Kenwood TK-66 Refurbishment

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by 65W200, Feb 23, 2013.

  1. 65W200

    65W200 Member

    Messages:
    55
    I have a Kenwood TK-66 receiver that seems to
    have developed a "drift" in the FM mode. The tuner
    will lose the station after being played a short while.
    Apart from that, the unit "seems" to function well.
    If anyone can recommend a good Kenwood repair
    person or outfit, I'd appreciate it. I do have a schematic
    for it that I could send along as well. This TK-66 is
    no powerhouse at ~ 22wpc, but its in pretty nice shape
    cosmetically. Its probably 20+ years overdue for a
    checkup. :D My longtime trustworthy repairman just
    recently retired.....:sigh:
    Thanks.
    John
     

     

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  2. Watthour

    Watthour Electron Rancher - JS3600

    My first WAG is that there may be capacitors fading in value and breaking down from heat. The second guess is that some transistors in the front end and/or IF could be doing the same thing.

    Here is one take on the subject:

    http://www.audiokarma.org/forums/showpost.php?p=1371977&postcount=8

    It appears that the unit contains almost all Germanium transistors (2SA234B, 2SA354, 2SA355, 2SB54, 2SB89A, 2SC281B, 2SC317, 2SC458LG, 2SC458B, 2SC494, 2SC664, and 2SC734) and some Germanium rectifiers. Resolving the problems with those could be almost more trouble than it is worth, since re-engineering the bias networks to get replacement silicon devices to work properly could take quite a bit of time.

    If you can isolate problem components you might be able to focus your efforts on replacement of just those devices, but it will still take some effort. There is a slim chance that you might be able to find some NOS replacements, but that would be pure luck.

    In either case, there may also be an alignment required after any repairs, adding to the time and cost.
     
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  3. 65W200

    65W200 Member

    Messages:
    55
    Thanks for your reply. Sounds like I have a new
    cool looking paperweight. :scratch2::tears:
    John
     
  4. Watthour

    Watthour Electron Rancher - JS3600

    Not necessarily, but be aware that you are in for a little more than an easy parts replacement/swap.

    I have a very early Sansui SS receiver that also used some Ge devices, making it a greater challenge.
     
  5. 65W200

    65W200 Member

    Messages:
    55
    A "little more than easy" seems to be the norm for me so this
    comes as no surprise. Do you have any recommendations on
    who to speak with in regards to this unit? Thanks for all of
    your input.
    John:thmbsp:
     
  6. Sonic Purity

    Sonic Purity New Member

    Messages:
    4
    Greetings all,

    Signed up on Audiokarma specifically to post in this thread. My background includes about 15 years of working professionally as a home audio (“stereo” to most people) repair technician in the San Francisco Bay area. My parents bought a TK-66 new in 1968 or ’69 (i’ve forgotten which). I’ve repaired it 4 times:

    1) Once for no/weak FM in 1975 (when i was in high school).
    2) Second time was a total overhaul when i worked professionally (Resistance Repair, Berkeley, California), in 1984. Noisy controls and switches was the main complaint.
    3) Third time was January 2001: recap + 2SC458 elimination, to cure a return of noisy controls and switches not amenable to contact cleaning.
    4) Fourth time has been these past few weeks (late Nov. through early Dec. 2017), for FM fades in and out. NOT frequency drift: sensitivity comes and goes, visible on the signal meter. (Same on AM but no one in that household uses AM).

    I know this model and this particular unit very, very well. I’m hoping by sharing here in this thread, i can help someone out (whether it’s 65W200 or anyone else).

    Second guess for the winnnnn! At least that’s what has happened twice (1975 and 2017) in the family’s unit. Details below.

    2SA234B, 2SA354, 2SA355, 2SB54, 2SB89A: Germanium PNP, yes.
    2SC317, 2SC458LG, 2SC458B, 2SC494, 2SC664, and 2SC734: Silicon NPN. No rebiasing needed.

    In the 2001 repair, i replaced all the 2SC350s and 2SC458s with 2SC2320s. No idea if those are available any longer. They were the shop’s go-to low noise high gain NPN silicon small signal audio transistors, making preamp sections quieter and happier all over the place.

    Hopefully it’s already general knowledge here, but if it’s not: the Hitachi-made 2SC458 transistors (any or no suffix) in the square package with a bevel which were manufactured up through the mid-1970s love to leak and/or get noisy. The later ones from Hitachi or anyone else in the standard TO-92 package are fine. Back in the day back in the business, we replaced those original Hitachi flat-bevel 458s on sight—all of them, all the time (on a full service). They’re high beta, so whatever replaces them needs to be appropriately high in beta as well. I don’t know why i didn’t do that on my parents’ TK-66 back in ’84.

