Help with a bass amp

Discussion in 'Shock Brother's DIY Amps' started by LuckyJinx, May 24, 2019.

  1. LuckyJinx

    LuckyJinx TDPRI Member

    Posts:
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    Joined:
    Apr 2, 2011
    Location:
    Portsmouth, Shire of Hamp
    Hi,

    I'm not sure this is exactly the right place to ask, given I'm not actually building it, but it can't hurt to ask.

    I've bought an amp off ebay; a Marshall MB30 for the low, low price of £32 shipped. The catch, of course, was it being listed as not working, for parts. So I got it, and I'm going to have a go at fixing it, and I thought I'd ask here for some pointers on how to diagnose the problem.

    What I've done so far:
    - plugged it in, it powers on, LEDs turn on, switches work, the pots seem a bit stiff but functioning;
    - plugged in a guitar, verified that it does not produce any sound (as advertised);
    - put the amp in the "Modern" channel, fiddled with the compressor, saw the LED go from green to red (assuming that means the signal hit the threshold) for different lengths of the strum (that is where I got the "pots seem to function" from);
    - plugged a cable from a phone into the CD in jack, got no sound to the speaker either;
    - plugged headphones into the headphone out, no sound;
    - ran a cable from the line out to another amp, got sound from the other amp;
    - ran a cable from the effects loop send to the other amp, also got sound.

    That is where I've got so far. In my layman understanding of things, this all means the preamp is fine, the problem is somewhere down signal of that. Also, given the headphone out also not getting a signal, it seems the problem is not in the speaker itself.

    What can I do now to track down the malfunction? I'm assuming it's now a question of cracking it open and checking how it looks inside, but some pointers on areas that are likely to be faulty would be cool.

    Also, am I likely to have caused fresh harm to something by running those lines to the other amp (I used an instrument cable for this)? I did keep volumes low, and only did it long enough to verify there was signal going through.

    Thanks!
     
  2. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    Location:
    Yorkshire
    You're unlikely to have done any further damage to it. Try plugging a guitar in to FX return too, just in case it's a 'jammed' shorting socket ( unlikely, but a slight possibility ).
    It's pointing to the output IC as a highly likely culprit. Good news is they're cheap as chips. It may be a failed op-amp, but they're usually quite sturdy. Possible causes?, the speaker/speaker cable is damaged. If it is shorted, the output stage -may- shut down. Simple to check, just unplug it and meter it. The thermal paste hardens over time. Most ( not all ) power ICs have thermal throttling/shutdown, but in a case of runaway, all bets are off. It may have shorted at some point in the past, and that wrecked it. Dry joints are always a possibility, as are cracked tracks. There may be a capacitor or two around the output stage. Worth checking.
    While you're in there, reflow just about every joint you can find.

    Usual caveat about being careful in there. When unplugged from the mains, you may not have the voltages of a valve amp, but power supply smoothing caps can still pack a heck of a punch.

    Good luck in your hunting.
     
    Wally likes this.
  3. LuckyJinx

    LuckyJinx TDPRI Member

    Posts:
    72
    Joined:
    Apr 2, 2011
    Location:
    Portsmouth, Shire of Hamp
    Good to know, thanks. I tried plugging into the FX return, but it didn't do anything. Worth a try, though.
    Thanks for the pointers, I'll start by checking the cable and the output circuit and generally hunt around with the meter.

    I should probably search around for a schematic for the amp. I'm planning to leave it unplugged a bit before I take the first look inside.

    Thanks!
     
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