Yamaha CS-50 Great until plugging in wrong pedal

Synth Repair
1

I bought a Yamaha CS-50 in 1978 that’s worked flawlessly since. Several years ago I accidentally plugged other synth’s sustain pedal into the CS-50 volume pedal input and it immediate stopped working. No sign of life. Any ideas how this can be fixed?

Also, it’s lost several of the colored plastic pot sliders caps. Are these available for sale anywhere?

Thanks,

Tom

2

Only thing I can think off is the LED in the opto-coupler opening the path - driven from IC3. When you plugged in some other pedal you may have caused a short to the -15V that also holds the output of IC3 hight to light the LED (see below block diagrma and schematic of same area)
So - I think - check PSU first

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3

Thanks very much for the help. Pardon but the explanation is over my head – though I generally get it.

It sounds like you are recommending I check the power supply first, not the ICs or anything else. I don’t know how to check a power supply if it’s anything more complicated than a fuse. I’m a software guy. Would I need some sort of meter to measure the juice? If it is the power supply would it need changing or is repair to PSU possible?

4

Hmmm … I’m afraid you may need at least a meter (DMM ) to start with - sorry no fuses in that path and yes the Power Supply is repairable.

If you look at below circuit of the PSU - TR7 transistor can supply 6A. I do however suspect that if there indeed is no -15V then that 0.47Ohm resistor may have blown - may even be a fusible resistor and its purpose may be exactly that - to blow/fuse on high current or short circuit.

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5

Thanks for the detailed explanation. It sounds like my mission is to find someone local who I can share your info with and step through the repair!

Cheers,

Tom

6

It turned out to be a simple fuse. My son and I opened it up. He noticed a bag of little parts. To my surprise Yamaha has spare fuses in a stapled bag stored within the unit. Unbelievable. I checked the 3 fuses and one was blown. We replaced it and it’s 1978 all over again!

Time to watch Blade Runner for Yamaha CS-80 inspiration.

1 Like
7

GREAT WORK - looks like that short took the main fuse out - interesting !

8

Thanks for the confidence boost I needed to give it a try!