Calibrating and tuning a CS50 (and I assume the other nearest siblings) is an iterative process. There are pitfalls at every step, and when you finally reach the end you get a nice easter egg -a tuned 4 (or more) voice synth! In the last post I quickly diagnosed why my CS50 didn’t make a sound -having the TU (tuning) voltage jumped to ground. This post is about other problems. The CS50 circuitry has a lot of functions that are distributed among a couple of boards, so you may find you have a tuning problem originating in your SUB board, or a portamento problem originating from your PRA board. Well, I had a TU problem coming from my SUB board – no matter what I tried, I got 6.5 volts at the TU terminal -trim pots had no effect… this terminal is supposed to be 4.000 volts. Lots of head scratching and component replacement later I decided to just order a CS50 SUB board from synthparts.com -a handy side effect of CS50 voice boards being worth $400 – $500 each, making the CS50 worth about 2x it’s ‘complete working’ value in parts is that CS50 specific parts are CHEAP. I got a complete front panel and lower chassis in like new condition for $150 while I was at it. Too much money probably, but not bad really if you just want to fix your cheaply obtained synth that just happens to be worth more as parts.
Part way through a power supply recap. I am told the small value green caps don’t really tend to fail, and to leave them alone. After the recap it’s a good idea to look at the rail voltages again just in case. Continue reading