for increasing gain via R39 if need be, since it has been turned down in the Euro model because its filters sport lower insertion loss than the GDT types used elsewhere.

The stereo threshold is rather on the high side. I first unsoldered one end of R142 (33k) but that makes it rather low, with some propensity for rapidly oscillating between mono and stereo. I replaced it with a 47k eventually, which gives a stereo threshold as low as I would ever want while still switching to mono on anything really weak. One could still argue for 43k or 39k, I guess, but I didn't have any at hand.

FM and AM scan speeds and muting times are being controlled by the RC time constants on IC11's pins 23 (OSC1) and 22 (OSC2). As of stock, R305 = 39k and C309 = 2.2µ result in fosc = 16.6 Hz or 8.3 steps per second on FM. With 50 kHz steps, that's a bit slothtastic at about 0.4 MHz/s. Muting is a tad on the long side for my liking, too. According to the TC9147BP datasheet, the resistor value should be between 10k and 100k, so there is room for improvement, the question merely is how low you can go without upsetting the auto tuning. (Funnily enough, that's pretty much the same scanning speed as my old Grundig T7500. I guess it never bothered me much because I mainly used it for fixed station reception, using its 15 readily accessible presets with alphanumeric labels. Plus, that's actually in 25 kHz steps.)
After pondering this for a while, I decided to solder in a 22k in parallel to R305, bringing its effective value to 14k. (This is also the recommended route, as the part is right between two electrolytics and would be hard to pull out.) The result is as quick as I would ever want, bordering on twitchy, so I would certainly not recommend going any lower than that. You could try 33k-39k. Auto tuning still seems to work fine like that, even if I've noticed some asymmetry between tuning up and down that's likely to be related to the offset IF filters.

While this model has more flexible preset management than the predecessor, with 2 banks of 6 presets that can be assigned randomly, the IC would actually support up to 8 memory buttons each, 2 of which are now going unused. That's a bit of a shame. Now as it happens, my unit is missing its #5 button keycap, and it would seem that you could route some thin wires from some external momentary switches with LEDs through there…

The TC9147BP would also support AM auto tuning if supplied with a 450 kHz IF signal, but they did not bother to implement that on this model, for better or worse.

Checking Capacitors

When I pulled the unit out again for measurements, I took the opportunity to check the ESR of capacitors in the power supply section and whatnot, which yielded these results (ESR in ohms as measured by ELV ESR1 at 60ish kHz):

C61 100µ/16V 0.67
C64 10µ/25V NP 1.74
C80 47µ/10V 1.83
C81 47µ/10V 2.46 ?
C82 100µ/16V 0.69
C106 470µ/16V 0.19
C107 33µ/16V 2.23 ?
C110 (replaced) Elite 470µ/16V 0.05
C122 4.7µ/35V 2.47
C160 100µ/10V 1.31 ?
C161 10µ/16V 1.32
C162 10µ/35V 1.48 ?
C163 1µ/50V 4.84
C164 1µ/50V 5.38 ?
C165 47µ/10V 2.90 ?
C166 47µ/10V 4.74 X
C169 33µ/35V 1.72 ?
C170 1000µ/25V 0.05 ?
C171 220µ/35V 0.17
C172 330µ/50V 0.07
C174 22µ/16V 1.41 ?
C175 1µ/50V 4.11
C179 330µ/25V 0.19
C191 33µ/35V 1.42 ?
C196 N.P.

Not super great overall. This confirms my hunch that the unit must have had a fair few hours on it before being put away for an extended period of time. C166 is the cap after the 5.6 V regulator which always runs in standby, so seeing that being toast makes sense. The associated filter C171 seems to have a rather chill life after its 100 ohm dropper instead.

I haven't swapped anything yet but did end up resoldering the main regulator power transistor (Q29) and the main filter cap after seeing some intermittent low-level rectifier noise during measurements. Seems to have helped temporarily, but a low-level zoo of rectifier harmonics has come back that seems to be coming out of the detector… tuning voltage supply?

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Entry last modified: 2025-09-13 – Entry created: 2025-07-22

The Onkyo T-405X saga

Introduction

It occurred to me that I should probably include the tuner that rekindled my interest in the things to begin with. It all started with a desire to have something more hi-fi than my trusty Panasonic RF-1410LBS radio there (which now resides in the bathroom), preferably without breaking the bank. It occurred to me that I would be able to use a micro component tuner plus some active speakers with an extra AUX input. (I ultimate got some brand new Edifier MR3s, with which I've been quite happy. I never expected to be using the Bluetooth quite as much, but it has proven quite handy.) Please note that the following is based on a series of posts I made back in April of 2025 in this ASR thread.

Tuner wise, I took a punt on a slightly banged-up looking Onkyo T-405X from 1999, WHD 205 x 76 x 280 mm. These basically aren't worth anything on their own, I got mine for a tenner plus shipping. I believe that this model is largely identical to the preceding T-405 and succeeding T-405TX (I mean, it's the late '90s, how much do you expect to be changing in FM tuner tech over 3 years at this point?). The latter has a better-quality service manual available.

The tuner can be fully operated from its front panel without requiring connection to a matching power amp or receiver. I did, however, find out that you may still want one, or else you'll have to improvise when it comes to cutting power to the unit as standby power consumption is a bit dire.

I was most worried about the glaringly obvious scuff in the display window, but thankfully a bit of Polywatch polishing paste and toilet paper (I couldn't find my usual polishing cloth / handkerchief) made quick work of it and removed it almost completely, to the point where other blemishes are more prominent now. (I strongly suspect that the company selling Polywatch / Displex in tiny tubes for several Euros each is making a killing on the stuff, but you can't deny it's good for polishing acrylic.)

Internals

Tech wise, Europe got a Mitsumi 4-gang frontend (if a small basic one with no RF AGC, FE417-G02) plus 3 IF filters (2x MA5 = 280 kHz + 1x MZ2K = 150 kHz GDT) and transistor IF amps of the not-explicitly-limiting variety, with the rest around an LA1837 with a proper ("hi-fi") dual coil quadrature detector, overall reminiscent of Onkyo's contemporary receiver FM/AM sections. AM was, however, not equipped, not a major loss I imagine.

The display is a typical VFD with filter window. It's not super bright (possibly owing to old age) but adequate, and dims a bit in standby.

I've taken a look inside - not much going on in there. (You can get an impression of the Japanese model here.) As small as the unit is, the PCB doesn't even fill it completely, close to 1/3 of real estate remaining empty. The frontend is quite tiny and standing up. There are some holes in the side wall to facilitate alignment, it still doesn't strike me as easy though. There are no instructions for anything but the IFT after the mixer as that is the only part accessible externally, and those don't even make sense (you are supposed to peak audio output level, well good luck with that on an FM tuner – I would target R112 next to LA1837 signal strength indicator pin 11 instead, and even that still requires a very weak signal for a decent amount of discrimination, at least in my unit that seems to top out at little over 3.4 V).

As it turns out, peaking the frontend IFT seems to get things very close to best channel separation and stereo (L only / R only other channel) distortion already. That may be a testament to ceramic filter selection by the 1990s. Despite the lack of a dedicated separation adjustment, stereo separation seems very good at this point. The residual seems a bit buzzy though.

Power Consumption

Power consumption figures are as follows:

Settings seem to be kept at least for a while when power is removed, so that's good.

