Bose Professional 1800 Power Amp Help

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  • Nevillet
    Badcaps Veteran
    • Sep 2019
    • 272
    • United Kingdom
    #1

    Bose Professional 1800 Power Amp Help

    Hi All,

    I'm trying to fix this 40+ year old power amplifier which works but has a couple of issues:

    When powered up without an input signal and the volume pots set to zero there is a noticeable high frequency interference coming through the speaker outputs. If I switch off the amplifier the interference disappears. The massive smoothing caps (14,000uF/100V) continue to power the amp for some time after the power is cut. I've attached the service manual, some oscilloscope screen dumps of the speaker outputs with the power on & off.







    Any pointers would be most welcome.

    Thanks.
    Attached Files
    Tags: None
    • budwich
      Badcaps Legend
      • Jul 2015
      • 3097
      • Canada
      #2
      Re: Bose Professional 1800 Power Amp Help

      there appears to be some difference between the two channels if I understand the display. since it is on both to a degree, check the voltages on both rails both for the dc value and then with a scope to see what any "ripple" looks like. Feed in a 1khz wave and follow it into the input stage and compare before and after the op amp stage.
      Last edited by budwich; 07-27-2021, 08:49 AM.

        Comment

        • Nevillet
          Badcaps Veteran
          • Sep 2019
          • 272
          • United Kingdom
          #3
          Re: Bose Professional 1800 Power Amp Help

          Many thanks for that. Once I get back to it i'll report back. Thanks

            Comment

            • petehall347
              Badcaps Legend
              • Jan 2015
              • 4426
              • United Kingdom
              #4
              Re: Bose Professional 1800 Power Amp Help

              there are suggestions in the manual for oscillation

                Comment

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