Korg - Poly 61 +/- 15v Power supply over voltage
Sharing some technical notes after a successful repair of Korg Poly 61 that had cooked a bunch of op-amps on its synth board, thanks to a power supply that was way over voltage.
Background : The synth arrived with only one of it’s 6 voices audible, but it was badly distorted and feint. LEDs on the synth board indicated notes were being triggered but only one in six made a sound.
Power supply inspection showed that the 0.0033uF spark arrestor that is connected on the AC input had cracked open, which is a relatively common problem with these old supplies. A modern replacement was fitted.
Next check was the power supply DC output voltages, and while the +5v appeared to be OK, the +15v and -15v were both way off, closer to +/- 26V so I immediately shut off the power and disconnected the supply from the rest of the synth to prevent anything downstream getting cooked.
The only circuit diagram I could find in a well worn scan of the Korg Poly 61 service manual is almost illegible and unfortunately has a few errors. While troubleshooting I cleaned it up and corrected a mistake in the circuit around pin 5 of the M5230L, by checking it against the PCB.
After a bit of head scratching, I checked the 3 pin connectors that attach the power transistors to the board, and noticed the +15 and -15 had been swapped. It’s an easy mistake to make as they are both 3 pin plugs and sit next to each other. After double checking against the circuit, I plugged them in correctly and the voltages immediately normalized. Using VR1 I set it to approximately 15v before replacing all the electrolytic capacitors and reflowing the solder joints. With the transistors plugged in correctly, and some new caps it was easy enough to adjust the voltage to the +15v spec.
The more challenging part of the repair was the damage caused by the overvoltage to the synth board, with all 6 of the JRC4560 op amps on the voice outputs fried, plus one of the TL072 in the voice 3 DCO circuit and the 13600 in the filter stage.
A few hours work to fit sockets and replace the chips, but thankfully the other boards escaped undamaged.
Anyway, there is a bit more work to do yet but most of the heavy lifting has been done. I hope that some of the technical information I have compiled during this repair will be useful to others. The power supply circuit isn’t perfect but it’s way better than anything else I could find.