The issue of standby power consumption was ultimately tackled by installing a power switch in an extension lead. I would not be one bit surprised to find the odd tired electrolytic cap in there, mind you, depending on the unit's history.

Regarding the transformer hum – it seems to be majorly amplified by the case. It's hardly audible with the lid off, and none of the screws holding the transformer or transformer holder are loose in any way. That's thin sheet metal for ya, I guess. I may tap into my stock of bitumen mat. (And the T-405TX was even lighter?!) The transformer itself is a cute EI core job that doesn't look oversized one bit, which kind of is what you'd expect.

Comparison to Kenwood KT-80

And now the part that absolutely nobody has been waiting for, the great tuner shootout: Kenwood KT-80 (1980) vs. Onkyo T-905X (1999). Both silver-faced and FM only (only the US version of the Onkyo has AM as well), but one a slimline analog normal-width model featuring a pulse count discriminator, the other a PLL synthesized micro affair sporting a regular quadrature discriminator and RDS decoding. Both 4-gang tuners with 3 IF filters, neither featuring RF AGC.

They sound really, really similar. Even output levels are almost identical. The T-905X might be a bit leaner in the low end, but that's about it. I don't notice any noteworthy difference in distortion, despite the vast difference in specs. (Mind you, I think those are awfully pessimistic for what's inside the Onkyo. I guess they wanted to cover their off days when they couldn't be arsed to properly align the discriminator or happened to install badly mismatched filters.) The T-905X seems to have kept its alignment well - a sign of how mature FM tuner technology was by the late '90s.

Sensitivity wise, there doesn't seem to be much in it. I would have to have a proper antenna switch and all to tell for sure, which I don't. Physical location sems to have a greater effect on reception. The T-405X has some pretty major issues with 3rd-order IMD where it sits in the kitchen, to the point of even a medium-strength station getting swamped, so I had to make the antenna wire quite short. Mind you, the poor Panasonic radio was always struggling as well. RF levels on FM for the local flamethrowers just seem to be pretty gnarly there. That being said, given that there are no alignment instructions involving RF tracking (they only instruct you to peak the IFT following the mixer, which is the only part accessible externally), I suspect that the frontend may have been supplied prealigned, and heaven knows how good the alignment still is or ever was. Either way, the T-405X is definitely a hot one, for better or worse. I resorted to using a variable RF attenuator I still happened to have.

RDS on the Onkyo appears to work quite well and will dutifully display radio text when asked to, even if it struggles with some characters like the colon or umlauts. There are enough buttons on the unit for it to be fully usable even without a remote (it sports an Onkyo RI system connection for that). You can save a station to obtain the time from when in standby, and it will dutifully do so (along with the weekday) and display the time with dimmed brightness. (The weekday can be displayed when the unit is on.)

Selectivity is similar (neither being a thoroughbred DX machine) but it seems the Onkyo does slight better in 200 kHz separation as well as fending off the local flamethrowers over 400 kHz. You'd think that the Kenwood's combination of 1x 230 kHz GDT (MM) + 2x 180 kHz (MS3) should beat the Onkyo's 2x 280 kHz (MA5) + 1x 150 kHz GDT (MZ2K), but then the second IF amplifier stage in the Kenwood also is an IC limiter-amp and as such may be degrading performance when strong signals are involved.

Desperate times call for desperate measures, so I resorted to the method of listening to each others' LOs. The Kenwood's seems to be exceptionally clean, no hum, no signal modulation, no nothing (maybe not a surprise for an oldschool mechanically-tuned affair). (PS – I have since noticed that there is, in fact, signal modulation when tuned to a strong signal. And the hum is very low in level, getting even lower after installing a bigger mains filter capacitor.) The Onkyo's has some low-level hum and a hint of signal modulation from the currently-received station. Listening on the KT-80, the hum actually turns into a buzz (which is not present as such on my secondary receiver, a Sony ICF-SW7600, though that is a lot noisier), so looks like that's getting into the audio somewhere. I never noticed because volume is really cranked at this point, i.e. near max on low gain on my Atom Amp+ with HD600s... possibly 30-40 dB above normal. (The KT-80 has this super long audio path that literally runs halfway around the PCB, so odds are it's picking up something because of that.)

In the schematic for the FE417-G02 frontend, I noticed that there is a higher than average 10k resistor feeding the tuning voltage to the FM LO, where in other models I've already seen 4.7k, 2.7k or even a 2.2 µH inductor. Mind you, noise level didn't strike me as problematic either way, and 10k had been good enough for a number of other tuners using the same LM7001 PLL synthesizer with the FD636U12 5-gang frontend (e.g. T-4670/4700, T-4670/4500, T-4850/407). The LM7001(J) is common as dirt in '90s tuners.

Overall, the Onkyo T-405X is a proper little tuner. I do wish it had a proper off switch (and along those lines, a unit of unknown history may appreciate the odd new electrolytic), and RF AGC clearly wouldn't have hurt as handling of high RF levels is relatively poor. With the Euro spec model being a 4-gang tuner with 3 IF filters it would have some modding potential in the selectivity department, I'm sure, but there's probably better candidates in terms of basic, inexpensive tuners to mod that are easier to work on (like maybe a T-4210R or T-4211, which do sport RF AGC as do their predecessors going back to 1986 at least; even the T-4310R with its 4 filters and IF bandwidth switching is not at all expensive; mind you, I've recently seen an old T-4017 go for a tenner and even that was just for me bidding on it, and that's a 4-filter job with a 5-gang frontend approaching a T-4650/4500). Either way it has rekindled my interest in tuners, and I might get another silved-faced model for the bedside table. The local radio landscape at night seems to have gotten pretty depressing though…

Audio Measurements

With my audio loopback measurement setup now firmly in place, I had to check the audio performance of the T-405X as well. And hoo boy, this one's a doozy.

Frequency response is actually good, being essentially dead-flat between about 15 Hz and 3/4 kHz before gradually declining, more quickly so beyond about 7 kHz, reaching -1.2/-1.5 dB by 14 kHz before the pilot tone filtering kicks in (which is pretty much bang-on in both channels). No complaints there.

Dynamic range is fine but nothing overly special at 72 dB(A) in mono PE off. Power-related harmonics are more pronounced in this one compared to the T-422, with 150 Hz at -68 dB and 100 Hz at -71 dB. I reckon the 1000µ/25V filter cap is a bit tired. The original is a 12x20mm and diameter can't get much bigger. (My old Kenwoods had 16x25mm caps, that gives you some more options.)

Channel separation (L→R) comes out as 42 dB (60 Hz), 42 dB (1 kHz), 36 dB (7 kHz). The rather pronounced, broad skirts around the 1 kHz tone suggest a rather noisy MPX VCO and a PLL bandwidth of near enough half a kHz (hmm, the loop filter components on LA1837 pin 14 are pretty much exactly as specified by its datasheet, I wonder whether the capacitors are still alright?).

Now remember the THD specs (0.5% mono, 0.8% stereo)? Well, 0.5% is about all I could get, and it's nasty with a lot of high-order harmonics, too (the top ones being H2 H5 H7 H3 H4). Stereo only increases slightly to 0.6% (mostly H2 H3 H5 H7). So that's why the residual always sounded buzzy to me. Given that specs are what they are, they must have known about this. The story still isn't pretty even when I reduce deviation to 10 kHz (-12 dB), distortion even increases numerically although it becomes mostly H2 at about 2%.