    Note: no one at the shop ever noticed a problem with Hitachi 2SC460 transistors in that same case style. There’s an orange one at the Q1 position of the I.F. sub board for the meter, muting, and multiplex switching, which is still going great in 2017 on the ’rents unit.

    Hitachi didn’t seem to be having a good time back then, making reliable transistors. The first repair i ever did on this TK-66 was replacing Q6 2SA234B in December 1975, to cure no FM. Back then, not knowing better, i went with a generic ECG 160 R.F. germanium PNP transistor. Worked great, and still does! The ECG 160 (and i would hope the NTE 160) has 4 leads: emitter, base, collector, and case—just like the 2SA234B.

    In 1984, i looked into changing the generic out for a closer-to-original match. Well… the shop didn’t have any, and couldn’t reasonably get any—germanium PNP transistors were already super-rare in 1984. I left the ECG 160 in place, and it was fine.

    Note that AM is picked off after I.F. transistor Q5, thus the failure of Q6 only affected FM. This time around, both AM and FM were intermittently failing. I was fortunate catching Q5 in the act of having an open B-E junction, with far more than 0.3V across it. I was even more fortunate to have a NOS ECG 160 to install! Again, it worked great. If both AM and FM are fading or super weak or dead, odds are high that it’s one of Q3 to Q5 on the I.F. board. Q6 and Q7 are the same transistor later in that stage, for FM only. I normally don’t recommend generics when there are other options, but in this specific case, the ECG 160 (the 1975 incarnation and some from the 1980s) work great. I would hope the NTE 160, still apparently available for around $4 or so each in December 2017, would be equally good.

    Alignment is necessary for best performance. Fortunately for me, i was a professional who had a lot of practice aligning receivers, and was able to buy a Sound Technology 1000A FM signal generator, one of our shop’s (switching) test panels, and other accessories when different repair shops went out of business. Having the great fortune to own a Nakamichi T-100 Audio Analyzer, i have the needed distortion metering to complete the alignment setup. For AM this time around, i set the I.F. to 455 kHz with a frequency counter and a funky Heathkit SG-8 (which itself needed some repair before i could use it), then dialed in the R.F. with over-the-air stations.


    Other Problems

    Noisy controls
    Like the original Hitachi 2SC458s, this machine originally had a lot of little Elna electrolytic capacitors, wrapped in white plastic. White Elnas were as notorious as 2SC458s at our shop: replace on sight! They love to leak. In the TK-66, that leads to D.C. on the controls and switches, thus noise no matter how clean they are. Gray and other color plastic cover Elnas are not necessarily problematic. I didn’t 100% recap all electrolytics in 2001, but did get most of them.

    Re-capping the electrolytics, changing out the preamp transistors, and cleaning the controls and switches should lead to a mostly-noise-free machine.

    Power Amp Overheating
    Another post (maybe the one Watthour linked to) disparaged the build quality of the TK-66. Well, in my opinion, yes and no. Some parts of it are funky, but keeping in mind that this was an early solid state receiver, it’s not that bad. The chassis is solid, and i will tell you for sure that the heatsinking of the power transistors is so overbuilt that it can run a long time under conditions which would trash most receivers from the 1970s onward.

    Why do i write that? Because one part of the poor build quality was the horrible cheap-*** trimpots Kenwood used. This gets annoying anywhere in the unit, and would-be tragic in the power amp. Bias trimpots VR5 & 6 tend to open up, making this nominally Class B (maybe AB) amp try too hard to run Class A! This machine of my parents lives (to this day) mounted vertically in a custom-built cabinet. Throughout the early years up until the 1984 repair when i was stunned with how much current the receiver was drawing and how hot the power amp was running, our family took it for granted that the metal front panel would be almost too hot to touch, and there would be heat expansion/contraction metal tink sounds as it warmed up and once it was shut off. This is not normal! Replace the power amp bias trimpots with something decent! And then adjust as usual to just barely eliminate or minimize the crossover distortion notch. A properly-biased TK-66 runs pretty cool, with the outside of the output transistor cage being right about comfortably warm, above ambient.


    Quirks and Other Notes

    * Volume control tracking is bad. More typical of American receivers than later Japanese models, which often tracked very well.

