Onkyo M-508 Repair

Restoring an imposingly beautiful 500W/Channel Power Amplifier

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The Onkyo M-508 is undoubtedly one of the coolest looking power amplifiers out there.

Giant needles of watt-o-meter.. imposing heavy as lead stance on the shelf. Everything about this amplifier says "turn the volume down before you hit the power." It has impressive THD specs and pumps 200WRMS and 500W peak into 8ohms each channel!

Unfortunately, the engineers who did the PCB layout were obviously working for corporate penny pinchers. To save some trace, they used 4-terminal power capacitors (unkeyed) and used one in particular to make a jumper connection on the 1 layer PCB.

At very high power, over the course of many years of dedicated neighborhood irritation, this cap would heatup due to the copper loss through the connection.

Eventually it burst, and spewed yummy electrolyte all over the board. And so begins a multi-state multi-decade saga of repair.

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AZYON wrote 08/06/2018 at 16:27 point

Did you finish working on this guy? Just got a mint 508 myself, with woodpanels and all. 75USD, lol. Thinking of re-capping it and stuff, so wouldn't mind a log if you had one:)

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Ted wrote 07/23/2020 at 15:34 point

Hi Azyon, yeah it's a beaut.  I've had to shelve this for a long time, but renewed interest from co-workers especially has be thinking about another attempt. 
Mine had damage from the cap, it blew.. and in subsequent attempts to bring it back i didn't realize that the input JFET had been zenered.  This causes a really subtle failure mode where it's OK at the bias-point but if you push the power (50W peak in my case) it breaks down and the bias declares WWIII.

My co-worker recommended toying with it while having a 100W light bulb in series with the line supply.  That way if the bias goes nuts, it will just turn on the light bulb and not have enough juice to blow anything up (hopefully).

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Ted wrote 02/05/2015 at 14:26 point

I had it running over the weekend. Also blew it to bits again. Build log is coming soon

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