Yamaha Electone E-Series (PASS)

Just wanted to see if anyone on this forum has any experience repairing/servicing any of the E-series Yamaha Electone organs from the late 70’s. I won’t go into great detail to start, but I’ll just say that mine has problems with the synth voices. I’m finally at a point where I understand the architecture of how it works, but I’m finding that it’s extremely time consuming if not impossible to continue troubleshooting (because of how the digital boards are installed).