These synths have a set of overtones (5 as I recall) that can be adjusted for the timbral base and a simple analog subtractive circuit over that. I'll study the diagram when I get time.
Only problems with mine were contact cleaning related as I recall.
I stock remanufactured knobs for these machines! Slider or rotating small knobs 3.75 each large $5 each
I have a variety of parts that might help you on these as it appears Roland used many of the same parts those years. And also there are some discrete components that I usually stock.
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