American Audio VLP1500 Distorting on one channel

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  • dicky96
    Sun Seeker
    • Mar 2017
    • 1825
    • Spain
    #1

    American Audio VLP1500 Distorting on one channel

    Hi guys I have a VLP1500 amplifier here - one channels works fine and the other channel distorts even at low volume settings

    Protection and Clip LED are off

    There is something wrong with the BIAS voltages around T16 and T18 (in particular) on the bad channel

    I tested all diodes and transistors in this area in circuit with diode mode - both channels seem identical. Same in resistance mode across all the resistors here.

    The output DC voltages on pin1 and pin8 of the 5532D dual op amp at the input of each amplifier module are the same, and the audio waveform looks OK here, it goes wrong after that. Playing some audio I can see on my scope that the waveform around T11 and T12 is not 'centered' on the bad channel, it is larger in the negative direction - on the good channel it is symmetrical. I also tried swapping the amplifier modules over, the fault is definitely on the amplifier, not front panel, rear panel or wiring to it.

    The yellow trimmer pots are both good and set the same on both channels. The white trimmer pots are good and set slightly different on each channel. I tried setting the bad channel white trimmer pot same as the good one but it did not help so I set it back again.

    Also I hear a click on the speaker if I put my meter probe on base of T16 or T18 on the good channel but only on the base of T16 on the bad channel.

    Any suggestions and does anyone have a schematic?

    See pics and voltage readings
    Thanks

    Richard
    Attached Files
    Last edited by dicky96; 07-13-2022, 05:31 AM. Reason: correction and more info
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    • dicky96
      Sun Seeker
      • Mar 2017
      • 1825
      • Spain
      #2
      Re: American Audio VLP1500 Distorting on one channel

      Not sure why no one could advise on this one as I gave a lot of info in detail but no worries I fixed it anyway

      If anyone is interested in the cause of the fault here you go

      https://www.youtube.com/watch?v=NnS4irCLq5c
      Follow me on YouTube
      ------------------
      Learn Electronics Repair
      https://www.youtube.com/channel/UCFX...R8UZ2vg/videos

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