Yamaha CS-50 power supply issues

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evilxsystems
 
Posts: 406
Joined: Thu Jul 07, 2005 10:23 pm

Yamaha CS-50 power supply issues

Post by evilxsystems »

I recently bought a dead Yamaha CS-50 that was having power supply issues. It was blowing fuses, which I tracked down to a leaky capacitor that shorted the bridge rectifier... now after fixing that, I'm having issues with the -15 volt and -15S volt power supply. The -15 supply is giving me -28V and the -15S V is giving me +16 Volts. I don't really understand the power supply circuit once it gets to the dual op amp.
I'm wondering if the OP amp is bad? There's a board layout here:
http://www.cs80.com/cs50.html/CS50_powe ... layout.gif schematic here (on page 22)
http://www.evilx.com/cs-50servicemanualpart2.pdf
I'm reading -15 volts being fed properly into pin 4 of the op amp,
but by the time it's getting to the trimmer to set the exact voltage,
it's +16V, I'm also wondering if either Tr4 is bad or the zener diode
coming off pin 5 of the op amp or maybe one of the two power
transistors (2sa745 and 2sd203) for the 15 volt section are bad...not
sure where to go from here...other than replacing everything. Which might be tough given the thing is 32 years old...

-jason

LostTime
 
Posts: 43
Joined: Sat Mar 08, 2008 9:14 am

Re: Yamaha CS-50 power supply issues

Post by LostTime »

Just a little question:
Have you measured the voltage with a small load?
Maybe the volate drops to the right point not until a load is connected.

Just a thought :wink:

evilxsystems
 
Posts: 406
Joined: Thu Jul 07, 2005 10:23 pm

Re: Yamaha CS-50 power supply issues

Post by evilxsystems »

that's a good point. While looking through a CS-80 calibration guide I found that the +15S and +15 source and -15S and -15 need to be shorted to do the adjustment on the PSU. When I do this I can now get the power supply to output the proper values. However when I hook it up the rest of the synth...i only read roughly 1.3V and -0.7V coming from the power supply. If something was shorting the +15 V supply to ground in the synth section would that explain it? or is the power supply bad?

Unfortunately this thing is a sea of wires, if it is a short in the synth its not the power supply rails themseleves, they don't appear to be shorting, so it must be on one of the voice or control boards, which means desoldering about 32 wires on 10 different boards...BANNED 1976.

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oystein
 
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Joined: Wed May 27, 2015 11:01 am

Re: Yamaha CS-50 power supply issues

Post by oystein »

I see this is an old thread but did you get anywhere with the CS50? I'm having problems with the -15v rail on mine, so I'm trying to figure it out. Replaced tr4 tr5 tr6 4558 and the electrolytics so far.

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