Going through the passband in 12.5 kHz steps, even output amplitude is up and down by tenths of dB, and if anything distortion looks best at +/-100 kHz off. I have never seen anything of the kind, the IF must be a total mess. Now the part values suggest arguably less than ideal impedance matching at the IF amplifier inputs (I guess favoring sensitivity and selectivity), maybe that would do it?

Given how much better the (admittedly twice as expensive) T-422 does with the same filter complement, it seems wasted on this heap. They could have put in an MS3G instead of the MZ2K and it wouldn't have mattered one iota sound-wise while it would have been cheaper and selectivity would actually have improved. Likewise, a single quadrature coil would have done the job, no need for a hi-fi grade dual coil setup.

Well, let's just say this one's going to remain my kitchen tuner. It sure is interesting how that much distortion still seems to be mostly unnoticeable though.

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Entry last modified: 2025-09-08 – Entry created: 2025-07-25

Onkyo T-422 Intermission

Initial Repair and Testing

I fished a champagne color "spares or repair" specimen of this ca. 1997 midi-sized (27.5 cm wide) tuner out of the bay with a cut mains cord and some scuffs, so clearly a recycling special. Other than the inside of the top cover being very brown, indicating years and years in standby (or perhaps a fireplace in its former home?), there was nothing untoward to be seen inside, so I replaced the mains cable with one including a switch and went ahead with testing. I knew from the T-405X what Onkyo's idea of standby was like in these system tuners, and seeing 8 W of standby power consumption (only marginally less than the ca. 9.5 W in operation) confirmed my hunch.

There was a minor hurdle in that I did not fancy depinning the internal power connector, so I proceeded to cut the remaining cable stub and solder the new cable to that. This proved a bit more tedious than you think, not least due to the heatshrink fighting me, but ultimately and with an additional layer of electrical tape it all worked out. A few of the scuffs came off with some elbow grease, and Displex to the rescue for the acrylic display window (again).

Functionally, the unit works fine. I'm sure it could use a few new electrolytics, even Nichicons have a finite life span after all, and a bit of low-level hum I noticed when performing separation alignment may be an indication. Anything at this level of quality I'd have to order, the local store carries tinkerer grade parts only. They may have a matching heatsink for the 5.6 V regulator though, which is approaching 70°C Tcase and only sports the PCB footprint for one. The beancounters won again there.

Display brightness is comparable to the T-405X's in standby, but it's noticeably higher in operation. What that may mean in terms of initial brightness, VFD aging and voltages is anyone's guess. The filters appear to be quite different, the T-405X has a substantially darker one and looks better due to its lack of reflections from the VFD.

Circuitry

Circuitry wise, I kind of knew what to expect circuitry wise from the Japanese market T-422M. Which is good because I could not find a service manual anywhere. And by that I mean you can't even buy one. It's like it doesn't exist. I could name a few more models from the same era that share this fate, like T-4511 or T-4211 (although there is one for the latter's US version T-4040, not to be confused with the low-end 1970s tuner).

The frontend I think is pretty much the same compact 4-gang Mitsumi job as in the T-405X (and would actually be fully accessible for alignment), as is the filter complement (2x MA5 + 1 MZ2K). That being said, the first IF amp uses two transistors instead of one, and there's an LA1266 IF IC (which also handled the AM section) and an LA3401 stereo decoder instead of the combo LA1837. The RDS decoder is a µPC1346CS, as in my old TX-SV636; the T-405X sports a BU1922 instead.

Realignment

The above page mentions these alignment items:

+ there's R801: RDS 57 kHz filter alignment

Armed with an antenna plus the ELV SUP2 test RF generator kit that I assembled last weekend (spoiler alert: the 3/4h build time seems wildly optimistic), I checked over the alignment. Discriminator center tuning was about 0.37 V off (P121), and adjusting L101 very gingerly (it's a very delicate adjustment) brought it back within 10 mV. Maybe 10 mV of difference between top and bottom band edge, so as usual the reference xtal is fine. (I'm always doing this using OTA stations as they have very tight standards for frequency accuracy.)

Not yet being set up for loopback testing, I skipped L102 initially. (You can attempt to adjust for equal and maximum voltage differences between nominal - 50 kHz, nominal and nominal + 50 kHz, but it's a very delicate dance as there is a slight effect on center tuning as well, so you have to tune up and down all the time and correct center tuning periodically via L101.)

Instead, I went to stereo distortion and separation (frontend IFT, R201), feeding L-only / R-only 440 Hz sine into the SUP2 and listening on the opposite channel plugged into my Atom Amp+ with HD600. (I tend to unplug the channel with the generated signal from the amp so I can crank up the volume.)
Best separation was obtained in the position I originally found R201 in, which is to say pretty much the far left end.
This struck me as odd, so I investigated. From pin 4 of LA3401, there's R203 (47k) - R201 - C206 to ground. Total resistance is adjustable from 47k to 157k. I figured that I could just install a jumper wire across R203 and still cover the optimum found, so that's what I ended up doing. (If you remove the front – 2 bottom screws, side clips – plus 4 screws at the back, the main PCB can be turned over quite easily.) Afterwards, I found my optimum at about 59k total, so technically that would have been just within range even before and my mod is not actually necessary. It may prove handy should you ever decide to install some narrower filters. R203 should probably have been more like 22k to begin with, but I don't know what the US version requires.)
If separation is well-adjusted, distortion on the opposite channel becomes easily audible. The IFT ultimately ended up about 30° further to the left to minimize distortion. We'll see whether this holds up when my "legit" FM RF generator arrives.
Well, it has (a Levear VP-8193D), and I promptly had to redo IFT and stereo separation alignment. So don't rely on toy grade FM generators. The IFT ended up pretty much back where I found it (I take that as good sign), and it's a good thing I had modded the stereo separation circuit as the optimum position would have been out of range stock (about 37k total). Very nice separation now. I later confirmed C206's ESR to be 1.2 ohms, which seems reasonable enough for a 10µ/16V, so that's definitely not toast.

R801 again ended up pretty much exactly where it was to begin with. RDS sensitivity still seems a bit meh. The TX-SV636 never was the hottest in that regard either, so I guess the NEC IC just isn't the most sensitive. The T-405X strikes me as better.

Much like the T-405X, the frontend is quite prone to generating IM3, so I assume no RF AGC in this one either. While they would be accessible in this model, I'm not touching any varnished air coils – no thanks. At least image and IF rejection both seem to be excellent.

To Note

Unlike the newer T-405X, you can only set the RDS Accuclock station frequency (for automatic day / time adjustment) using the remote on the accompanying A-922 with a RI system connection, which is a bit of a bummer. It's working here anyway, so perhaps the unit automatically searches for one in the background as a fallback option. There were no presets stored or anything, so I'm pretty sure the goldcap was empty as received.

According to Hiro-kun, the time can be set manually as follows:

How to set the time manually

  1. While holding down the [MEMORY] button, press the [DISPLAY] button.
  2. Press the [DISPLAY] button several times until [ADJUST] is displayed, then press the [MEMORY] button.
  3. When [MANUAL] is displayed by pressing the [TUNING/PRESET] button, press the [MEMORY] button.
  4. After adjusting the day of the week with the [TUNING/PRESET] button, press the [MEMORY] button.
  5. After adjusting the time using the [TUNING/PRESET] button, press the [MEMORY] button.

