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Old 9th January 2013, 08:03 PM   #1
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Join Date: Jan 2006
Location: Deerfield Beach, Florida
Default Onkyo M504 Opamp upgrade

I am looking to upgrade the opamp and possibly the metal oxide resistors on the input section on my Onkyo M504 amp. The current Opamps are M5218 Mitsubishi, also they are inline 8 pin package. Attached is a schematic of the service manual. I noticed their is a mosfet of some sort on the input also. Not sure how the opamp fits in with this. Need a little help understanding this. Also where can I get a converter board for an inline 8 to DIP 8 converter? Thanks to all that can help me. This amp has always sounded "thin" in the bass, I suspect the opamp and input caps have something to do with this...
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File Type: pdf hfe_onkyo_m-504_service.pdf (689.0 KB, 57 views)
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Old 9th January 2013, 08:40 PM   #2
jaycee is offline jaycee  United Kingdom
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Join Date: Apr 2004
Location: Norwich, UK
The opamp you mention is not in the audio path - it is used as a servo. Changing it will not change the sound. The FETs form the first part of the amplifier stage. It is a bit hard to tell this because the schematic is rather untidy.

I would suggest renewing any electrolytic capacitors in an amp of this age - particularly the 8 large power supply capacitors. This could well be why you are finding the bass lacking.
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Old 9th January 2013, 08:47 PM   #3
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Join Date: Oct 2010
Location: Ojai, CA.
I agree with jaycee. It's not the servo or the input stage. The usual culprit for lack of base in "vintage" amplifier is the electrolytic capacitors. The electrolyte in the capacitor will over time degrade and even dry out. Replace them with the appropriately rated capacitors.

Nice amplifier you have when it works well. Good luck on the repair.
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Old 9th January 2013, 09:27 PM   #4
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Join Date: Jan 2006
Location: Deerfield Beach, Florida
Thanks for the input. Looking further, I notice that many of the smaller caps in the M504 are of the styrene variety. Would any of these styrene caps be benificial to an upgrade to polypropylene. Their are some smaller value electrolytics, not sure if they are in the sound path. I have a good assortment of Elna Silmic II's laying around, not sure if it would be benefit to swap any of these out.
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