Circuit Overview:
CPU controlled analog with 'oscillators' amounting to digital pulses being driven to an op amp to kick things off. From there on the signal path is typical digital controlled analog.
Service Tips:
Many common issues with these. Aftertouch tends to go bad. Here is a page describing the problem and a sometimes fix. The feedback resistor also should be modified often in the op amp gaining up the signal from the AT sensor. I seem to recall 3x the value being used. 150K I think. Look for occasional bad solder and lots of dirty switches!
Parts:
Roland may still have some parts for these. And also there are some discrete components that I usually stock.
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