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Thread: Hartke LH500 blowing fuses

  1. #1
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    Hartke LH500 blowing fuses

    A friend of mine brought an LH500 which started blowing fuses after it fell during rehearsal. I'm more into tube amps and have little experience with SS amps so bear with with this one.
    When disconnected from the main PCB the rest seems to be working OK and amp doesn't blow the fuse so it looks like a power amp problem.
    The speaker output measured open circuit. All transistor on top of the heatsink measured ~1 Ohm between all 3 legs. I lifted only one of them from the PCB and it measured OK. The SA1837 on the heatsink measured short circuit between emitter and collector.
    At this point I'm wondering if I should measure all of the transistors individually?
    Any advice would be appreciated.

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    A schematic would be handy.

    Output transistors are paralleled so a short in one means they will all measure shorted on that side with a DMM, or near-shorted due to the fractional ohm emitter resistors. A shorted transistor can take out one or more driver components depending how it fails. I power up a fuse-blower with no load connected (a bulb limiter is handy if you don't have a variac) and monitor the output voltage at the speaker output. If I'm seeing the amp with a hard + or - DC voltage it's usually the output transistors on one side. If no DC either its shorting rail-to-rail or a PSU issue.

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