More Repair Work

During alignment, I noticed a bit of low-level hum, no doubt due to one of the filter capacitors being a bit tired. With my new ESR meter, I found the 1000µ/25V to be at about 0.1 ohms, which is still decent but about 3 times what it should be. A 47µ/100V (should be in the negative voltage supply for the display) measured 2.5 ohms, that's quite dead. A 100µ/16V at the frontend was up to 0.55 ohms, again roughly 3 times what it should be. A lot of the smaller caps I checked still seemed perfectly reasonable though.

The 1000µ/25V actually measured 0.05 ohms directly at its pins on the bottom side, which I considered serviceable enough. I did, however, swap out the very tired 47µ/100V for a 100µ/63V (the part actually sees something in the 40s tops, I think they only wanted to maximize its size for longevity). A dark dust spot on its side is hinting at the reason for its demise, the dropper resistor next to it (if the T-405X is any indication, that feeds a 27 V zener). Unfortunately I had neglected to buy anything close to a 100µ/16V, so that'll have to stay for now. (I did buy a Jamicon SS series part eventually, but at 0.45 ohms of ESR that hardly seems an improvement. The TK 100µ/35V at around 0.2 ohms seems more promising provided I can make it fit.)

All the capacitors mentioned seem to have counterparts with the exact same values in the T-405X, so I think it's safe to say that they have quite a bit in common.

The big ribbon cable, a rather tight friction fit affair, sent me on a bit of a side quest, as the blue reinforcement tab at the end came off and had to be superglued back on.

I did end up installing a lightweight push-on heatsink on the 5.6V regulator after resoldering it. Not the last word in heat dissipation but better than the bare regulator I'm sure. To nobody's surprise, the old joints quickly gave way while test-fitting the heatsink, so they must have been quite brittle and weak.

Performance and Sound Quality

Now a DX machine this tuner is not – as previously mentioned, the little Mitsumi frontend appears to be the same as in the T-405X and is similarly prone to RF intermodulation, so the basically good sensitivity is kind of moot and an external adjustable attenuator is a recommended accessory. The two 280s and one 180 kHz GDT filter provide a compromise bandwidth somewhat on the wider side, so the unit struggles a lot more with 200 kHz separation than the KT-880L and still comes in behind the KT-990SDL in Wide. Mind you, it still just about manages to pull it off, which is more than you can say about twin 230 kHz GDT (MM) filters like in the KT-3050L's (or T-4650's) Wide bandwidth.

RDS sensitivity is generally fine but not as good as the T-405X's which uses a different IC. I think the µPC1346CS just isn't the last word in that regard. My TX-SV636 receiver uses the same chip and came misaligned from the factory to boot, which made the RDS super deaf; I corrected that many years ago but sensitivity only became "fine" even then. At least the T-422 sports radio text which the (otherwise much quicker) KT-3050L doesn't, rather a must-have feature in my book. Much like the T-405X, the RDS Accuclock function requires the tuner to be turned on and back off once after initial powerup before it actually acquires the time.

I was, however, very positively surprised by sound quality. Post repair, stereo separation mod and alignment, it's getting super close to the KT-900 subjectively. Not bad for something that had barely escaped recycling! I can't say I'm terribly surprised, the filter complement should be little worse for group delay than the three MMs found in Onkyo's entry-level full-size tuners, and the LA1266 with dual coil quadrature detector and LA3401 stereo decoder are shared with numerous other good tuners, leaving only the 19/38 kHz output filters (and postdetection filter) as a variable. I did have to modify the stereo separation adjustment circuit for best results; this should only be necessary on the Euro spec model.

These guys were Not Cheap (basically twice as expensive as the T-405X) when new, and I guess it did show somewhere after all.

Audio Measurements

Once I finally had a loopback measurement setup cobbled together, there were some positive and negative surprises. For starters, I found 1 kHz distortion at 40 kHz deviation mono at 70 dBµV to be around 0.1% with little room for improvement, as H3 was slightly dominating H2 already with nothing else of note. I may try to tweak detector distortion next time I have the unit open. L-only distortion in stereo came out a little under 0.3%, with channel separation for the fundamental reaching the mid-50s of dB although at that point distortion is actually a few dB louder, so you could say low 50s. An excellent value either way, my little mod probably paid off here. Noise level came out pleasantly low, approaching KT-900 levels and trouncing the KT-3050L in mono.

Now frequency response starts out essentially dead-flat to about 6-7 kHz but then starts to shelve down to around -3.3 dB which is then maintained to around 15.5 kHz. So upper treble would be a bit subdued, but not bad. Still, it barely misses its spec like that (30-15,000 Hz ±1.5 dB). (I had noticed something similar on the TX-SV636 with my then much younger ears.) Also, the pilot tone filter is decidedly out on the left channel with a notch at ca. 19.7 kHz, while the right one is close to spec at little over 19.1 kHz.

Alignment brought the pilot tone filter back in line, with only 1 dB of difference in notch depth between channels. 1 kHz mono distortion dropped to 0.07% at 40 kHz deviation, now dominated by H3, with only about 0.1% worth of mostly H2 in stereo L-only / R-only. Channel separation came out as 51/53 dB (60 Hz), 53/58 dB (1 kHz), 44/43 dB (7 kHz), very good overall. Dynamic range measures up to 77.8 dB(A) mono no PE, which I reckon is substantially better than spec, likewise the more than 72 dB(A) L-only no PE.

The low-level hum I noted actually is almost exclusively odd-order mains harmonics dominated by 150 Hz (-74 dB), so unless that's somehow generated in the power supply section (the T-405X shares the same quirk) it may be coming from the transformer or even mains wiring coupling into something. There is not much in terms of bridge rectifier harmonics, 100 Hz being dominant at -87 dB, supporting my findings of still-decent filter capacitor ESR. Perhaps the unit isn't as high-hour as I previously thought, with a fireplace in the previous owner's home being responsible for the unusual darkening inside instead.

This model is actually the one with the hottest output out of everything I've tested so far, easily 2 dB louder than average.

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Entry last modified: 2025-09-06 – Entry created: 2025-08-10

Kenwood KT-1100D: Triumph and Tragedy

Initial Assessment

Another eBay find, a cheap KT-1100D from ca. 1987, S/N 76Kxxxxx. A few scratches but came with AM loop (and the original price tag: DM 995, equivalent to an Onkyo T-4270/4087). Same loop as KT-880L, case construction seems similar as well. The electronics are much more complex, obviously.

Unfortunately, the unit turned out to be rather dead. Powers up (sticky power switch, may be broken somehow?) and seems to operate, emits some low-level hiss on FM and AM, but no reception and no trace of LOs on either band. 21 W from the wall, spec is 18 W on 220 V.

Going by fingerprints on the inside of the top case, it appears I'm not the first in there. Checked the VT/GND test points - 1.5 mV. Well, that's obviously not going to work. Not shorted though. Time to poke my nose into the service manual to check where the tuning voltage supply comes from…

Repair

Following tuning voltage power, I tested at R266 and found one side shorted to ground (single-digit ohms). Now what could that possibly be?