    * Rec. Out loading can throw off high frequency response of the entire receiver. If you’re testing frequency response and want accurate high frequency results, you may need to disconnect anything on the record output jacks.

    * These receivers were partially hand-built. Weird things sometimes happen. This time around, i found a funky solder joint on the I.F. board, which mostly looked OK but upon closer inspection was not. Resoldering improved AM & FM sensitivity.

    * Weirdness #2: a truly bizarre wire lead/flux bridge between preamp input pin 2 and B+ at C19, putting +18V on the right channel Mag/Tape/etc. input. Repairing this cured a right channel noise in all modes other than Tape Monitor. Chances anyone else will ever see this are vanishingly small; the point of reporting it here is to get you to be on the lookout for truly weird things like this.

    * Even when nominally in mono, the multiplex circuit isn’t pure mono. Maybe this is a failure i’ve not found. In practice, it works as a form of blending, producing a smoother transition between full stereo and hard mono.

    * Adjusting the AM signal meter level may throw off the FM meter, muting, and MPX mono R.F. switch point adjustments.


    Mods

    AM Pop Elimination Mod
    Even with perfect caps, transistors, and wonderfully clean switches and controls, a standard TK-66 still produces a loud POP out of the speakers when switched into AM mode, at any volume setting. Those whacky Kenwood engineers threw a little extra filtering in for AM, in the power amp. C32 & 33 charge up suddenly when S1-5r closes, creating the pop. The solution/mod: add a 10MΩ 1/4W resistor across each of contacts 1&2 and 7&8 of S1-5r. This pre-charges the capacitors, so they won’t pop. That high a resistance will not have any meaningful effect on frequency response in non-AM modes. Remember: a TK-66 is mid-fi at best.

    FM MPX Light Mods
    The FM multiplex circuit of the TK-66 is typical of its era: primitive. The Stereo light tends to flicker, which does nothing for its longevity. During the 2001 repair, i studied the circuit and decided it would be wise to double the values of C24 and C25, electrolytics filtering the base and emitter of FM stereo light switching transistor Q7. This was a worthy improvement, with no side effects.

    Finally in the last few months of 2017, the original FM Stereo incandescent light bulb which had been working extremely hard for a majority of its 37-38 years of life burned out. Already fed by half-wave filtered D.C., this was meant for an LED mod. It’s so easy it’s silly:

    1) Change R34 in series with the power supply to pin 13 of the MPX board (Q7’s collector) from 150Ω to 470Ω (i used a 1/2W). Interestingly, the unit here did not have a resistor R34—it was a direct wire connection.

    2) Hook up a standard red LED with proper polarity in place of the original incandescent lamp. I used solid wire for its stiffness, which allowed the old-timey red LED to point straight at the red plastic lens. This probably would have been sufficient, but just in case the receiver might get jostled, i put some heat shrink tubing (red of course) around the LED and the back end of the red plastic lens. With or without the heat shrink, the end result looks basically identical to the original (with less flicker thanks to the earlier mod).
     
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  7. Sonic Purity

    Sonic Purity New Member

    Messages:
    4
    Part 2 (end)

    Real-World Specs

    Output Power
    I laugh every time i see the “FET 60W” sticker still on the front panel. This was obviously before the FTC (i think it was?) power specification rulings. This receiver since first measured in 1984 has always done no better than 18W/ch. into 8Ω resistive, both channels driven, at the 1% distortion point. It may be that if the main filter capacitor in the power supply were replaced, it might get closer to the 22W/ch. others have reported.

    FM Tuner
    Sensitivity (mono, 3% distortion): around 13 to 15 dBf
    Muting: adjusted for 20 dBf
    MPX mono switch: adjusted for 25 dBf R.F. level, 5.5% pilot injection level
    FM stereo separation: 34 dB @ 400 Hz


    Summary
    Is it worth spending even a fraction of this amount of time on a primitive early-generation solid-state receiver with a paltry 18W/ch. output from its capacitor-coupled power amp, with many germanium transistors, some of which may tend to fail, whose specs get left in the dust by so many other makes and models? Well, when it’s installed in a custom cabinet of an audio-only system, hanging vertically from its faceplate (which few receivers after about, say, 1973 could do), working with the same Dual 1009F and Scott S-15 speakers interfaced with it since it was new, and that 1985 Magnavox FD2041 CD player that keeps going and going and going (my intent when purchasing it for them when it was new) and the very nice B&W bookshelf speakers out near the (CRT, in 2017) TV, its Mono Out is driving a custom amp which drives a single speaker in the master bedroom (that’s a whole other story), and when one’s mother (my father died almost a decade ago) clings to older tech (including her hardwired copper pair house phones), and further when one’s own life is so off the rails that one has time for these sorts of shenanigans plus the skills, tools, and parts, yes it is.

