Microphonic caps

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by ramonneke76, Feb 14, 2015.

  1. ramonneke76

    ramonneke76 Member

    Messages:
    198
    Joined:
    Dec 31, 2012
    I did the 'chopstick' test on my 66 bf Bandmaster.

    There are some caps and one resistor that are a microphonics. Also the volume pot of the normal channel is noisy (still noisy after cleaning).

    The 3 caps on the right (2 blue molded and the tiny brown one). Could explain the noisy volume pot?

    Also the earlier replaced 1.5M ohm resistor (the white non carbon comp). According the schematics that has to be 1M ohm I believe.

    Wise to replace these components?

    [​IMG] pic one (3 caps on the right)

    [​IMG] resistor
     
  2. swiveltung

    swiveltung Member

    Messages:
    14,567
    Joined:
    Feb 5, 2011
    Location:
    PNW
    Fenders often will do that... and it's hard to figure out what's driving the microphonics... I do know that often it's not the caps. First I would try a new tube in V1.
    In what way is the Vol pot noisy? scratchy? Hum?
     
  3. Jeff Gehring

    Jeff Gehring Silver Supporting Member

    Messages:
    4,190
    Joined:
    Jan 17, 2008
    Location:
    Kansas City area
    Check the high side of the 'normal' volume pot for DC voltage, you may have a leaky cap in the tone stack. The 'usual suspect' there being the 250pF disc cap. If there is DC leaking through to the volume pot, it will sound scratchy no matter what, and a failing/failed cap could be at the bottom of your microphonics, too.
     
  4. ramonneke76

    ramonneke76 Member

    Messages:
    198
    Joined:
    Dec 31, 2012

    Thanks. The pot is scratchy. I can try to clean it again. The tube in v1 is new but I have more tubes I can try.
     
  5. ramonneke76

    ramonneke76 Member

    Messages:
    198
    Joined:
    Dec 31, 2012

    The 250pF is the loudest when I tap on it. I'll check the DC. DC should be 0 on the volume pot?
     
  6. EFK

    EFK Supporting Member

    Messages:
    1,023
    Joined:
    Jan 18, 2009
    Ceramics are always "microphonic" when you tap on them. It doesn't mean the cap is bad, however - you will have to test it to determine that. In regard to the blue poly caps, same thing: if you have the volume up, it's typical to hear tapping through the speaker if you're tapping on the cap, but again, it doesn't mean the cap is bad or needs to be replaced. Those caps are on the input stage so of course they are going to be sensitive; you would likely hear tapping on any cap in that location, but it doesn't matter as we don't play the amp and tap on caps at the same time. Leaky caps, on the other hand, should be addressed and definitely will contribute to noise and scratchy pots and as noted above the small ceramic is usually the problem. I can't remember the last time I saw one of those blue caps need replacement.
     
  7. Jeff Gehring

    Jeff Gehring Silver Supporting Member

    Messages:
    4,190
    Joined:
    Jan 17, 2008
    Location:
    Kansas City area
    Yes, there should be zero volts DC on the volume pot, or very close to that.
     

Share This Page