• Onkyo M-508 Repair

    01/31/2015 at 15:03 0 comments

    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. Since I was nearby and heard the event happen (while it was off no-less), we removed the offending cap and restarted the unit.

    It lasted about 30 seconds before the lack of a positive voltage rail and inevitable leakage forced the output stage into saturation. Ka-Boom!

    Many many years later, I pulled it back out to finish the repairs.

    1. Begged original schematics from a support center

    2. Traced the error and replaced all blown transistors

    3. restarted it and enjoyed about 2 minutes of musical perfection (after the calibration sequence was completed)

    4. BOOM! Dead again... what's up this time?

    ... Well.. It was made in the 80s.. and has electrolytic caps all over the design.. take a wild guess! Yep.. every electrolytic was dry as a bone.

    Starting over..

    1. replace every single cap in the unit!

    2. replace all blown transistors

    3. I'll let you know how it goes.

    Pictures to follow.