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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 10th Jul 2021, 9:42 pm   #1
samjmann
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Default Marantz PM-48. Protection circuit.

A friend asked me to look at this amp that he'd owned since the 90's. The fault was that the protection relay clicked in and out at random. This fault had slowly got worse until now when the amp would barely run at all. The midpoint on both stages was 0.3mV... perfect!
When the volume was advanced it would trip off, worse on loud passages.

All of this appears to be nothing more than 2 slightly loose grounding screws on the main board. Each of these seems to form decoupling for the protection circuit. They were both loose by about half a turn. Loosening off either screw brought the fault back.

I cannot find a circuit anywhere on the Web for this this amplifier. There are are a few rather inconclusive threads on other forums regarding this fault, others too have not found a schematic.

I've only repaired about four Marantz amps in my time, two of them have had grounding issues similar to this!!

SJM.
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Old 3rd Aug 2021, 6:14 pm   #2
vinrads
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Default Re: Marantz PM-48. Protection circuit.

That's a good one well done ,thanks for the info , Mick.
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Old 6th Aug 2021, 8:38 am   #3
John_BS
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Default Re: Marantz PM-48. Protection circuit.

Thanks for posting. Obscure fault!
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