Archive of the former Yahoo!Groups mailing list: Discussion about the Korg PolySix synthesizer

Hi - here's how to check the opto... http://huebnerie.de/blog/2013/07/polysix-photocoupler-myths/

"This leads to an eady way to check whether a tuning problem is related to the photocoupler circuit at all: measure the voltage on pin 7 of IC18. Within regulation it should be somewhat between -2 and -6 volts, -3 are more normal.
When the loop breaks open due to failure of the photocoupler or related parts, it will be stuck on the maximum negative output, something around -13 volts. There must always be a small current through the LED, otherwise the loop does not work and the TUNE HIGH preset has no effect!"