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Repair and disassembly guides for guitar amplifiers.

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Harmony H535 amp blows fuse

I have a Harmony 535 guitar amp that instantly blows the fuse with all the tubes pulled and the amp still in stand by.

There is no sparking or arcing and no popping sound. I need to know how to test the filters and both transformers please. I do have a wiring schematic for this amp.

I have 35 years of experience fixing Major Appliances, I use a Digital Multimeter and I am well qualified on safely tests on live circuits. I have made a dim-bulb test so I can test with out blowing the fuse

Answer this question I have this problem too

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I will buy the amp if you are interested in selling it.

=== Update (03/30/2020) ===

Willing to buy


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1 Answer

Before you start repair, discharge all capacitors in the amp because some may have DC 100V+ stored in them.

Test all the capacitors and diodes (if any) to see if any of them has shorted.

If that doesn’t do it you may have to do a “live” test.

For example, first disconnect all the outputs of the power supply transformer and turn on the amp. Since transformers rarely go bad, I expect the fuse to stay intact. Then you can reconnect the rectifying circuits back to the transformer and disconnect the output of the rectifying circuit, and test again. And continue adding more of the circuit back until the fuse is blown. Since you have the schematic this shouldn’t be too hard. Just be sure to unplug the amp and discharge the capacitors before you touch anything else.

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Don Thompson will be eternally grateful.
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