Yamaha CS50 #1251 Part 2: closer, much closer

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.

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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

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