Pulsing buzz varies speed with Volume/Bass. Any ideas?

Discussion in 'Solid State' started by tusntuk2, Sep 7, 2017.

  1. tusntuk2

    tusntuk2 Member

    Messages:
    56
    Location:
    Arkansas, USA
    Kenwood KR-5600 receiver, just finished recapping Audio and Power Supply boards. After resetting bias it started making buzz/hum in a rhythmic pulse that would vary speed as either the Volume or Bass control was moved. Right channel only. Maybe some transistor oscillation? Haven't had one vary speed like this before. Any ideas where to look or what to check? Voltages I have checked so far have all looked pretty good.

    KR-5600 has op-amps for preamp section, and audio controls on the PCB with power amp. Tuner seems to work fine, unaffected by the problem. Any help would sure be appreciated! It's a good puzzle.
     
  2. Binkman

    Binkman Addicted Member

    Messages:
    5,967
    Location:
    Raleigh, NC
    Greetings;
    Retrace your work. At least a hint is the bass and vol. circuits and one channel. has the bias changed since adjustment?
     
  3. tusntuk2

    tusntuk2 Member

    Messages:
    56
    Location:
    Arkansas, USA
    Thanks for the quick response Binkman. Bias - that could be the achilles heel here, because the noise did not start until after I adjusted the bias to factory spec. I ran the amp on big speakers and got it pretty warm, then adjusted, waited, adjusted, waited, let it cool down a bit, checked again and thought it was pretty close, about 39 and 39.6mv, so I left it there. That's when I checked the phono and aux and found the noise. I don't get the noise cold, there is no noise in the FM. Amp seems strong, it can crank the three way 12's. I am still rechecking my work, polarities, values, etc, but no luck so far. The amp got warm before the noise, no noise cold or little test speakers. No other clues, but checking voltages now.
     
  4. tusntuk2

    tusntuk2 Member

    Messages:
    56
    Location:
    Arkansas, USA
    Hmmm - amp got hotter on noise side, bias jumped to 80+mv, other side at 50mv. Heat sink for R output trans was warmer than L output. How do I check bias transistor Qm8? I am still using factory bias pots, they seemed to make adjustments OK.
     
  5. Binkman

    Binkman Addicted Member

    Messages:
    5,967
    Location:
    Raleigh, NC
  6. tusntuk2

    tusntuk2 Member

    Messages:
    56
    Location:
    Arkansas, USA
    Checked my work on electrolytics on the audio board, and everything checked out. One discrepancy from the S/M was using 100uf/50v at Cm 29,30, not the S/M 47uf/50v. I took the values for reordering parts from actual parts on the board, and it seemed the 100's there were original (factory). I also used 474k /50v film caps on inputs Cm3,4, but I wouldn't expect a problem there. New bias trimmers and KSA992's for Qm7,8 should arrive soon, so I have my fingers crossed. Any other ideas?

    I found reference to a similar problem, http://audiokarma.org/forums/index.php?posts/10743899/ (hey, how do I copy a link to a post?) called "New receiver rhythmic feedback traced to speakers" where they discuss amp load causing the oscillation, but no definitive answer. I had installed 10,000 uf caps for mains, too, so there should be plenty of reserve.
     

Share This Page