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Solid state preamp output problem (...I think...)

Discussion in 'Amps [BG]' started by patiopup, Jan 6, 2013.


  1. patiopup

    patiopup

    Joined:
    May 6, 2011
    Not a dedicated bassist, but I do mess around with it and I enjoy lurking on the site from time to time.

    The deal is this: I've got this SS Fender Bassman 150 that I picked up cheap used a couple of years ago. Recently I plugged back into it after not having messed around with it for a bit and had very little volume output when running through the regular input. Nothing abnormal about the sound, just not much of it, and the gain ("volume") and EQ seem to be behaving normally.

    Here's what I've done so far:

    * Bridging effects send and return with jumper: low volume.
    * Ran drum machine into line-in: normal volume and normally responsive to master volume control.
    * Ran multi-fx unit, looper, and drum machine into effects return: low volume on all counts.
    * Ran bass into regular input and used XLR line out to another amp, post-processing: low volume.
    * Ran bass into regular input and used XLR line out to another amp, pre-processing: normal volume.
    * Removed head, looked around inside: no obvious damage or loose components.

    The way I figure it, the problem is with the level of the signal after the pre-amp (and, in fact, after the effects return). The power amp section performs fine so long as I bypass the preamp.

    Any thoughts on how I might narrow it down to something I can repair on the cheap? The amp isn't worth the cost of a service tech, but if there's something I can track down with a multimeter or by swapping out cheap components, it'd be a laugh to give it a go.

    As it is, I'm left with a powered cab that I can run with a multi-fx -- not that it sounds appreciably worse than it did with the preamp working...
     
  2. patiopup

    patiopup

    Joined:
    May 6, 2011
    Had a quick thought after typing that up, and tried one more thing: I linked the effects send to the line in, and, voila: all loud again, with full access to the EQ section, compressor, etc. Since I don't use the effects loop, this makes it a fully functional amp again...

    Would still be interested in a proper fix if anyone has a thought.
     
  3. BassmanPaul

    BassmanPaul Gold Supporting Member

    Joined:
    Aug 25, 2007
    Location:
    Toronto Ontario Canada
    You need to get a schematic for the unit. That way you can identify the points where things work and the points where they don't. You'll should then be able to identify the area where you have to look.
     

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