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Thread: Mesa Boogie Nomad 45 switching pops

  1. #1
    Supporting Member Alex R's Avatar
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    Mesa Boogie Nomad 45 switching pops

    Jeez this amp is a pain. It came in with some oscillation, so i did a cap job. Yep, everything came out to get the board out with all the pots hanging off it, put it back and it was fine. Except no reverb. That turned out to be the opamp, unusually, and I managed to replace it without taking the damn board out again .

    I was bench-testing it and suddenly the volume dropped. Traced this to the J175 P-channel fet on the bottom left of the FX loop page on the attached schems. I assumed this was there to effect the mute, but weirdly the speaker mute switch still muted the amp without it. Well I replaced it with an BF245A and the volume came back... and now the amp makes a loud pop when channel switching. It pops on all channels and also when disengaging the reverb with the footswitch (but not when engaging the reverb). It pops with the footswitch unplugged and you use the switch on the back channel to change channels. It's a really loud pop... I took out the BF245A and it still pops without it.

    I'm out of patience so I'm off to eat. If anyone can save me the trouble of trying to work out what's up with the damn cursed godforsaken thing now, or indeed what I have done to f it up, I would be grateful beyond words!

    Sometimes it's so nice to know you're all out there, guys
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  2. #2
    Supporting Member Alex R's Avatar
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    no replies... oh god I hope it's not that difficult, I've already sunk more than enough unchargeable hours into this beast... what do you think about changing out the LM3914 IC?

  3. #3
    Supporting Member Jazz P Bass's Avatar
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    Ahh, the Medusa Amp.
    A real treat to work on, I agree.
    First off, the speaker mute switch has nothing to do with the channel switching mute.
    It simply removes the signal to the PI.
    The LM3914 "may" be at fault, I have not had any Pop issues on the amps that I have seen.
    I would be looking at the "Patented Mute Circuit" of the switching matrix.
    Maybe a bad cap in there.

  4. #4
    Supporting Member Alex R's Avatar
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    Ah... so the J175 I took out might be the one at the bottom right of page 8, which is a switching mute (didn't realise this). I'll get a J175 and see if it does the trick.

  5. #5
    Supporting Member Alex R's Avatar
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    ...and thank you!

  6. #6
    Supporting Member Alex R's Avatar
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    BTW I don't like switching circuits that run off heater supplies. HT can short to heaters and cause deeply wearisome damage to switching components, traces, etc.

  7. #7
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    J175 is a P channel FET unlike BF245A which is N channel.
    If you can hear switching pops probably you have a bad mute FET or mute circuit is not working.

  8. #8
    Supporting Member Alex R's Avatar
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    Yeah I REALLY messed up, first I mistook the switching mute fet for the speaker mute fet, then I pulled the BF245A out of my P-channel bin and assumed I'd put it in there correctly... serves me right for getting annoyed with an inanimate object. But you would have to be a saint not to be annoyed by the hassle involved in taking that board out!

    The mute fet shorted, that's what caused the volume drop. Don't know why it did that...

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