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Kenwood KR-7200 Right Channel Drop

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by MiKR7200, Sep 23, 2016.

  1. MiKR7200

    MiKR7200 New Member

    Messages:
    29
    Hey guys,
    I picked up this KR-7200 a few weeks ago and have had intermittent right channel audio cut out ever since. It seemed like at first it was working longer before cutting out, and has starting occurring faster after power up lately. I have feverishly cleaned all pots especially volume/balance with deoxit, working the knobs a lot in between. I have cleaned the contacts numerous times on the speaker protection relay to no avail. I've tested dc offset which shows around 6.7mv left and 52mv right. I went through all the capacitors on the main amp board and replaced those that looked suspect (mostly 47uf and 220uf). The larger 220uf had an obvious leak so I was pretty excited that I had figured out the problem, yet just soldered them in and within 10 minutes of playing the right channel drops again. :( One thing I have figured out is that when the channel drops, if I turn off power, and turn it right back on its back on again, then a few minutes later the same thing again. Could it be a back relay? I've looked everywhere for cracked solder joints and cant see anything. It's getting frustrating at this point, but I know that I'm technically inclined, I have a service manual, I have tools, I'm just baffled and don't want to hand over $70 at the local shop for a bench test alone.
    Any help would be greatly appreciated! Thanks!

    Mike
     

     

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  2. MiKR7200

    MiKR7200 New Member

    Messages:
    29
    A bit more information:
    Sometimes the right channel will not just cut out immediately, but become distorted for a while and then at some point drop completely. Even when the right side is pretty clear, it always seems a bit lower in volume than the left for some reason. I really appreciate any suggestions. Thanks!

    Mike
     
  3. MiKR7200

    MiKR7200 New Member

    Messages:
    29
    So last night I spent a few more hours (unsuccessfuly) troubleshooting the problem. I swapped the differential pair transistors q2&q4 on the right with q1&q3 on the left. Brought the left voltage offset up to 36mv, and the right down to 6mv. Maybe I'll just replace those with new ones, but I don't think it's part of my problem since the right continued cutting out. I readjusted the bias on both sides to around 18.5 (the closest I could get to 20 with those trim pots). I also desoldered and removed the protection relay and found nothing unusual. No pitting of the contacts or anything. I continue looking for a bad solder joint or something that stands out, but still no luck. Not sure what to do next other than recapping the rest of the main board and maybe some of the resistors.
     
  4. Watthour

    Watthour Electron Rancher - JS3600

    Does this occur in all modes? In mono mode? Tape Mon?
     
  5. MiKR7200

    MiKR7200 New Member

    Messages:
    29
    Not sure with mono, but does occur in all input modes am/fm phono etc.
     
  6. Watthour

    Watthour Electron Rancher - JS3600

    The reason I asked about mono mode is that the L/R channels are combined at the main amp by the mono switch. It the problem happens to be upstream somewhere, such as the mode switches, this would be a way to isolate it.
     
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  7. MiKR7200

    MiKR7200 New Member

    Messages:
    29
    I just got home. Going to play it until it starts acting up and I'll check mono (right) and post back. Thanks. There's also a 680ohm register on the main amp board that appears to have been overheating and even discoloring that area of the pc board. I have some replacements on order. Not sure why it got so hot though. The resistance across the resistor is still good, but visually it doesn't look good.
     
  8. MiKR7200

    MiKR7200 New Member

    Messages:
    29
    I just confirmed that the right is cutting out in mono mode when the selector is set to "right". Does this indicate that the problem is in the main amp?
     
  9. MiKR7200

    MiKR7200 New Member

    Messages:
    29
    Here's the resistor I mentioned... Not sure if it's normal, but I put a temperature probe on it and it was about 150 degrees F. In comparison everything else on the board is running between 70 and 90.
     

    Attached Files:

    Last edited: Sep 24, 2016
  10. Watthour

    Watthour Electron Rancher - JS3600

    If that is R47 (as it appears to be) it is the dropping resistor for the protection relay coil. It must pass all the current for the relay coil and thus may get fairly warm without anything being wrong. It COULD be replaced by a 2W resistor on stand-offs just to get the heat away from the board for safety.
     
  11. MiKR7200

    MiKR7200 New Member

    Messages:
    29
    It is r47. I'm replacing it tomorrow when it arrives in the mail. Is it possible that the relay appears fine visually, but is not functioning properly? I'm really stumped about where to go from here. Like I said, when it cuts out if I cycle the power switch it comes right back on loud and clear. Then cuts out again shortly after.
     

     

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  12. Watthour

    Watthour Electron Rancher - JS3600

    The nest time the right channel cuts out you can attempt to jumper the relay contact if you suspect them. This can be done at Pins 1 and 18 of the X13-1240 relay board.

    FWIW, I've had the speaker rotary selector switches on Kennys of this era which have "gone stupid" in the manner you describe. Have you tried connecting the speakers to the B or C terminals, or monitoring with headphones?
     
  13. JimmyEJames

    JimmyEJames Super Member

    Messages:
    1,258
    Clean selector and mode again
    Push button filter switches tape monitor loudness too very often problems here
    Speaker select too
    JJ
     
  14. MiKR7200

    MiKR7200 New Member

    Messages:
    29
    Watthour,
    I've tried all 3 speaker terminals with the same result. Haven't tried headphones but I will do that to rule it out.
    JJ,
    I'll reclean the switches and post back.
    Thanks to both of you for the help.

    Mike
     
  15. MiKR7200

    MiKR7200 New Member

    Messages:
    29
    I took the front panel off and thoroughly cleaned all the pushbutton switches with deoxit. I also replaced that r47 resistor with another 1w metal film. I think I'll go with a 2w and see if it runs any cooler. I just don't want the board getting any further heat damage. Im not certain that the problem is resolved, but I ran it most of the day yesterday without any right channel issues. I'll just have to wait and see if it starts going out again. I wonder if one of those buttons such as the tape monitors could have been the whole issue?
     
  16. JimmyEJames

    JimmyEJames Super Member

    Messages:
    1,258
    Those push buttons are an issue more than 50% of the time probably more especially on Kenwood
    I say because of what I have owned and read
    Also seem to never get properly cleaned by a lot of people : - (
    Metal on metal switches can be a lot harder to get clean than carbon tracks
    When it happens again hopefully it won't but if it does pump the push button switches rapidly and see if clears up
    Not hard usually to identify pin down
    JJ
     
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  17. MiKR7200

    MiKR7200 New Member

    Messages:
    29
    I took the front panel off and thoroughly cleaned all the pushbutton switches with deoxit. I also replaced that r47 resistor with another 1w metal film. I think I'll go with a 2w and see if it runs any cooler. I just don't want the board getting any further heat damage. Im not certain that the problem is resolved, but I ran it most of the day yesterday without any right channel issues. I'll just have to wait and see if it starts going out again. I wonder if one of those buttons such as the tape monitors could have been the whole issue?
     
  18. MiKR7200

    MiKR7200 New Member

    Messages:
    29
    Not sure how I double posted there, but thanks JJ, I'll remember that.
    Mike
     
  19. DR_Simpkins

    DR_Simpkins Active Member

    Messages:
    209
    Location:
    West Virginia
    I'll almost guarantee it's one of those buttons , I posted on here years ago trying to figure what was causing it. Lots of spraying and working the switches back and forth and buttons pushed in and out a zillion times fixed it.
     

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