    Hoping this helps (and can you tell i write novels?),

    ))Sonic((
     
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  8. Moving Ahead

    Moving Ahead Active Member

    Messages:
    323
    Location:
    Australia
    Nice write up.

    I got a TK 66 about a year ago. On first inspection it was cooking in one channel. The bias on these units can go really high. I replaced trim pots and re biased and it runs cool. (I suspect this be the case with the TK series)
    I've got to say I can still remember the cursing when the unit was on the work bench especially the power supply.
    The TK66 might not break any records but what it does do, it does very well. And if you put it in its historical context it beats many a younger contender.
    Furthermore there are not many other units that can compete with its bachelor pad looks. All in all I love my 66.
     
  9. quaddriver

    quaddriver 120 What's per channel Subscriber

    good work, I got a couple 'tekes' in the closet needing attention. One I did 7 years ago then playing in the shop it went dark - never looked into why. A tk 55, a 140 and another that escapes me...their day will come
     
  10. Daniel G.

    Daniel G. New Member

    Messages:
    2
    Strangely, I just found a TK-66 in a box in my basement. It turns on, seems to tune in AM and FM stations, but there is no sound. I noticed that the schematic has a "Jack A" in the power transistor circuit. "Plug A" supposedly has wires connecting all the pins. But Plug A on this unit has no wires and no connections between the pins. Does anyone know what the purpose of Jack A is? And should I connect the pins as shown on the schematic? If the pins should have been connected, I can't imagine why they are not, or how the unit could function like this, if it ever did.
     
    Last edited: Dec 31, 2017
  11. Daniel G.

    Daniel G. New Member

    Messages:
    2
    Update:

    I see that Jack A is test point. Apparently the wires are separated to facilitate current measurements. I wired Plug A as shown in the schematic, and Voila . . .it works, and pretty well, at that. Why Plug A was unwired I'll probably never know.

    Thirty years in a box clearly didn't stress the components. Now if I could only figure out how to remove the front panel so I can replace the burned-out bulbs . . .
     
    Last edited: Jan 1, 2018

     

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  12. Moving Ahead

    Moving Ahead Active Member

    Messages:
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    Location:
    Australia
    If you can work out the test plug matter I'm sure faceplate removal will be a breeze. Just take care of the silkscreen when cleaning. The bulbs are festoon type. You'll need some replacement foam strips to stop light bleed.
     
  13. djez

    djez AK Subscriber Subscriber

    Messages:
    46
    Location:
    Las Vegas, NV
    I just bought one of these, but I don't have any schematics or service manual to get the procedure to adjust the DC offset and bias. If anyone has either of these in addition to the procedure for both adjustments, I'd appreciate it. I'm a relative newcomer to the vintage repair/restore game, so I'll probably need idiot proof instructions for properly doing the bias. I'll be ok on offset as long as I know which pots are for which adjustment.

    Thx, Dave
     
  14. dlucy

    dlucy dlucy67 (Doug) Subscriber

    Messages:
    1,858
    Location:
    Philadelphia, PA
    I have a pair of TK-140X units (good-looking non-working one and a parts donor one) and I stopped working on them after realizing they were stuffed full of Germaniums. @Sonic Purity 's post above gives me more hope for these pretty-but-unobtanium units. Thanks for all that history!
     
  15. djez

    djez AK Subscriber Subscriber

    Messages:
    46
    Location:
    Las Vegas, NV
    Sonic,

    I'm looking for some help on a unit I just purchased. I'm fairly new the the repair/restore game so I may need some directions a little more specific than some.

    I see there are 6 trim pots on the amp board. I believe VR's 1 & 2 are DC offset and 5 & 6 are for bias? Curious what 3 & 4 do. My offset was too high so I used 1 & 2 to get it to normal, but noticed the heatsink was pretty warm so I'm sure I have a bias problem. I see you recommend changing the pots & would like your opinion on what I should buy to replace them & if they're all the same values. I'm also going to need step by step bias instructions as to where my test leads go to measure bias & whether there's a mV level or just watch for crossover distortion on the scope which I'd need help to set that up.

