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  1. #21
    Computer "Specialist" Agent_24's Avatar
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    Default Re: Electronics Experts Wanted...............Amplifiers.....

    Quote Originally Posted by SurferJoe46 View Post
    Would it be a gamble to just piggy-back another opamp over one of the suspect ones? Opamps themselves are pretty cheap.
    IF you have a bad opamp generating the noise, adding another over the top wouldn't get rid of it. You'd need to pull the bad one first. And you'd have to have a suspect to begin with - there's a lot of them on the boards...

    You really need to narrow the noise down to a specific part of the circuit first. If you can unplug the preamp board and at least narrow it down between preamp and power amp, you'll eliminate half the circuitry already.
    Freezer spray, hot air, and signal tracing would then be the next logical step.

    In any case, you better be good at soldering, because it looks like almost everything here is surface-mount and pretty fiddly.

    With regards to sending it away for repair - you could probably get away with sending in just the boards. Any decent shop would have a power supply, speaker, etc to hook up for testing. Ask them about it.


    Edit: I see this amp has a headphone output. Have you tested the signal coming out of the headphone jack? That is before the power amp so if there's noise coming out of that too, then your problem is probably in the preamp section. That might be easier than unplugging cables.
    Last edited by Agent_24; 10-12-2018 at 12:09 PM.
    Non-system disk or disk error. Replace and strike any key when ready.

  2. #22
    Smiling Down On Youse SurferJoe46's Avatar
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    Default Re: Electronics Experts Wanted...............Amplifiers.....

    OK - results from some testing.....................

    I plugged in a headphone set and the noise is still there. BUT.............

    I then quickly - before the noise went away of it's own volition - pressed the buttons for the special effects and one of them made the noise go away. When re-pressed the sound came back.

    The other Special EFX buttons only changed the tone of the noise - to be expected as they are supposed to boost bass, boost trebel, boost midrange, etc. etc.

    The button that shut the noise OFF is called the "TT Boost" button. Now I have to remember what that button is for. I've got a pdf on it somewhere her in my files... BRB

    Here's a video - spoken in some form of a foreign language I think https://youtu.be/thwRuLtniRc

    .... but the "TT" stands for: "Trans-Tube". So now I think that the problem is in the post-amp or EFX side of the amplifier.

    This may well be on the inside of the cab where the driver is... and as yet I have not seen that part but I know it exists since there's a LOT of wires going in to the cabinet part, not just the two for the driver - but also some nutserts that are holding some hefty screws coming up from inside the cab.

    I suspect another PC board and possibly some heavy components to handle the Wattage inside there.

    I didn't take pictures of that area - but later I might - I just remember a lot of ORANGE and possibly some YELLOW wires going into or out from that area.

    This may be easier than I originally thought it might be to repair if it's just an EFX module of some kind to replace.

    Peavey is notorious for having a lit of NOS for even their very most antique gear - so that's a bright spot for me.

    I need a bright spot or two right about now. I'm old.


    "Life" is what happens to you while you're busy making other plans


  3. #23
    Computer "Specialist" Agent_24's Avatar
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    Default Re: Electronics Experts Wanted...............Amplifiers.....

    The Preamp and EFX circuits appear to be on the same PCB as the controls - the 2nd board in your photos that you can only see the back of.

    If the noise only appears when TT Boost is enabled, then the problem must likely lie in that section.

    That TT Boost button corresponds to S1 on the board, which is called OVERDRIVE SWITCH (A/B) in the schematic.


    When this function is on, the audio is diverted through the circuits made up with opamp U2.
    Aside from that opamp, the circuit also contains two transistors (Q7 and Q6) as well as a double-diode, D5. These components would be first on my list of suspects.
    Non-system disk or disk error. Replace and strike any key when ready.

  4. #24
    Smiling Down On Youse SurferJoe46's Avatar
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    Default Re: Electronics Experts Wanted...............Amplifiers.....

    OK - that's a relief ---- that it may only incorporate one or two bad components.

    I tried to utilize the XLR Output, but I need a male-female and tonight I could only find an XLR-to-TS 1/4" jack - which leaves one circuit out of the signal to my 1000 Watt Peavey (another unit all together) amp.

    Tomorrow's a new day and I'll find one of the many XLR-to-XLR cables that I cannot find at the mo ---> and make the correct connex.


    "Life" is what happens to you while you're busy making other plans


  5. #25
    Smiling Down On Youse SurferJoe46's Avatar
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    Default Re: Electronics Experts Wanted...............Amplifiers.....

    Is there any programming needed when an opamp is replaced?

    I've never worked with them and I think they need to be programmed for a special purpose and not some generic value - right?


    "Life" is what happens to you while you're busy making other plans


  6. #26
    Computer "Specialist" Agent_24's Avatar
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    Default Re: Electronics Experts Wanted...............Amplifiers.....

    No - you're thinking of things like microcontrollers (such as the one which performs the tuning function) or EEPROMs, flash memories, PLDs, etc

    An opamp is just a simple analogue IC.

    You'd only need to change things when replacing an opamp is if it's one with offset null pins, and which need to be adjusted for the circuit to work correctly.
    In this case, the opamps are all NJM4580, which don't have an offset null feature anyway.

    If it turns out you need to replace U2, note that it's the NJM4580V variant - in an SSOP8 package.
    Non-system disk or disk error. Replace and strike any key when ready.

  7. #27
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    Default Re: Electronics Experts Wanted...............Amplifiers.....

    Quote Originally Posted by SurferJoe46 View Post
    Is there any programming needed when an opamp is replaced?
    I doubt your issue is due to failing opamps. I wouldnt start replacing them unless you are pretty damn sure they are the cause of the issue, you could end up making
    another issue on top of the one you have.

    Perhaps try a hairdryer or hot air gun & some freeze spray, see if you can make it fault by heating or cooling components
    Unplug & replug all the cables
    Pull out all the boards & look for bad solder joints (use a magnifying glass)

  8. #28
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    Default Re: Electronics Experts Wanted...............Amplifiers.....

    try a search or two, you may get some clues were to start looking :-)

    https://www.google.com/search?q=peavey+MAX+112+faults

  9. #29
    Computer "Specialist" Agent_24's Avatar
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    Default Re: Electronics Experts Wanted...............Amplifiers.....

    It could also be that the switch that enables/disables the TT Boost circuit is dirty/faulty.
    Non-system disk or disk error. Replace and strike any key when ready.

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