[KT-1100D PLL LPF amp schematic]
KT-1100D PLL LPF amp schematic.

Who would have guessed, it's another shorted capacitor. C161, 33µ/35V. Measured straight across it, yep, as shorted as can be.

I had initially determined R266 to be open, so when I found that the nominally 28 V supply struggled to climb above 15.5 V, that sent me on a wild goose chase in the regulator section. Turns out R266 was very much alive (it might just have some corroded legs), and our short was dragging down the regulator. Oops.

My stash yielded a 47µ/63V to replace C161. A bit big but still fits.

That brought the unit back alive, though it deemed center tuning to be about 100 kHz above actual station frequency. It took me a while to find discriminator coil T9 on the middle of three vertical sub PCBs, but performing its alignment per service manual brought center tuning back to where it should be.

Power consumption post-repair: Standby 1.76 W, FM 16-17.5 W, AM 16 W. (Actually that's not even standby, just a secondary-side power switch. If they had made the parallel spark killer capacitors a smidge bigger, they could have exploited that for keeping the memory alive. I guess they figured that given the tuner IC's general frugality, the early 18 mF 5.5 V backup capacitor would be plenty.)

I may have to modify the tuning speed à la KT-880L, while it has a tuning knob the actual tuning IC (TC9157AP) does not accept input very rapidly, and turning the knob quickly makes things go slower if anything.

Alignment and Drama

The center tuning discriminator was the most obvious item that had to be corrected. Other than that:
The LO trimmer cap could use the usual slight tweak.
MPX VCO frequency was just barely out of spec.

AM alignment seemingly was not significantly off for a change. Despite the lack of extra RF amplification, sensitivity still is good. The variable AM bandwidth feature is neat but also very necessary, as filtering is not great (one SFU450E + IFT again). You can just about get rid of 9 kHz hets but things get rather muffled then.

I should arguably have called it a day at this point, but buoyed by my previous success, I proceeded with RF alignment, which I did manage to improve slightly.

And that's when it happened: I had noticed that the first RF coil made quite a difference on the KT-880L, so when the same wasn't really happening here I was confused and tuning back and forth repeatedly  and that's when the alignment tool suddenly slid down and the core proceeded to get semi-stuck. Oh no. A look with the magnifier confirmed my worst fears, a cracked ferrite core. There is a possibility it already was before, but it's definitely worse now.

At least it seems to be stuck / tuned where it's supposed to be, so I guess that counts as escaping with a black eye. I am convinced that it must be possible to glue the things (despite reports of little success), but in this case it may best be left alone. Why does it always have to happen on fancy tuners… (This type of coil only seems to be shared with other relatively high-end models either   990D, 7020, 6040. Well, 5020 and 3050 too, so those might be good candidates for a parts donot. A single one also remains in the KTF-3010 which in turn is very similar to the KT-3080.)

A suitable parts donor has now been purchased. The only problem is that if I can get that running again, I might have to get a parts donor for the parts donor…

Notes

I must say I'm not a fan of this signal-dependent variable high blend / stereo blend feature (slider above memory buttons). You seemingly cannot fully disable it on weaker signals. Something doesn't sound quite right in the stereo department either way. (My ears did not deceive me, see alignment part 2. With stereo alignment and auto-stop sensitivity corrected, I don't actually mind the variable stereo blending now.)

The S-meter is a bit of a joke, too, it's supposed to illuminate the 7th segment by 40 dBµV and seems limited on the bottom end as well. That's even worse than older Kenwoods. (Update: With auto-stop sensitivity adjusted, you can get about 52 dBµV maximum out of the meter, similar to some of my other Kenwoods.)

The variable AM bandwidth feature is neat but also very necessary, as filtering is not great (one SFU450E + IFT again). You can just about get rid of 9 kHz hets but things get rather muffled then. Despite the lack of an additional RF amp, sensitivity still seems good.

In order to fully disassemble the front, you'd need to remove some push rivets   not a fan. IFT alignment also calls for a very obscure instrument, a genescope (combo sweep generator and scope).

Service docs do seem more organized and less rushed than in previous years.

I believe this model basically is an evolution of the preceding KT-990SD. The mainboard has gained several subassemblies. This construction is trying to be a bit too clever for its own good for my tastes while attempting to hit a certain price point, and I am not a fan of the tradeoffs this generates. That's certainly a downside to the tuner wars.

More Alignment

With my Levear VP-8193D RF generator and some experience realigning a bunch of other tuners under my belt, I could tackle the KT-1100D's stereo issues.

For starters, despite the alignment mishap, it is actually among my most sensitive tuners. Some IM3 was noted in DISTANCE mode. Both comparable to the KT-880L, I'd say. LOCAL will obviously make it handle stronger signals.

Turns out the order of alignments is as given for a reason, plus you can actually perform most of the distortion and separation adjustments by ear listening to the residual signal on the other channel using stereo L-only / R-only modulation.

L25 (subcarrier 38k, #9) was substantially off, easily 15-20°. This has a major effect on L-R amplitude and consequently stereo separation. While alignment instructions say go for minimum distortion, this would appear to equate to maximum L-R amplitude.

I proceeded with the stereo distortion alignments VR5 and VR7 (#13/14), then carefully went over their mono counterparts VR4/VR7 (#11/12). You can try feeding in an L=-R signal and sliding the stereo blend all the way to the left which sometimes helps, otherwise L-only / R-only is fine. (Do make sure to sanity-check with both channels actually playing, sometimes listening to just the residual can be misleading. Here it generally seemed to line up.) You can go over these several times. I also went back to L25 for another tweak. Eventually even the detector distortion adjustment had a noticeable effect.

The actual channel separation pots actually didn't need much tweaking at all. Narrow separation VR1 needed a bit. Narrow distortion as well.

Auto-stop sensitivity as found was quite a bit higher than the specified 12 dBµV and had to be turned a fair bit. The S-meter can only be turned down to 52-53 dBµV while still achieving full deflection then. I suppose you could turn the auto-stop back up if you wanted coverage to higher levels. At least this has pushed the threshold for automatic stereo blending to lower levels now.

I find distortion compensation to be a double-edged sword… forget about temperature compensation, optimum adjustment points will vary with deviation (doing it at 40 kHz vs. 75 kHz would literally move the optimum for detector distortion from one end of the pot to the other) and RF amplitude, although amplitude dependency wasn't too bad in this model.

Overall, I would say that comparing '80s tuners by ear without prior realignment is a complete crapshoot. Especially Kenwoods. I've found so many alignment issues it's not even funny.

Checking Capacitors

Given the stench when I first opened the unit and a rather yellowed transformer, I had my suspicions about this being a fairly high-hour unit. After I had recently gotten an ESR meter, I decided to poke around in the power supply section, which yielded these results (ESR in ohms):

C109 2200µ/35V 0.02
C110 1000µ/35V 0.04 ?
C114 33µ/35V 1.03 ?/X (not marked on PCB, at R184)
C115 100µ/35V 0.36 ?
C117 1µ/50V 3.54
C118 1µ/50V 3.56
C119 10µ/35V 0.63
C120 10µ/35V 1.12 ?
C122 10µ/35V 1.25 ?
C124 10µ/35V 1.14 ?
C129 330µ/35V 0.10
C131 330µ/50V 0.09
C132 100µ/50V 0.23 ?
C134 100µ/10V 1.12 X
C135 1µ/50V 3.58 
C136 10µ/35V 1.22 ?
C137 10µ/35V 1.23 ?
C138 47µ/10V 1.27 X

Looks like all the big guys are fine or at least close, same for the 1µ/50s. Not sure what's up with all the 10µ/35s, either C119 is a positive outlier or all the others have degraded very consistently, either way my new 10µ/63s are around 0.9 ohms so I probably wouldn't bother. C114, C134 and C138 all seem be toast, and I'd probably replace C115 as well. One can discern a theme… they are all smallish, medium capacitance, low to medium voltage parts, almost all located close to heat sources like regulators or zeners.

