KA-7002 help!

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by birchoak, Mar 6, 2011.

  1. birchoak

    birchoak Hi-Fi Nut Subscriber

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    672
    Location:
    Amesbury, MA
    Howdy Folks,

    Been battling to keep the left channel on. Lots and lots of D5. De-fluxed main power board. More D5, on everything, including me (ha ha, just a joke). Tried jumping PRE & MAIN, tried jumping up and down, tried religion ("Please God, let it work this time").

    It is a BEAUTIFUL unit, very clean inside and out, and so well made it brings tears to my eyes every time I touch it. I will work to fix this problem until I die, if only to touch that extraordinary volume knob.

    Now listen, I am a bear of Average Skill and Intelligence. I don't know how to use a scope, don't have a signal generator, sometimes leave the house without pants on. Any "simple" advice & troubleshooting procedures for common folk would be greatly appreciated. The seller has kindly offered to send me a schematic, which will probably only confuse me more but will make me look impressive to my wife as I ponder it (upside down, of course) over a Newcastle Brown.

    To summarize: Unit extremely clean inside and out (like, maybe one piece of dust inside it). Left channel drops in and out with a tiny "tic" or "pop". Protection relay clicks reliably and I carefully cleaned relay contacts with clean paper. D5'd every single pot, switch, RCA MANY times over 2 weeks. Looked for obvious cracks, cold solder joints on boards. There is a ding in the upper right rear corner of the right side panel, maybe from UPS but maybe not as unit was extremely well-packed. I love this thing and I will not stop until it is working perfectly, even if I have to pay to have it done, buy another one, or contract TRIO to commence re-production of the 7002.

    Cheers, and much thanks ahead of time for any and all advice you folks see fit to pass on. :yes:

    Birchoak
     

     

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  2. Bob91343

    Bob91343 Super Member

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    2,012
    There isn't a whole lot more you can do. If you have a schematic diagram you could take a clip lead and progressively short each pair of switch contacts that are supposed to be connected, but that has perils of causing major problems if you slip or make an error.

    No substitute for an oscilloscope. The switches that are most likely to be culprits are those that are seldom used, but that's a shot in the dark. If you buy a cheap used 'scope it will pay for itself many times over in the course of the years. And you might learn something useful.
     
  3. tcdriver

    tcdriver AK Subscriber Subscriber

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    I really get what you say about the look and feel of the KA-7002. Those are really beautiful units. I have never owned one myself, however, I have a few of the KA-6000, which was the predecessor to the KA-7002, which have a similar look and feel.

    Troubleshooting:

    First, you need to find out if the problem is in the preamp or amplifier section. This is easy to do. Run the preamp outputs into another amplifier connected to your speakers. If you still experience the dropouts, the problem is in the preamplifier section. If, on the other hand, you no longer have the left channel dropouts, the problem is in the amplifier section and the preamp section is OK. Either way you will have isolated the problem to half of the unit. Give this a try and let us know how it turns out.
     
  4. Bob91343

    Bob91343 Super Member

    Messages:
    2,012
    You can transpose the left and right pre-out/main-in and see if the problem switches channels. This avoids the need for more equipment.
     
  5. tcdriver

    tcdriver AK Subscriber Subscriber

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    Yes, you can. :thmbsp:
     
  6. birchoak

    birchoak Hi-Fi Nut Subscriber

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    "First, you need to find out if the problem is in the preamp or amplifier section. This is easy to do. Run the preamp outputs into another amplifier connected to your speakers. If you still experience the dropouts, the problem is in the preamplifier section. If, on the other hand, you no longer have the left channel dropouts, the problem is in the amplifier section and the preamp section is OK. Either way you will have isolated the problem to half of the unit. Give this a try and let us know how it turns out."

    That is great advice; I didn't think of that. I will definitely try it and let you all know. I like the idea of narrowing the fault search parameters.
     

     

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  7. birchoak

    birchoak Hi-Fi Nut Subscriber

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    Another good idea

    Yes, I will try this as well. Thank you.
     
