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Yamaha CS-10 Noise source and External input not working

Posted: Tue Sep 18, 2018 7:56 pm
by bionary
Hello,
I ordered a CS-10 from a person in Japan, I live in Michigan. When the synth arrived, everything worked except the Noise source and the External signal in. I have all the setting set properly on the front panel to access either source. The user said that everything worked properly before they shipped it. Does anyone have any idea where I could start troubleshooting this problem?

Re: Yamaha CS-10 Noise source and External input not working

Posted: Tue Sep 18, 2018 9:49 pm
by Mooger5
Had a look at the schematic. Three probable culprits come to mind:
A 68K resistor in the VCO board leading to the wiper of the Ext/Noise Level potentiometer, the potentiometer itself, and the Ext/Noise selector switch.
Oddly, the components in the serv manual have no reference number, just the value, but the resistor should be easy to locate.

There is a test point, TP5, but is of limited usefulness.

Another cause would be a loose wire, due to the journey the synth went through:
The signal wire carrying the Ext/Noise to the VCF also carries the the VCO outputs, so if the waveforms can be heard then the wire is OK.
A wires from the power supply to the extenal input preamplifier and noise circuit opamps should be verified, if the opamps have their own power busses.

In case the 68K resistor is the offending part, replace with one with same ohmic value but higher wattage tolerance.

Good luck.