The tab of regulator IC14 also looks a bit bronzed from heat; I found some small, lightweight clip-on TO220 heatsinks locally that seem like they would be a good fit once C138 has been replaced.

Update: I did up replacing C138 and C114 (using Samwha RD 47µ/35V, ESR 0.17-0.19 ohms) as well as C134 (using Jamicon TK 100µ/35V, ESR 0.18 ohms), and the regulator got its clip-on heatsink and solder joints touched up. I still have various 100µ/63V with ESR of 0.07-0.08 ohms that could replace C115.

Mods

I noticed that inductors L15 and L16 (10µH RF chokes in the LO / in its tuning voltage supply) are instead equipped as resistors R318 (39k) and R319 (3.6k) in this European model, which I can only imagine would have been a cost-cutting measure. From a noise perspective this means that the top half of tuning diode D8 in particular sees a fair bit of thermal noise, and I can't imagine that's doing the LO phase noise much good. Even if the effect of R318 is effectively halved (meaning the equivalent of 9.5k added to R319), that would still equate to it dwarfing R319's contribution. They only need L15/R318 to begin with because of C29 (33p), which I think is supposed to linearize the capacitance vs. tuning voltage charactistic towards the bottom end and as such may result in better tracking.

L15/R318 looks decently accessible, so I might just buy such a choke (or rather a handful) and install it.
Update: I failed miserably at getting R318 out, which as usual was well and truly in there with thoroughly bent-over leads and not easy to reach from the top even with needlenose pliers. An attempt at soldering a choke across it at the bottom also failed. Then I had to call it a day and finally get all the crap off of my dinner table as my vacation was drawing to a close.

I have thought about how one might implement a small trickle-charging circuit for +5V memory backup in order to get some mileage out of the remaining voltage when the unit is turned off. Basically all it would take is a zener follower in series with a Schottky diode connected to the cathode of D39, and another small transistor with bias resistors that shorts out the zener diode when the unit is powered up. It all only needs to be super low power (literal microamps, if that), so I might even use a BC550C for the follower. The follower may not even be needed at all, and when using a lower-voltage zener like a 5.1V at 100 µA or so, explicitly turning it off may not be required either. Three parts sounds like a plan.

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Entry last modified: 2025-09-01 – Entry created: 2025-08-10

A "spares" Kenwood KT-3050L

Introduction

After my KT-1100D alignment "oopsie", I was looking for a suitable parts donor. That's when this ca. 1992 KT-3050L crossed my path, sold as "spares or repair" with known issues and only a power cord stub. Since it didn't seem in terrible shape cosmetically, I figured I could also try to get the thing going again first as it's actually a fairly nice tuner for a low-midrange affair, with two 230 kHz GDT filters in Wide plus two additional 110 kHz ones in Narrow and a comprehensive RDS implementation (only the discriminator merely is of the basic quadrature type).

Initial Assessment

The unit arrived as described. There was no shortage of dust and dirt, which I kept on cleaning throughout the course of the ensuing repair. The back plate around the power cord grommet was slightly bent as the previous owner had apparently tried to remove it from the outside with pliers and brute force, and guess what, the strain relief did what it was supposed to. I removed the grommet by pushing on its side from the inside woth a flathead screwdriver (it's reasonably easy once you know how) and fitted a temporary power cord.

The unit would take a while to "wake up", and would run in standby absolutely forever from that point on, but if you turned it on it would start to suffer from bizarre freakouts after several minutes, with the display going blank and coming back, which would get worse and worse until the unit eventually settled back into standby.

When it did work, it seemed to operate fine, minus the discriminator being about +50 kHz off. That was triggering my OCD, so I ballparked L20 using the tuning meter, which turned out to be surprisingly accurate (just a few tens of mV off when actually using the assigned test points to measure).

Diagnosis

The power supply area looked a bit toasty around the zeners for the -33 V supply but generally all the voltages were there and seemed stable. Power ca. 3.5 W standby, 7.5 W on. I can't really approve of a design that even uses the VFD for standby indication, mind you; no wonder the display isn't exactly super bright after all these years.

I noticed some voltage fluctuation on the +5 V on the end of R238 during the "freakouts". That's a line marked "CE", and it is designed to be quickly pulled down to ground through D43 following a power failure. During normal operation it is being kept high by D67-R238, with D43 being reverse-biased as the voltage at the anode of D31 exceeds 10 V.

[Schematic excerpt of power supply section]
Excerpt of power supply schematic showing CE line.

It goes here on the control PCB, the input of reset IC IC11:

[Schematic excerpt of reset circuit]
The reset circuitry.

IC11, a PST529D, is designed to keep its output pulled low unless the input voltage exceeds roughly 4.2 V, at which point it lets go and R298 pulls the output high. C197 shapes the resulting step into a pulse that briefly turns on Q46 and thus pulls the reset line low. With its /CE input being high and /RESET briefly going low, the main micro wakes back up. When power goes back off, only /CE goes low and the main micro goes to sleep.

I was initially barking up the wrong tree and suspecting C197, which is confusingly mounted in an electrolytic footprint but definitely is the 100n film cap given in schematic and parts list. Oh well, having the front off was no loss in terms of accessibility and would come in handy later. I was initially hesitant about pulling the big ribbon cable, but it turns out the connector on the main PCB is a mere friction fit affair. After some initial crunchiness, it wasn't too bad. I had a lot more trouble with the T-422's.

Then I saw Hiro-kun mentioning C140. I decided to check it using my new ESR meter, and sure enough, 10.3 ohms. Going through the power supply section yielded these measurements:

C137 330µ/25 0.20 ?
C139 330µ/50 0.11
C140 4,7µ/50 10.30 X
C141 33µ/50 1.0 ?
C142 47µ/50 0.53 ?
C145 10µ/35 2.38 X
C146 10µ/35 0.68
C147 2200µ/25 0.04 ?
C148 47µ/10 0.73 ?
C149 1µ/50 4.12
C150 47µ/16 0.23

So only two obviously bad caps, a bunch more questionable ones (which you will find are associated with the always-on parts, no surprise) and a few which seem OK.

You will notice that our worst offender C140 (4.7µ/50V) is directly associated with the CE line, and +5.6 V regulator output capacitor C145 (10µ/35V) isn't far away either. That wouldn't have been helping matters.

Repair

I had to remove the main PCB to access the bottom side, but overall it wasn't a terrible job, 4 screws at the back side and half a dozen on the board. I left the power cable connected, it wasn't too much of an issue. Overall the unit has turned out a lot more service-friendly than I initially thought in general. Pulling apart the front didn't yield any major issues either.