    I also see 1 trimpot on the preamp board that I'm not sure what it does.

    I only see 1 orange 2SC458 transistor on any of the boards so not sure if I'm missing some. On my Sansui I've used KSC1845's so wanted to see if those would work.

    Thanks in advance,
    Dave
     
  16. Moving Ahead

    Moving Ahead Active Member

    Messages:
    323
    Location:
    Australia

     

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  17. djez

    djez AK Subscriber Subscriber

    Messages:
    46
    Location:
    Las Vegas, NV
    Thanks for the response. I have the schematic, but it isn't clear what each VR does. I'm pretty sure 1 & 2 are offset, but would like to know what the other 4 on the amp board do, what the test points would be for bias, and what mV level I'd need. Hopefully you're better than me at deciphering schematics.
     
    Last edited: Apr 16, 2018
  18. Sonic Purity

    Sonic Purity New Member

    Messages:
    4
    Hi Dave and Everyone,

    Busy right now, so this’ll just be a partial answer. I’ll endeavor to fill in the blanks (if no one else has) sometime in the next week or so.

    First, there were production changes during manufacture, so not all units are identical. Each one originally shipped with a schematic correct for that unit, along with the owner’s manual (at least my parents’ did). Numbers of transistors etc. will vary and types may vary. Example: my parents’ unit has two IF boards, but the official Kenwood TK-66 manual only shows one, with a different board number.

    Power amp VR1 & 2 are indeed D.C. offset adjustments, for symmetrical clipping since this is a capacitor-coupled output amp design. Kenwood wants them adjusted for symmetrical clipping with a 16Ω load at 1 kHz. I’d be inclined to rough adjust for symmetrical clipping with no load, turn the volume back down, apply my standard load (8Ω), then re-check and fine-tune. Keeps from stressing the amp so much under full load if it’s way off. People who lack ’scopes can do about as well measuring actual power supply voltage (collector of Q3 or Q4 can be convenient), then adjusting each channel for half that voltage on the + capacitor side of the output, which conveniently is the same as the collector of Q2 (left) and Q5 (right).

    To my amazement, Kenwood provides no instructions for bias pots VR5 & 6. I don’t remember if i checked for a current value (voltage across the appropriate .47Ω resistor(s) in practice). Quite sure i viewed crossover distortion, and likely monitored current at the same time. (Keep in mind this was the mid-1980s, and i didn’t write down specifics.) Some amps of this period could not have their crossover notch removed without being biased too high and running too hot. I don’t remember for the TK-66. I think one could make the notch just disappear and be OK, but i’m not certain.

    Remembering that this was early solid-state design, VR3 & 4 were part of the primitive protection circuit. It did not operate at all like newer ones. From memory, it did something like make the audio chop in and out at an attention-getting rate. Zero information on adjusting those from Kenwood, and zero notes of mine regarding whether i even tried. We can see from the schematic that the trimpots adjust the voltage level being measured across the driver transistor resistors at which Q9 will turn on, which appears to kill the 22V supply line to Q1 and Q8.

    The trimpot on the preamp board is the MPX separation pot, because why put it on the MPX board?o_O

    VR1 & VR2: 30kΩ
    VR3 & VR4: 10kΩ
    VR5 & VR6: 500Ω
    All linear taper (the B on the schematic).

    That’s all i have time for today.

    ))Sonic((
    (not even time for the color sig.)
     
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  19. djez

    djez AK Subscriber Subscriber

    Messages:
    46
    Location:
    Las Vegas, NV
    Thanks Sonic,

    I appreciate your time & will patiently wait for any more info you might have. I’ll see what I can do with getting some adjustments dialed in now that I know for sure what ones to trim.

    Thx, Dave
     
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  20. Moving Ahead

    Moving Ahead Active Member

    Messages:
    323
    Location:
    Australia
    001.JPG

    Classic bach pad looks.

    Firstly, if you're new to electronics I'd suggest you get familiar with the dangers with working with live equipment.
    Secondly use 2 DMM with mini grabbers for left and right channel adjustments. keeping your hands away from the open unit.
    VR5 & 6 Bias. 14mV -measure across R 19 and R 20, 0.47ohm.
    VR1 & 2 adjust center voltage- 24volts. measure positive lug C17/C18 1000uF and chassis. Main supply is 47V measure positive lug C23 2200uF.
    You can check no DC at speakers terminals with test speaker (must be) attached.
    VR3 & 4 I made sure D1 & D2 was receiving equal voltages.

    Good luck
     

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