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Thread: 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

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    Last edited by 76jimmy; 04-01-2020 at 06:31 PM. Reason: Link

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    Supporting Member Jazz P Bass's Avatar
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    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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    Member Jon Snell's Avatar
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    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.

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

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

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    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).

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    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.

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    Lifetime Member Enzo's Avatar
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    For future reference, try this file.
    Attached Files Attached Files

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    Weird, that PDF is also missing half of pg4.

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    Lifetime Member Enzo's Avatar
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    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.

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    Lifetime Member Enzo's Avatar
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    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.

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    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.

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    Supporting Member Jazz P Bass's Avatar
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    Here is a copy of the 60-60 schematic.
    It includes the full circuit.
    Peavey-Classic-60_60-schematic.pdf

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    Quote Originally Posted by Jazz P Bass View Post
    Here is a copy of the 60-60 schematic.
    It includes the full circuit.
    Peavey-Classic-60_60-schematic.pdf
    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.

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    Supporting Member Jazz P Bass's Avatar
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    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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    Member Jon Snell's Avatar
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    I would check the NFB loops, If one channel has the output transformer not ground or floating or NFB from the wrong amplifier goes to the other amplifier, it will sound like a fog horn and be uncontrollable.
    Look for a broken link on the sub board.

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    Jazz, The 50/50 page looks like the same board as the 120/120, good find.

    Jon, I'm an amateur/hobby amp repair guy. I did find the problem was a bad cable that connected pin J200, J204 and J206 to the patch board. I would like to know what a NFB loop or floating NFB is? Like I said I found the faulty component but I still don't know why. Stranger is why each channel operated independently but nut in stereo. I'm only asking to help my understanding.

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    Member Jon Snell's Avatar
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    My apologies.
    NFB stands for Negative Feed Back, (known as CH1 or CH2 Feedback in this amplifier, points J300 and J304) and this amplifier uses it to help control the non linearity of the amplifier. It helps to control the back ground noise and overall gain of the amplifier.
    If a ground connection breaks then there may be too much NFB and after a certain point, NFB becomes PFB or Positive Feed Back. (Think microphone pointed at the main speaker and a high output whistle/whine is produced).
    The amplifier turns into an oscillator and typically "Takes Off" at anything from and including almost any frequency.
    If the NFB from one amplifier feeds into the other amplifier, due to poor connection within a connector, (J200 etc), either or both amplifiers will turn into high powered oscillators of uncontrollable power.

    I hope that goes a way to explains it for you.

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