Ultimately I only replaced the two obviously bad caps, using CapXon GL 10µ/63V for both that were briefly formed at 60 V just in case. That did the trick. (The antenna connectors also seemed a bit wobbly, so I gave some attention to their solder joints as well.)

Out of circuit, the old C140 still measures 1.66 µF, that's definitely toast. The old C145 still measures 9.66 µF, but the comparison with C146's ESR shows that it is quite tired indeed.

During subsequent testing I noticed that the "3" key wasn't registering. The multimeter confirmed that its tact switch made absolutely no contact whatsoever. Thankfully it turns out it's a run of the mill 6x6mm affair with short stem, and the local parts store had a few of the things in stock (kudos to them).

Finally I also fitted a new switched power cord (adding a knot for overkill strain relief) and threaded the grommet back in. Some hammering with the rubberized grip of my pliers straightened out the back panel to some degree.

Alignment

Once discriminator center tuning was brought into spec, distortion adjustment L21 was found to be close to optimum +/-50 kHz symmetry already, requiring only the slightest tweak. (Later it turned out that it was actually substantially off, and wide distortion could be dropped by an order of magnitude.)

Wide separation and distortion adjusted fine and weren't too far off to begin with, narrow distortion is a bit "eh" not the least due to a ca. -15 to 20 kHz passband offset that you can't correct (there are zero distortion adjustments for narrow, period). When you've got 110 kHz filters going, that sort of stuff obviously matters. I don't think the LM7001 PLL has any facilities for IF fine-tuning, it seems to be running in basic "fref = 50 kHz for FM, 9 kHz in AM" mode here. They didn't even bother to implement 1 kHz for longwave even though the chip could do it and the band was notoriously plagued by off-grid stations… shows you how much of a priority it was. This was one of the last models to be offered with longwave coverage anyway.

I had one hell of a time trying to get the FM frontend to track properly – if tuned as described the high end would always be worse, with all tuned circuits being off in the same direction. Ultimately I had to deviate from the specified tuning voltage range (2.0 to 11.5 V). I lined up the tuned circuits for good sensitivity on the high end at 11.5 V and then retuned the LO inductor until things were tracking on the low end, went back to the high end to retune the LO trimmer (a bit scratchy but not unworkable, btw), rinse and repeat. Final result was about 2.37 V to 11.435 V, with the high end actually being more sensitive than the low end now. So in a way this unit is the opposite to my Grundig FineArts T-904, which picked up sensitivity just from readjusting the tuning voltage range to spec.

I was surprised to see that this model only uses single varicap diodes rather than the usual pairs. It does not seem to negatively impact strong-signal handling, which seems at least as good as the KT-880L's, but I still have my doubts about things like LO harmonics.

The unit did not come with a loop antenna and does not take the kind of spade terminals used in earlier ones, so I refrained from aligning the AM bands.

Notes

It's a nice if perhaps a hair dark-sounding tuner in Wide (see measurements) with sensitive and quick RDS decoding, and the "Active Reception" logic generally seems to be doing a good job picking the right bandwidth. Narrow yields somewhat quieter output due to auto-adjusted separation. Looong muting interval.

Memory retention time does not appear to be more than a day, so I guess the goldcap (which "conveniently" sits under the VFD) has seen better days and could stand being replaced. It seems to be quite small, so I hope the larger-diameter types I can buy locally would still fit; on the other hand there is a 100 ohm resistor in series so placement is not exactly critical and it could be relocated just about anywhere. There still is parallel capacitor C173 (100µ/10V) that should be checked for excessive leakage and quite arguably replaced by a type of greater voltage handling.

Power consumption is about 3.6 W in standby, 7.9 W on FM and 7.1 W on AM.

These don't seem to be selling at all right now. I mean, it's vacation time and the tuner market is in the toilet in general, but still. I paid more for this grubby spares unit than people are willing to pay for a working one in good condition.

Audio Measurements

Once I finally had a loopback measurement setup cobbled together, I tested this unit as well. It immediately flunked the 1 kHz mono distortion at 40 kHz deviation at 70 dBµV test, posting over 0.5% of H2 in wide. No wonder I thought it sounded a bit warm. I immediately popped the cover and started adjusting L21 for distortion and L20 for offset with realtime FFT on screen. With a substantial tweak of L21, H2 went down by about 27 dB, with RMAA reporting a THD of 0.024% now (which remains dominant H2 with nothing of note past H3). This is starting to rival KT-900 distortion levels and is actually better than spec. (I wonder whether discriminator distortion was ever set properly from the factory.) L-only distortion remains at a good level at about 0.04% dominant H2. Channel separation is 60/49 dB at 60 Hz, 54/48 dB at 1 kHz and 35/35 dB at 7 kHz, like spec.

Given the tight and slightly off-center narrow filters, a mono THD of 0.27% there is no surprise and roughly in spec still. Things don't get any better with L-only, reaching approximately 1.4%. Channel separation drops to about 34 dB, again like spec.

I think the de-emphasis time constant is slightly high, much like the nominal part values would suggest (620p * 82k = 50.84  µs) – there is an ever so slight downward tilt starting around 3-4 kHz that reaches -0.5 dB by 7 kHz and -1 dB by 10 kHz, gently dropping to about -1.8 dB by 15 kHz before the pilot tone filtering starts to set in around 16 kHz. (Or it might just be the pilot tone filtering making itself felt. According to simulation, a deemphasis error that results in -1 dB by 10 kHz would give -0.5 dB by 3.18 kHz, and I'm seeing -0.2 dB at best.) Either way, it's in spec. Not as good as the KT-900 though.

With the excessive distortion dialed in, I actually no longer think the unit is dark-sounding at all.

I took the opportunity to adjust the L/R pilot tone filters which had been about 250-300 Hz high, which dropped pilot tone leakage levels by about 6 dB. Those remain rather mediocre even then at about -60 dB when a bunch of my other tuners can do -80 or not far off anyway.

Much like the specs suggest, mono SNR isn't particularly high at about 75.5dB(A), even the little Onkyo T-422 manages to beat this one by 2 dB. Stereo is closer though.

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Entry last modified: 2025-09-06 – Entry created: 2025-08-27

A Kenwood KT-990SDL Needs a Tuneup

Initial Assessment

This 1985 (S/N 52Kxxxxx) KT-990SDL, essentially the direct precursor to the KT-1100D as the second highest in the lineup, is another I snatched up cheaply on the 'bay, another very nondescript auction that didn't even show it working. I had actually missed it the first time round but apparently the highest bidder had bailed and it was relisted, even netting a bit more for the seller this time. It arrived chucked into a big box with a bunch of larger and smaller styrofoam pieces, which I'd call protection by hopes and dreams more than anything else, but amazingly enough it arrived externally unscathed, previously existing dirt (which may also account for two initially sticking memory buttons) and styrofoam bits notwithstanding. Did I mention that I hate styrofoam with a vengeance? The stuff clings to everything. The unit cleaned up well, though the musty basement smell took a while to dissipate and the top case shows the same issue with stickiness that I previously observed in the KT-880L, clearly Kenwood had some paint issues in those years.

The Japanese market counterpart to this model appears to have been the KT-2020.

This unit turned on fine for a change, appearing to tune in stations. It also produced output. Borrowing the KT-1100D's AM loop antenna with holder (the antenna being the same T90-0111-15 also shared with the KT-880L), mediumwave seemed alive as well, although the longwave band was very quiet.

