Vintage amp, no sound, looking for suggestions to get it going

Discussion in 'Electronics & Electrics' started by badmofo, Oct 28, 2018.

  1. badmofo

    badmofo Member

    Oct 10, 2011
    Hi guys,

    Not sure where to post this exactly so will start here. My brother-in-law knows I fix old PC's so has presented me with an "opportunity" to fix one of his mate's old guitar amps which recently stopped working. Details are scarce but from what I'm told they were messing around with the voltage selector around the time it stopped working - why anyone would do that is beyond me but there you have it.

    So it powers up - lights come on and I can here very low static from the speaker. I've tried some basic things like checking the fuses (all look good), plugging in a speaker to the external speaker out, tapping things randomly with a screwdriver (you never know your luck). The caps look good too BTW.

    So there's life in it, but the level of static doesn't change at all when you crank up either of the volume knobs - actually none of the knobs have any impact on the static, and I'd expect these 40 year old pots to have some crackle in them at least. And there's zero crackle when you plug things into the input jacks - I don't have an electric guitar but have tried a microphone and my iphone playing tunes.

    Any thoughts? My totally noob guess is that they've partially killed the PSU with their dicking around with the voltage selector - again, WTF would you touch that?

    Pic attached if that helps.

    Attached Files:

  2. paulvk

    paulvk Member

    Mar 10, 2010
    Last edited: Oct 28, 2018
  3. merlin13

    merlin13 Member

    Apr 12, 2003
    back in BrizVegas
    What, the mains input selector?? If so then anything/everything related to the power supply stage is chronically suspect to start with. Have you not poked around with a multimeter yet?

    Get onto the Service Manual.

    Guitar player. Step up from a Drummer, which is a step up from a Roady... 'nuff said.

    Oh, for future reference - probably not a good idea to plug expensive audio items into mains-powered audio gear that may have had the arse blown out of it without some serious multimeter diagnosis first... had to autopsy more than one bit of additional gear that had swallowed a grenade where nasty Bities had shorted back to the actual input and/or output sockets.
  4. elvis

    elvis Old school old fool

    Jun 27, 2001
    That's what I'd be doing first. Check voltages to see if they're in spec. Maybe you've blown one rail but not another. As always, be careful around the 240V stuff.

    From there, googling your model and "common fault" typically yields a winner for me. I haven't had a single amp ever that died of something uncommon, and sometimes I even get lazy and just replace the part that 99% of owners have issues with, and it all comes good without testing.

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