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9200 Poweramp makes pop noises and hums

Discussion in 'The Workbench' started by Schwarzwaelder, May 6, 2021.

  1. Schwarzwaelder

    Schwarzwaelder New Member

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    Hi, I'm new here and am having problems with my 9200. It is on Channel A and scared the shit out of me. It doesn't matter if I play it or just let it sit without a signal, after 15 to 30 Minutes it pops really loudly. And from then on it keeps popping until I switch the standby switch off. Today I tried it a second time after I switched it off after that behavior with first preheating it again and then switching Standby on. It didn't take 10 seconds for it to do it again and after I switched Standby off again it started humming really loud! Like a 50Hz hum, so I switched it off immediately.
    I was running Channel B for over an hour today with Standby on just to see if it makes the same thing, but it was fine, only one of the 5881s was slightly glowing on the outer plate. Interesting is though that when I was running both Channels at the same time, Channel B was loudly popping too shortly after Channel A made the first pop. Before that when only B was running it was fine for over an hour. According to the pre-owner the 5881s shouldn't be very worn, so I can't believe that the tubes should be going bad now. I am no newbie to electronics and circuit design, but I never dealt with tubes as this was before my time. I noticed the Amp has these huge Capacitors 220u 400V at the back and I think they might be for power supply filtering. My question now is, could it be that they are going bad and cause me problems like that random popping after warmup and the following hum?
    I hope some tube professional here can help me out.
    Thanks in advance!

    -Schwarzwaelder
     
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  2. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    tubes can go bad any time...

    it could be a cap, but, I'd try tubes 1st...
     
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  3. Old Punker

    Old Punker Well-Known Member

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    Yeah, tubes can last anywhere from 10 minutes to 20 years! :agreed:
     
  4. Schwarzwaelder

    Schwarzwaelder New Member

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    Thanks! But I might change the big electrolytics anyway. 27 Years is a lot for them usually. I've heard they should be changed every 10 to 15 years.

    I have another question. Has anybody here successfully biased one of the 9200s? If I remember it right it has a bias pot and a balance pot on the underside. What does the balance pot do?
     
  5. neikeel

    neikeel Well-Known Member

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    Sounds like tubes to me, most likely one of the PI tubes (simple cheap dual triodes x2 ECC81 and x2 ECC83) I would try those then test the 5881s as there are 8 of them do that first before ripping into the filter caps. You should not only know what you are doing but know how (or who) to test it properly before shotgunning so many parts blindly :2c:
     
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  6. Schwarzwaelder

    Schwarzwaelder New Member

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    Good point with the ECC81 and ECC83 tubes. They are still the first ones my amp has, so it could be likely they are getting worn out.
    Don't worry, I won't be shotgunning blindly. I have the schematic here and have been studying it for the last two hours and indeed the big caps are for the anode voltage of the 5881s. If I now imagine they are letting the power supply ripple through it goes directly to the output transformer, which could explain this immense 50Hz hum that nearly blew my speakers out:)
     
  7. Schwarzwaelder

    Schwarzwaelder New Member

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    And I know from the past that failing Capacitors can be a real nightmare, because they become more and more random in the way they misbehave. Some short, some go open circuit. So that is why I am considering replacing those. Wouldn't be a big deal to do so
     
  8. Schwarzwaelder

    Schwarzwaelder New Member

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    Hey guys, I just wanted to give you an update on the situation. I just changed the filter caps and even though it did reduce the idle hum my speaker cabinet makes, it didn't solve the problem with the pop noises. So the next step is changing out the ecc81 and ecc83 tubes. I'll keep you updated.
     

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