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Old 19th July 2009, 04:00 PM   #1
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Default Trying to save my Onkyo 504 amp

I just joined this forum hoping someone can help me with my 10 year old M 504 power amp.

A week or so after a lighting strike near my house, I turned on my seldom used stereo system and it was silent. After checking it out a bit I determined the only problem seemed to be in the power amp. All the other components were fine, the 504 amp was lit up and looked OK, but had no output. I have since noticed that the "servo" light on the front of the amp is not lit. It appears that the "protection circuit" actuated and then failed to reset. There is no evidence in the amp of anything being burned or otherwise damaged.

Any ideas on the cost or difficulty of fixing it? To me it's a cool lookng amp that I have enjoyed over the years. I hate to just throw if away if it can be fixed for maybe !00 dollars or so?
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Old 19th July 2009, 08:46 PM   #2
djoffe is offline djoffe  United States
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Default a start...

at the minimum, you will need a schematic or service manual...

here's a site that had something close, I think, but you'll have to look further for the actual schematic...Now, if the 5060 was anything close, you had better know what you're doing, or take it to someone who does...

http://www.audio-circuit.dk/Schemati...060-pwr-sm.pdf
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Old 20th July 2009, 03:57 AM   #3
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As I recall, the M504 and 508 are very similar.
http://www.audio-circuit.dk/Schemati...508-pwr-sm.pdf

The M504 is a very good amp, however it is known for it's problems with the large PSU caps, which tents to blow due to the fact that their working voltage is only app. 5 volts above the actual rail voltage.

The reason why the protection circuit won't reset could be caused by serious DC voltage on the output. You have to check this before proceeding with any other repair try.
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Old 4th September 2009, 04:21 PM   #4
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Default Onkyo M-504 Amplifier

I read a post of yours regarding problems with the servo circuit on an M-504 Onkyo. My meters are also pegged. There does not appear to be any voltage present on the amplifier output. If I ground the output of the protection circuit, the speaker relays pull in and the unit will operate normally, but the meters are still pegged. Any suggestions ? Thanks, Pete
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