  8. birchoak

    birchoak Hi-Fi Nut Subscriber

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    Location:
    Amesbury, MA
    KA-7002 Update

    Hope no one's been holding their breath on this one; since I last tinkered with this little beast my wife and I have had a son (who is, believe it or not, more fun than vintage audio). Ahem.

    Seems like Deoxit keeps working over time. Bob91343 made the excellent point that the most seldom-used switches are often the culprits. Turns out the guy I bought it from had kept the NORMAL/SEPARATE switch on SEPARATE (for how long I do not know). When I fired up the 7002 last month I had both channels. Finally. I think that switch was oxidized something awful and that I probably didn't work it back and forth enough after Deoxit'g initially.

    So...I now have a fully functioning (I think) KA-7002 that produces quite a bit of hiss/static/whatever unless a source is playing through it. Should I think about replacing the filter caps? Is that a logical first step to keeping this little baby healthy? I don't see anything that looks like a power supply; if anyone has done a recap on this unit I'd love to hear about it.

    Cheers and good night.:beerchug:
     
  9. birchoak

    birchoak Hi-Fi Nut Subscriber

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    Location:
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    KA-7002 update: every time I walk past this beautiful little hunk of ailing audio sweetness, my heart sinks. "It's just sitting there, waiting for a deft hand to bring it back to sonic perfection. Why am I giving up on this thing?" Two weeks ago, I visited a friend who has been restoring a Datsun 280Z in his garage. He is four years into it and just got the engine back into the chassis after completely rebuilding it. His only prior experience was rebuilding a lawn tractor. I complimented him on his tenacity and sheer daring, then shared my ill-fated attempts to bring back the 7002. He said, "If you really want to fix something badly enough, you'll figure it out."

    That was a wake up call. I dragged the 7002 off its shelf and once again admired not only the build quality of the amplifier but the parts themselves. The faceplate is nearly 1/8" thick and the graphics are all embossed, not silk-screened. Every knob is solid, milled aluminum--not metal shells over plastic--and the main power board slots in and out of its connector so easily that . . . I thought, "Well, this board would be so easy to work on I might as well recap it and see where that gets me." An amplifier this old needs recapping anyway, regardless of other problems. The recap was very easy but made absolutely no difference to the distorted sound problem. Nuts!

    Undeterred, I tracked the problem to the preamp section and ordered the appropriate replacement capacitors. Poking around on the net, I found that 2SC458 transistors are notorious for going bad and making unwanted noise. Could they be the culprits in my crappy sound? Who knows! I ordered replacements. On a roll, I also ordered parts for the protection board (completely different from the publicly available service manual at Hi Fi Engine but a thread here on AK set me straight--EchoWars is, quite simply, the man) because I saw scorch marks under one of the transistors (actually a regulator, according to EW) and figured that ain't good. Completely amped up now, I ordered new trimmers for the power board because adjusting the originals accurately and with any sort of confidence that they will retain their settings once adjusted has proven to be near impossible.

    While I'm waiting for the parts to arrive, I'm going to clean and polish the knobs and faceplate again and see about milling new sideburns for the little beast. I'm a carpenter by trade and one thing I can do fairly easily is craft new wooden parts and cases and such. I will report in as progress dictates.

    Never say never! Never give up! Keep at it!
     
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  10. tcdriver

    tcdriver AK Subscriber Subscriber

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    Very likely the 2SC458 are the culprits. Good luck with the rebuild.
     
  11. Blue Shadow

    Blue Shadow I gotta get me a new title

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    Seven years, that seems to be about right. I have a few items that have been shelved while I learn more and recently repaired some that were waiting 7 years. Hope you get the KA working right soon. Nice series of great that early Kenwood ATC stuff.
     

     

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  12. spark1

    spark1 Well-Known Member

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    The null switch is a notorious source of channel drop out and noise on that amp. Also, tape monitor switch is a common source of channel drop.

    2SC458 and 2SC871 transistors commonly become noisy with age. They are found throughout the 7002. Be sure of correct pin-outs when you replace transistors. Replacing them took care of residual noise and hiss in my case.

