CS50 – CS for CraftsmanShip

After all those plastic synths and heavy metal profile construction with no chance to operate in disassembled state this Yamaha CS50 was a real pleasure to work on. Nice condition and just a minor fault: the external modulation input did not work anymore. There is not much in between the input jack and the modulation source switch – just an OP amp and a capacitor, and unfortunately no protection resistor. This means the OP amp is blown when a high level signal is applied to the modulation input with its level pot turned fully clockwise. After replacing the OP amp and adding a series resistor to protect its input the external modulation was back again.