KA-7002 alive again

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by dstariha, Feb 10, 2019.

  1. dstariha

    dstariha New Member

    Messages:
    20
    Location:
    San Francisco Peninsula
    This unit, s/n 420432, was purchased to help me figure out what is wrong with another unit I own. This unit was untouched prior to me having it, however, it tripped the protection circuit at low levels, less than a couple of watts. After replacing one xistor and a cap in the protection circuit it now trips at higher levels but well short of maximum.
    However, on sine waves from a sig gen it can attain full+ output.
    Any thoughts on why it trips on musical low notes but not sine waves across the audio range.
    I have noticed that the pre-amp section audibly resonates at around 1kHz.
    In an earlier post it was suggested that pf caps across the outputs (B-C) might alleviate this problem. It did not. Also the r chan runs considerable hotter than the left even though the offset and bias settings are very close to each other and the spec.
    Thanks for all the help in the past and for what may be offered now.
     

     

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  2. rickl

    rickl AK Subscriber Subscriber

    Messages:
    210
    Location:
    Twin Cities
    I don't have a KA-7002 nor I have I've seen one. I did look at the schematic on hifi engine.

    I'd probably measure the voltages at pins 1,3 and 2,4 on the protection board to see which channel is tripping the protection relay. then try to see what is wrong with that channel.

    A lot of switches to clean too. You didn't say but hopefully all switches have been cleaned a few times. this is the first thing to do before replacing anything.

    The schematic does list expected voltages. look for any that are not close. Since you are probably using a DBT so nothing gets fried, the measured voltage will be lower than what is on the schematic.

    this might take some more thought since dc offset and bias are within spec. my guess you will figure it out. just don't be too vague when asking for help. it is great you replaced a transistor and cap on the protection board but there are more than one of each so it is hard to know what you did.
     
  3. dstariha

    dstariha New Member

    Messages:
    20
    Location:
    San Francisco Peninsula
    Thank you for your response. The switches have been cleaned as suggested by yourself and others. The unit was made in two (at least) separate versions with the s/n break at 410501. The earlier version had the main amp (driver board) as a separate board from the protection board. My units have them combined. I replaced Qe16 (C-E short) and Ce17 in the protection section. Indication are that the protection circuit now operates correctly.
    The focus of my question is the disparity in operation between the unit's response to sine waves vs. music. (It trips at low music levels (at low frequencies) but can reach full output+ with the sig gen.)
    Last night I tried to track down the microphonics (?) in the pre-amp section. Everything looked good so I cleaned the board. adjusted the wire dress and tightened the wire wrap connections. This morning I cannot repeat the symptoms. I also rechecked the offset and bias, they are fine.
    I do not yet have an o'scope so am limited in that regard.
    I am going to use my other integrated amp to drive the power amp section. This should isolate the problem to either the pre-amp or power amp. I hope to post the results soon.
    This site shows both versions of the schematic. The last page shows the newer units with combined driver and protection board (X07-1020-00).
    http://akdatabase.org/AKview/albums/userpics/10007/Kenwood KA-7002 Schematic.pdf

    Thank you again and I hope to gain more of your advice when I start on the other unit I own.
     
  4. rickl

    rickl AK Subscriber Subscriber

    Messages:
    210
    Location:
    Twin Cities
    kind of a snarky response but https://sfbay.craigslist.org/eby/ele/d/sunnyvale-tektronix-oscilloscope/6802032614.html

    i tried to follow the schematic with your serial number but the grammys were on and while i did not enjoy everything it was distracting enough.

    The microphonics might be a noisy transistor. I'd check the naughty transistor list (dlucy might have it in his sig). I had to replace a few in a kenwood reel to reel that was causing drop outs after a while.

    this might be difficult to diagnose with only a DVM and desire.

    I'll try to pay attention to your thread and help were I can.
     
  5. dstariha

    dstariha New Member

    Messages:
    20
    Location:
    San Francisco Peninsula
    Thanks, ricki. I may have found my answer to the vexing music vs sine wave problem. I have what sounds suspiciously just like a 60 Hz. resonance in the pre-amp section. I drove the amp section with my Mac's pre-amp out. Sounds fine. So the problem is somewhere in the pre-amp section. I am going to look at the DC filter capacitors first. I feel kind of foolish about this hum, clearly the protection circuit was able to react to the resonance. If I turn the volume way up on an open aux input the amp will reach a resonance point and the protection circuit trips. If I terminate the input I can turn the volume up full, I still hear it but much attenuated. 'Crude but effective' one might say.
    It is nice to know that at least half the integrated amp works just fine.
    I do not think your first line is snarky, actually helpful. I have not looked to buy an o'scope because I may be able to pick one for free. Of course, a relative is involved and I have fix it first. The good news: It is a tube job. Being older they are a little easier for me troubleshoot and repair.

    Thanks again for all your help.
     
  6. rickl

    rickl AK Subscriber Subscriber

    Messages:
    210
    Location:
    Twin Cities
    Good job narrowing down the problem.

    Since you are using the AUX input, make sure you don't have a broken ground on the input. They get connected/disconnected often and might break the connection to the board. Sometimes hard to see and just reflowing the solder works. Select a tape input and see if the problem goes away. If the problem is not there, I'd look at the AUX and not the PS.
     

     

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  7. dstariha

    dstariha New Member

    Messages:
    20
    Location:
    San Francisco Peninsula
    I thought I might just wait for the parts, mainly caps but your suggestion has shifted my focus to the wiring. I noticed a couple places where the wiring bends around sharp corners. I can work on that also. And I might as well go through the switches again.
    Thanks again.
     

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