Ampeg PF-350 low volume and other issues

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  • Camarada78
    Member
    • Jan 2021
    • 28
    • Brazil
    #1

    Ampeg PF-350 low volume and other issues

    Hello,

    i have an Ampeg Portaflex PF-350 on my bench for repair. It works but with very low volume, 1/3 of what you'd expect. Much lower than any 100-150w amp i have around.

    It has SMPS and a class D amplification. Looking inside i found two issues:


    - found R94 and R98 burned near the Q10 and Q13.
    - found that i'm gettin +80v/-80v instead of the +55v/-55v shown on schematics.

    The overvoltage is probably the reason why the R94/R98 keep burning. But i cant find what is causing it. the SMPS side seems ok. I'm getting 176v before the transformer.

    What i've done so far:

    - since Q10 and Q13 were ok, i tried beefier resistors. also burned.
    - changed Q10 and Q13 (both 33n25)

    What would make the SMSP release 80v instead of 55?

    Anyone experienced with this amp?

    ***
    I've annexed the schematics but i could not find the revision I actually have.

    Most schematics online do not have R94/R98 in series with C84 and C97 (as per annexed picture).
    Attached Files
    Tags: None
    • R_J
      Badcaps Legend
      • Jun 2012
      • 9567
      • Canada
      #2
      Re: Ampeg PF-350 low volume and other issues

      The resistors are blocked from DC voltage by the capacitors C84 and C97, for the resistors to go open, the capacitors are likely shorted.
      The +/- 55 volts is unregulated so it may show higher when the power amp section is not operating
      Last edited by R_J; 04-22-2023, 10:19 PM.

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