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Old 25th May 2019
Lives for gear

You may have more luck in the synth subforum.

Since it's working on poly I (mostly) but in Poly II you only get sound when multiple keys are pressed, that would indicate that you have some voice chips that aren't working.
Are they in sockets or soldered? Check the solder joints and/or socket connection.

Put the synth into test mode by turning it on while holding the "Key Transpose" button. Turn the sustain up on the amp envelope and turn the LPF all the way up and turn the noise all the way up.
Press the Poly I and Poly II buttons together.
Now, when you press a key, the display will tell you what voice chip is being played. Press any key six times to run through all the voices. The ones where you get no sound will be your culprits.

My guess would be the first few chips aren't working properly. Are they replacement clones or original rebuilt?