    I did a major rebuild of my 7002 recently. All that point to point wiring makes access difficult...especially to the filter boards and tone board.

    Great looking amp, especially when stacked with matching tuner (see my avatar). Sounds pretty good, too. Not as good as some of my better amps, but certainly better than many from the era.
     
    Last edited: May 18, 2018
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  13. birchoak

    birchoak Hi-Fi Nut Subscriber

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    Well, thanks for your interest, folks. I recapped the preamp board, temporarily screwed it back in to test the sound. Nope, no difference. I replaced the 2SC458S (what a PITA--lots of squinting, checking that I didn't make any unwanted solder bridges on that tight board). No difference at all. I replaced the remaining four transistors on the board (apparently the replacements for the 458s also serve for the 871s?). Again, not a bit of difference to these tired old ears. Finally, brandishing a new regulator for the protection board, I swapped that new transistor in and, Hallelujah, no more awful distortion, no more noise. I had finally conquered the 7002! Of course, I promptly undid all of my progress by attempting to install new Bourne trimmers on the power board. That was a big mistake. The amp would not turn on and I fried a bunch of 250v/4A fuses trying to make it stay on. Nuthin' doin'. Checked for solder bridges, etc., burned out components. Nothing. I put the original trimmers back in--still blowing fuses like a popcorn maker. I adjusted the trimmers to zero-no current passing through--still nothing. Rats! From king to pauper in one move. Sigh. If I start the unit without the power board slotted in, it will stay on, so I'm reasoning that the problem is some kind of imbalance in the power board, unwittingly introduced by yours truly. Before I give any more blood, does these things sound as good as their build quality suggests? Am I chasing a pot of lead at the end of this electronic rainbow?
     
  14. spark1

    spark1 Well-Known Member

    Messages:
    993
    Location:
    The Great Midwest
    Well, don't expect audio nirvana....but in good working order they sound quite nice. It is not in the same class as several of my other integrated amps (sansui au919, optonica 4545), but certainly holds its own against my rebuilt marantz 1060 and sansui au7900 units.

    BTW, the board you are having difficulty with is the driver (aka power amp) board. The power and protection boards are on the underside.

    Have you tested the driver and output transistors?
     
    Last edited: May 18, 2018
  15. birchoak

    birchoak Hi-Fi Nut Subscriber

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    Location:
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    spark1, thank you for weighing in. Guess I was expecting audio nirvana (mostly because I've worked so hard on this little bugger; nothing related to reality I'm sure). Power board is underneath? Huh. Well, I don't know how to test driver/output transistors but the thing sounded fine just before the Trimmer Incident. Did I blow those components with the replacement trimmers somehow? I have a multimeter; how do I test? At this point, I'm thinking maybe put the 7002 away for another 7 years, take a little break from it.
     
  16. spark1

    spark1 Well-Known Member

    Messages:
    993
    Location:
    The Great Midwest
    Don't get me wrong...the 7002 is a good little amp. It was a high-end unit in its day, but solid state amp designs improved dramatically throughout the 70's and well into the 80's. As I said, I think its sound quality is better than much of what was produced in its own era, in the "golden age" of the 70's and 80's, and in the decades since. But, it's not in the uppermost tiers of performance. Stated another way, it's performance is well above average but not in the top tiers...perhaps 80-85th percentile, to throw a number at it.

    Bottom line, it's a solid performer with exceptional build quality and great looks. I've been through a lot of gear over the years...this one is staying, based on a combination of nostalgia and performance

    I assume the protection relay closes (clicks) when powering up without the driver board? If so, I suspect that you may have caused a problem on that board - or in the outputs - when replacing the trimmers. Are you sure that you connected the new ones correctly...it's easy to make a mistake with them.

    The fact that it keeps blowing fuses suggests that you you've created a short circuit on the driver board or have blown/shorted the outputs. You've got to find out where, and correct the underlying problem before replacing any blown transistors.