KT-990SDL vs. KT-1100D

While the FM frontend (4 gangs with an extra tuned buffer for the LO) and IF filter complement (2x / 4x MS3G = 180 kHz "GDT-lite" for us Eurotypes, MP3 = 250 kHz mid-level GDT for North America) are much the same as in the successor, there also are a number of differences: The 990SDL still sports a tuned RF preamp for the AM ranges, as well as longwave coverage (which moved increasingly down-market in the following decade – 5020L, 3050L, 2060L, the exception being the 6050L which demanded a price premium over the plain 6050). In return, it lacks a switchable FM RF attenuator, despite the presence of a display indicator (I believe the entire display section is shared with the KT-1100SD). Where the KT-1100D has a slider for FM stereo separation, the same controls AM bandwidth in this model (which the successor moved to a pot on the bottom left). There is, in fact, no mono/stereo switching at all here, just automatic stereo blending at lower levels. Also, there is a memory bank selection switch below the power button, which was later moved to two buttons underneath the display. The tuning lock button next to the tuning knob was later moved below the power button. The old model still has a hard primary-side power switch as well. Also, there basically is just one big main circuit board with a separate power supply, nothing like the wealth of sub PCBs in the KT-1100D, and the layout is very different as well.

Alignment, take 1

FM alignment was uneventful (apparently I wasn't the first one in there either) but did appear necessary. Sensitivity picked up. This was the first time I've seen a PLL detector being substantially off, over 0.6 V. Distortion alignment proved decidedly more finicky than in the newer model, as not only was there a fair bit of variation with input level (I guess effective passband changes as IF amp stages saturate), alignment by L-only / R-only and listening to the opposite channel residual could be decidedly misleading, you generally need to listen to both in order to avoid nasty surprises.

The longwave band needed major adjustments but peaked up fairly well. Makes me wonder whether it ever was aligned properly to begin with, or how much of a tumble the unit took on the way. I took the liberty of peaking sensitivity at 162 and 252 kHz, given that the range past 279 kHz has not been used in donkey's years. It seems nice and sensitive now.

The mediumwave band threw me a major curveball – I could not get the prescribed 2.3 to 20.0 V tuning voltage range no matter what I tried. Once I hit 2.3 V on the bottom end, the top end would not go below about 20.6 V. Since both L and C are fully adjustable an the tuned circuits, I eventually decided to deviate from the given range and settled on 2.3 to 22 V first and 2.3 V to 21 V on a second attempt. (As found, the tuned circuits seemed to peak at around 21.5 V up top.)
Note that despite the similar circuitry, this model is never going to be quite as hot as the KT-880L as about 12 dB of attenuation have been included, presumably for better handling of strong local stations. If you don't have any, you could consider shorting out R138. This is similar to the preceding Basic T-2 / KT-1010(B).

You would think that given the filter complement, this tuner in WIDE would about equal the KT-880L, but nope, 200 kHz separation is not as good then. It pulls ahead in NARROW much like you would expect. It seems at least as resistant to RF IM3, which makes sense as the RF preamp and mixer seem to be even more chill, with the grounded gate first IF amp picking up the slack.

I can't shake the impression that the sound on this as well as the KT-1100D has a certain kind of artificiality to it, which I think may be related to the stereo decoder. Stereo sound seems kind of… tubby. Possibly uneven stereo separation? It gets better after alignment, but hasn't quite gone away on the KT-990SDL in particular. I guess checking the handful of bipolar capacitors in the signal path couldn't hurt but I'm not holding my breath. I tend to prefer tuners that are playing it straight instead of being a gizmo attack … the KT-900 sounds lovely, and even the little Onkyo T-422 gets awfully close to that past repair / mod / alignment (even if it's not exactly a DX machine, obviously).

Measurements and Realignment

With a loopback setup finally cobbled together (Focusrite Forte headphone out @-9 dB into Levear VP-8193D into DUT with a BNC to IEC adapter cable back into Forte line-in via a RCA to TS cable, controlled by my trusty Latitude E6330 on battery to avoid ground loops and using RMAA 6.4.5 with output level set by cal tone, deviation 40 kHz mono, pilot tone level 6%, RF level 70 dBµV given that FM over cable is no longer a thing here), I could go about checking my distortion alignment. And well 

Do not, I repeat DO NOT align (mono) discriminator distortion by ear. I was greeted by 0.68%, basically all H2. Oops. The FR (swept sine) test suggested distortion dropping towards the treble, but still, not great.

So a realignment of distortion was in order. That dropped mono distortion right into digital sources territory – 0.005% dominated by a bit of H3 around the -90 dB mark. Mind you, my signal generator is specified at ≤0.05% at 75 kHz deviation, so I would take anything much below 0.02% with a grain of salt. Plus, distortion also varies with signal level as effective passband characteristics change, so if you've got H2 dialed out at 70 dBµV, it'll reappear at 80 and eclipse H3 once again, but 0.016% THD still is first-rate. L-only distortion reveals little besides VCO phase noise and whatever else may be causing the rather elevated noise floor I'm always seeing when L-R comes in.

Narrow distortion remains very sensible at 0.07% in mono, dominant H2. In fact, if I tune my signal generator 12.5 kHz lower, it even drops to 0.016%, with H2 and H3 about equal.

Dynamic range in mono, PE off scores an excellent 81 dB(A). That being said, the output still isn't as clean as it could be - there is a slew of low-level odd-order mains harmonics in the output that make me suspicious of transformer wiring. Few are reaching above -80 dB, but still, I've seen cleaner from the likes of the KT-900. (Maybe this is why the KT-1100D has a secondary-side power switch instead?)

In Wide, I measured channel separation of 48/48 dB (60 Hz), 63/68 dB (1 kHz), 45/48 dB (7 kHz). You could do better on the extremes if the filters actually were proper wide, but this is at a good level either way. Narrow slightly drops separation to 43/47 dB (60 Hz), 43/62 dB (1 kHz), 39/43 dB (7 kHz).

Frequency response is good, with neither channel hitting -1 dB before 15 kHz up top and only the slightest variation, only the KT-900 still slightly beats that. On the bottom end, some coupling capacitors in the signal path appear to be adding up (and a bit more so on the left channel), but even then -0.5 dB in the mid-20s is nothing to be particularly concerned about.

Audio output level is on the lower end of things, about 2 dB below average.

There seems to be something funky going on in the stereo decoder, as just switching the generator to L+R makes the left channel drop half a dB while right channel output goes up by the same amount. If I didn't know any better, I'd say composite signal is leaking into the sub decoder output somehow. How and where, I don't know. This is a fairly complex analog multiplier affair, too, and I haven't yet found a detailed theory of operation for it (although studying the NJM4200 datasheet gives you an idea), so good luck troubleshooting that.
I speculated it could be a DC offset on the 38 kHz signal, but there was none of note. I did find one apparently leaky buffer JFET in Q90 (there is no way it should be reading 1.5 kOhms G-S even in-circuit; fortunately the source follower circuit means that it's effectively bootstrapped), and coupling capacitor C256 sure was appreciative of some reforming after only having seen a few millivolts for 40 years even if wasn't our culprit.

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Entry last modified: 2025-09-06 – Entry created: 2025-08-29

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