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Peavey CS400 one chanel down

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  • #16
    OK, so you had to pull the IC to see signal at the input pins. You need to specify when you do that. That is a fine way to see that the signal path is intact, but if we are led to think that IC is in place and we still see the input, it will lead us astray.


    I might try banging some sort of noise signal into the base of Q6 just to see if it makes it to the output.
    Education is what you're left with after you have forgotten what you have learned.

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    • #17
      Originally posted by Enzo View Post
      I might try banging some sort of noise signal into the base of Q6 just to see if it makes it to the output.
      I will do that. Thanks again.
      Interested in old, rare, unusual or just plain bizarre music equipment?
      www.nicosonic.com

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      • #18
        Now we have some progress!
        Applying a signal to the base of Q6 got me no or very little and distorted output, but applying signal to the collector of Q6 produced a strong and clean output. So I guess Q4 or Q6 could be at fault. Can I replace Q6 (7204) with a 2N3439 (as per Peavey's cross reference list) even though it is not the same package?

        The amp may not be repaired yet but I am happy that I am finally going somewhere. Thanks Enzo!
        Interested in old, rare, unusual or just plain bizarre music equipment?
        www.nicosonic.com

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        • #19
          Package doesn;t matter as long as the leads get in the appropriate holes. Unless it is something that has to bear against a heat sink or fit in a space or somethinig.

          Those pairs of transistors engaged in some sort of personal embrace can be difficult to check in place, a bad Q4 could squash whatever Q6 is trying to do for example. Pull them and check them.
          Education is what you're left with after you have forgotten what you have learned.

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          • #20
            I couldn't access the forum yesterday... Server down?
            Anyway, I replaced Q4 and Q6. I have now more output but it is still quite distorted. I also have to correct myself: when I apply a sine wave signal to Q6's collector, the output is fairly clean and strong but still has a slight kink in the signal. I am going to replace CR9 and CR8 and check Q5 and Q3 out of the circuit.
            Interested in old, rare, unusual or just plain bizarre music equipment?
            www.nicosonic.com

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            • #21
              After having the amp on the back burner for a (very) long time, I finally got it to work. The culprit was an open resistor, namely R22 (47k). This little component got me wasting a lot fo time and effort...
              Thanks to everyone who helped me on this one!
              Interested in old, rare, unusual or just plain bizarre music equipment?
              www.nicosonic.com

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