Korg Poly-800 - not turning on, need some help with repairing

Synth Repair
, ,
1

[KORG [Poly-800] – [doesn’t turn on]

I have Poly-800 (bought for funny price) in very good condition. It won’t turn on. I’m electronic engineer and checked all power supply section (for me no issues). Here’s what checked:

+5V voltage on C100 (present)

-5V voltage on C11 (present)

+5V power supply (between Vcc and Vss) on IC24 (80C85A) and most of important IC’s is ok.

It seems to be ok from power supply side. Voltage is tracing from PIN11 of CN6A to PIN10 and then regulated to 5V. Some transistors (as Q1, Q3) are warm but not very hot. No fried diodes (as D2 or D24..). Replaced some 100uF/16V capacitors to new.

I checked that pin no. 36 of processor is still at HIGH state. Do you think it should be microcontroller reset problem? The reset should come from PST518 (IC40) to terminal no. 36. Should I try to short RESET IN and Vss of 80C85 for a short while?

Maybe it’s ROM memory problem? Should the synth at least turn display on even if there’s ROM issue? There’s 27C64 8KByte. Does anybody dug deeper in Poly-800 that only power circuit?

2

If you have a CPU reset problem, I would expect the symptom you describe. Typically everything needs to be running for the display to come up with “11P”. ROM issue would absolutely cause this, CPU or RAM issue. Do those two chips have good power and ground? You kind of need to diagnose this from the CPU “outwards”, not easy because it’s fairly complicated machine. I had a machine where someone modified it to use a CR2032 memory battery instead of the alkalines but did it badly and when the battery died the synth didn’t boot because they had cut the trace for Vcc to the RAM.