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Broken amp, need diagnosis help

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  • Broken amp, need diagnosis help

    So I bought a peavey classic 60 power amp on CL thinkin' I got a good deal on a cheap power amp, but lo & behold, the seller packed it like shit, and it arrived broken.
    I wanna see if I can salvage the situation, so anything you can do to help is greatly appreciated.

    A 22uf electrolytic cap got smashed in and one leg broke off the board, a little mylar or film cap beside it also got 1 leg detached.
    A power tube got smashed to bits and both preamp tubes got destroyed.

    I resoldered both 22uf back on, although the 22uf's leg snapped right at the cap's cylinder and the only way I could solder it was to hold the connecting part with tin, I'm waiting for a replacement part I ordered to come in.

    Put in new tubes, fired it up, as soon as I turn the standby switch to play, I hear loud erratic crackling from the speaker, this is without anything plugged into the input, turning the master doesn't seem to affect it.

    So... what should I look for now?
    I noticed a few places on the back of the board the trace got scratched up, would this be the culprit?

  • #2
    I tested the voltages of everything on the board, it seems only the power input and the various fuses have voltages on them, everything else reads at 0 volts.
    Including all the pins in the power socket, caps, resistors, everything.

    That means the board is somehow not getting any power?

    Comment


    • #3
      Well, there are those internal fuses. Are they blown?

      A dented in high voltage cap is not a good idea to powr up. Even if you welded a wire to it, I'd fear internal damage.

      Make sure the meter is set for DC, if you get AC voltages at the fuses and forget to switch the meter, that DC looks a lot like zero AC to the meter.

      Does the power indicator light come on?
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        cracked traces would also be suspected with this much blunt force trama.

        Comment


        • #5
          Originally posted by Enzo View Post
          Well, there are those internal fuses. Are they blown?

          A dented in high voltage cap is not a good idea to powr up. Even if you welded a wire to it, I'd fear internal damage.

          Make sure the meter is set for DC, if you get AC voltages at the fuses and forget to switch the meter, that DC looks a lot like zero AC to the meter.

          Does the power indicator light come on?
          Hey, I tested the internal fuses while they're plugged in, they all are ok.
          Should I remove them and test that way?
          I think I did forget to switch to DC, so let me try that again.

          The power indicator comes on, standby light comes on, when I plug in the tubes all tubes light up.

          Is there any chance something big like the transformers could've blown?

          Comment


          • #6
            When you run a signal through the amp, do you hear the output tubes "singing"?

            Comment


            • #7
              No singing, no matter what I do all I hear is loud irratic crackling.
              But I did find just now that if I pres down on the top of 1 of the power tubes the crackling stops completely.
              Is this a grounding issue then?

              Comment


              • #8
                I agree with CHIP, cracked traces or failed solder, probably on tube sockets & components that help mount the pc board to the chasis. I'd check continuity from the offending tube socket (makes sure amp unplugged & power supply is 0V unplugged) pins to the next solder point on the board. While in there, I would reflow all tube sockets as precaution.

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                • #9
                  ok, so I'll check all tube socket traces. If any parts of the trace appears to be shorted, what do I do, patch it up with tin?

                  I ran some tests on the voltages in the tube sockets, I got
                  Power Tubes
                  (all in vDC)
                  V1 = 0
                  V2 = 30
                  v3 = 525
                  v4 = 0
                  v5 = 55
                  v6 = 0
                  v7 = 30
                  v8 = 0

                  Preamp Tubes
                  Only 4,5,9 have 30vdc, everything else measured 0, shouldn't I get plate voltages here?

                  And V4->V9, V5->V9 = 6vac

                  All values apply to both sockets in the pair.

                  Comment


                  • #10
                    I usually use solid core copper, usually extracted from coax cable line. This ensures there's enough gauge to transfer higher voltages.

                    Pins 4,5 & 9 are heater pins and should read around 6.3 V AC.

                    I would definitly look at solder connections first.

                    Comment


                    • #11
                      You guys have been amazing!
                      I've already made progress, touched up a few shoddy areas of the board and now when I plug a mp3 player into the input and put my hand on top of the power tube, I get a clear signal, but if I let go loud crackles ensue once again.
                      In my limited understanding, this means the board is not groundded correctly right?
                      So where should I check now? I don't have a schematic and can't figure out where the board should be grounded to the chassis, I don't see a typical bare wire screwed down to the chassis like the other tube amps I've tinkered with(both of them).
                      Last edited by silentrage; 01-09-2010, 04:02 PM.

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                      • #12
                        so, all of the solder connections are solid and have continuity on that tube you are pressing, correct? if you put that same tube in the other socket, does the problem follow?

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                        • #13
                          I swapped the tubes and get the same problem, if it helps the crackling seems to need a build up. Right after I turn the amp off standby the signal stays clear for a few seconds, then it starts to crackle loudly, of course it vanishes as soon as I put a finger on either tube.

                          Comment


                          • #14
                            Still sounds like a solder connection to me. I would touch up all the solder connections on all the large components on that same board. Large load resistors, filter caps, and even pc plug pins (if any).

                            Even if a solder connection looks good, applying fresh solder may reveal a lifted trace or cold solder connection.

                            Comment


                            • #15
                              Gotcha, I might as well just systematically rework every joint on there.
                              Thanks!

                              Comment

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