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Peavey Classic 120/120 dual mono repair

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  • Peavey Classic 120/120 dual mono repair

    Sorry for double post
    Peavey Classic 120/120 dual mono
    Hey guys. Figured I'd use the forum to troubleshoot this amp with you guys. Peavey tech support is down so any advise, tips or tricks are much appreciated.

    Little back story. I purchased this amp from GC about 6 months ago. When it arrived it was in severely damaged condition wasn't packaged well. I returned it to my local GC and the store manager was fine with giving me a full refund including shipping charges. I asked what he would do with the amp and he said "dumpster". We settled on a salvage price and I left the store with the broken amp.

    One of the output transformers had broken the welds on the mounting plate, few broken pots on the front panel and a well dented enclosure. Couple tack welds, replaced broken pots and knocked the tin back into shape. I completely disassembled the amp checking all the components for obvious damage and reassembled the amp checking the wiring with the schematic. Success! Or so I thought. The amp fired up and was working fantastic.

    This week I got a hold of a multi effects pedals with stereo output and couldn't wait to try the 120/120 in stereo or dual mode. Hooked everything up. Threw the switches in they're appropriate positions turned it on both channels 1&2 seemed to go full output with trumpet like tone Loud AF. I'm trying to describe the tone of the sound. It was freight-train-like. The thunderous output would be turned off by putting either channel in standby. The problem persistent even with known good tubes. Mono operation seems to work %100. The full output feedback sound only starts off standby when a signal or chassis vibration is given and both channels one and two are on. In stereo mode both channels work fine if only one is taken off standby at a time. Soon after I take both off standby it goes into feed back and not immediately, sometimes I can get both to output at low level for a short time then feedback again. If I remove the preamp tubes from either channel I can take both off standby without the full output feed back and of course only one channel will output that still has the tubes in it.
    Link to schematic:

    120/120 schematic
    Last edited by 76jimmy; 04-01-2020, 06:31 PM. Reason: Link

  • #2
    You will have to call Peavey for a schematic.
    The manual can be found here: https://www.manualslib.com/manual/12...?page=7#manual

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    • #3
      From what you described I would check for a hairline crack in the ground side of the input jacks. If the ground is lifted totally because there is a crack or break in the circuit, it will sound like a fog horn. From memory, the volume pot grounds are grounded through the front panel, as are the tone controls. Why it is OK when you turn one side off is a mystery without further investigation.
      Study your service manual.
      Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

      Comment


      • #4
        Thanks for the replies. I think I fixed the link to the schematic. I definitely think it's on the input side of the amp being if I pull any of the preamp tubes on either channel the foghorn turns off. Curious thing is why both channels work independently and not at the same time is what is making me scratch my head. Come to think of it I don't think I pulled the input/output board on the back of the amp. Something to check.

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        • #5
          Couldn't figure out how to post a pick here is a link

          https://photos.app.goo.gl/DyoxW4MnzEzU4p1g8

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          • #6
            The most critical part of the schematic (for this problem) is missing. (left side of pg.4 of pdf)
            Does anyone have the complete schematic of the 'pwr amp patch board' ?
            I assume that is where the stereo mono switch is and showing the NFB routing (to presence pots).
            "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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            • #7
              Didn't notice half the page was missing until I had Staples print it for me full size. I narrowed in on the input circuit. First I found the new pot I replaced was bad so that's always frustrating. Lucky I ordered an extra when I replaced it the first time. Problem wound up being a bad cable that went to pins , J200, J204 and J206. The cable didn't any cuts in it but it must have gotten pinched by the busted transformer. Didn't think it was going to be simple as a bad cable. Thanks all for your input. Helped me narrow in on the input and pots.

              Comment


              • #8
                For future reference, try this file.
                Attached Files
                Education is what you're left with after you have forgotten what you have learned.

                Comment


                • #9
                  Weird, that PDF is also missing half of pg4.

                  Comment


                  • #10
                    That is weird, it didn't used to be. The layout drawing is intact, we can draw out the circuit from that, there is not much on that board.
                    Education is what you're left with after you have forgotten what you have learned.

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                    • #11
                      I just checked my dealer support site, and that half page is missing there too. I know my paper copy is intact, but it is not accessible.
                      Education is what you're left with after you have forgotten what you have learned.

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                      • #12
                        Agree, there was enough info on the page to figure out what went where. Thanks again for the help. Super glad I got this working for a piece of wire and a pot.

                        Comment


                        • #13
                          Here is a copy of the 60-60 schematic.
                          It includes the full circuit.
                          Peavey-Classic-60_60-schematic.pdf

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                          • #14
                            Originally posted by Jazz P Bass View Post
                            Here is a copy of the 60-60 schematic.
                            It includes the full circuit.
                            [ATTACH]57668[/ATTACH]
                            That model doesn't have the mono option so no stereo/mono switch. That's the part that is missing in the 120/120 schematic.
                            "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

                            Comment


                            • #15
                              Ok.
                              How about the Classic 50-50 (page 3/ stereo switch): Classic 50-50.pdf

                              Why did Peavey have so many models?

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