    Here is a good primer on transistor testing: http://audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/
     
    Last edited: May 19, 2018

     

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  17. birchoak

    birchoak Hi-Fi Nut Subscriber

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    spark1, how kind you are to indulge me in my madness with this thing. Are the outputs those big silver flying saucer things that are bolted to the board? I will pull them and test them. It's so easy to work on that board; thank God the problem seems to be confined to it at this point! I was pretty careful with the trimmers but...probably not enough. I was really excited about doing this thing all the way (it was 1 a.m.); obviously, I should have quit while I was ahead.

    This amp is like that girlfriend I definitely should have broken up with but kept hanging in there, because, "if I do this one more little thing everything will be fine; she's so darned pretty and well-built I gotta do it." Not a lot of objective thinking here. It's those knobs that keep luring me back to the pain; quite possibly the finest twiddlers on any piece of hi-fi equipment. If I can't fix this amp I'm going to put them on a chain and wear them around my neck. At least the volume knob.
     
  18. petehall347

    petehall347 the brandy coffee man Subscriber

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    go back here and look for things that cant happen . " I promptly undid all of my progress by attempting to install new Bourne trimmers on the power board. That was a big mistake. The amp would not turn on and I fried a bunch of 250v/4A fuses "

    will be a simple mistake like trapped wire or i heard of some screws longer than others doing this sort of thing with some things .. just thinking out loud here .
     
  19. petehall347

    petehall347 the brandy coffee man Subscriber

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    i think i found same thing as you did ..." If I start the unit without the power board slotted in, it will stay on"
    trimmers are the key .. are they the same value as original ? were they installed right way round and turned to min before power on ?
    fuses blowing is not a good sign .. best check output transistors next .
     
  20. spark1

    spark1 Well-Known Member

    Messages:
    993
    Location:
    The Great Midwest
    The output transistors are mounted on the heatsinks, held on by two screws each. Keep in mind that the case of these transistors is the collector, which needs to be isolated from the metal heat sinks. Accordingly, you will find a thin mica insulator between the case and the heat sink, and with a very thin layer of thermal compound between the case and the mica, and between the other side of the mica and the heat sink.

    For testing and short term use at low outputs you don't have to worry about re-applying new thermal compound, but after you've found the problem and fixed it, you should clean off the old compound and apply new. Also, keep in mind that the mica insulators are fairly fragile...if you break or crack one, you will need to replace it. Some very knowledgeable folks use silicone pad replacements, while others prefer the micas.

    The output transistors, assuming they are original, should be 2SA649s and 2SD218s (one of each of the left channel, one of each on the right). If you have to replace them, I think OnSemi MJ21193 and MJ21194 would be good substitutes, but my expertise is quite limited, so you would want confirmation from someone who really knows what they are talking about!

    The driver transistors are on the main amp board (aka driver board or power amp board)...they are Q9 and Q10 (2SC680BL) and Q11 and Q12 (2SA656). Make sure you note their orientation before reinstalling them after testing (assuming they are good). I don't have appropriate subs in my notes from my own KA-7002 rebuild, so if they are bad, you will probably need to start a thread seeking substitution advice on those transistors.

    If you haven't already done so, get the service manual here:

    https://www.hifiengine.com/manual_library/kenwood/ka-7002.shtml

    It's free, but you do have to register first (also free).

    Which trimmer resistors did you change out...the 1k ohm (VR1 and VR2, for center voltage adjustment), or the 500 ohm (VR3 and VR4, for idle current adjustment)? Or both pairs? Did you confirm proper values and that the new trimmers had the same turn configuration as the old ones (max resistance at full counter-clockwise). If the old ones have not been turned since removal, measure them and assure the new ones are set to same resistance before installing.

    Final thought...if you are going to be digging around in this amp, you should probably build a dim bulb tester. It's quite easy and cheap, and can really help to protect you from damage-causing mistakes. You turn the amp up through the DBT, which a) gives you a quick indication of any shorts, and b) limits current draw so as to avoid short-circuit damage.
     
    Last edited: May 19, 2018
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