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Bose Professional 1800 Power Amp Help

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    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

    #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.

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      #3
      Re: Bose Professional 1800 Power Amp Help

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

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        #4
        Re: Bose Professional 1800 Power Amp Help

        there are suggestions in the manual for oscillation

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