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Ampeg PF500 class D section repair

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  • Ampeg PF500 class D section repair

    I picked up this amp for next to nothing mostly to get more experience with SMPS and class D amp repair. I had an SVT7 Pro but was never able to get the kinks worked out of that one and sold it off as a fixer to get my money back. This one there is really nothing to lose.

    When I first got it the fault light was on fully, and the fan did not come on. Probing voltages I found the low voltages that come from the U2 switcher to be present, but not the +/-82V high voltage rails. U5 was not switching. Was trying to decipher the different protection modes but wasn't having much luck, so I decided to just check for shorted diodes or transistors. Found the output FETs Q10,Q11 to be dead shorts. Pulled those out and powered up and the fan comes on and the 82V rails come up. so some progress.

    With no FETs installed I checked and U1 is not putting out any output. I don't see a mechanism that would disable U1 from the FETs being removed, but perhaps am missing something. The voltage on the CSD of U1 is odd, varying from 2-6V or so, going up and down every few seconds. The output from U3 555 timer is low, so that should not be causing the CSD voltage to vary. From my reading of the datasheet for U1 the CSD will float up to a working voltage unless it is pulled down externally to cause a shutdown (It's not my favorite datasheet as far as clarity).

    The DC voltages I measure on U1 (relative to chassis ground) are:
    1 (VDD): 10.2V
    2 (CSD): 2-6V varying
    3 (IN): 0.78V
    4 (VSS): 0
    5 (NC): 0
    6 (VREF): -78.1V
    7 (OCSET): -79.8V
    8 (DT): -83.2V
    9 (COM): -83.2V
    10 (LO): -83.2V
    11 (VCC): -69.6V
    12 (NC): 0
    13 (VS): 0.91V
    14 (HO): 0.91V
    15 (VB): 16V
    16 (CHS): 3.3V

    Any thoughts? Is U1 toast? I check Q7, Q18, Q22, Q23 and none of them check as shorted. I do note the VB (pin 15) is reading 15.1V more than VS (pin 13) and the datasheet calls out a min VS of VB-15V.

    schematic: PF500 MAIN POWER SUPPLY SECTION 2034544-XX-REVJ03-SCH.pdf
    U1 datasheet: https://www.infineon.com/dgdl/Infine...535676143e2799

    Thanks in advance,
    Greg
    Last edited by glebert; 03-08-2020, 05:31 PM.

  • #2
    Are you using a meter? This is a digital amp, it runs at high speed. Your meter won't respond to 50kHz or such. Scope this. Pin 2 might be a varying duty cycle signal your meter interprets as a voltage. Output Pins 10 and 14 could have a digital signal.
    Education is what you're left with after you have forgotten what you have learned.

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    • #4
      Originally posted by Enzo View Post
      Are you using a meter? This is a digital amp, it runs at high speed. Your meter won't respond to 50kHz or such. Scope this. Pin 2 might be a varying duty cycle signal your meter interprets as a voltage. Output Pins 10 and 14 could have a digital signal.
      Yes, used a scope to verify no switching. Using DMM to measure voltages. The varying voltage at pin 2 is slowly varying using either.

      edit: went and added pin names to the pin numbers to make it easier to follow, especially since so many of the datasheet values are referenced to some other value.
      Last edited by glebert; 03-08-2020, 05:32 PM.

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      • #5
        Doing a bit of research, I have found others who said that the IRS20957 may not start without a load. I don't think it is a load issue in this case though, with the other components on the outputs of the IRS chip in this amp. The other issue might be that I think the output of the IRS chip is used to clock the PWM conversion of the audio signal at U16B. In probing the audio in to the IRS chip it is not being correctly converted. Maybe the IRS will shut down if the PWM input is essentially missing?

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        • #6
          "The other issue might be that I think the output of the IRS chip is used to clock the PWM conversion of the audio signal at U16B"
          As far as I know, that is simply the feedback path.

          Also, Pin #3 is an audio input.
          Internally it triggers the PMW of the IC.

          I would replace the Ic.

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          • #7
            Originally posted by Jazz P Bass View Post
            As far as I know, that is simply the feedback path.
            Since the feedback is before the inductor that signal is going to be full rail switched, so it can't be just negative feedback of the audio.

            Also, Pin #3 is an audio input.
            Internally it triggers the PMW of the IC.
            That is what I used to think, but the application note for the IRS chip says it is PWM external to the chip. https://www.infineon.com/dgdl/an-114...53559a256f10e3

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            • #8
              Probed the output of U1 when powering up to see if there was any switching going on but it seems pretty dead. Will order a new one with the FETs, and some of that chip quick stuff. Haven't used it before, hope it works, but since this was a Hail Mary anyway it is all good.

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