Question about Electro-Harmonix 12ay7 Tube Microphone Preamp

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A big hum of 100 c/s in the output,i need the schematic for repair

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You must be on 50Hz power. You don't really need a schematic to repair the unit. Since the hum is twice power line frequency, the likely cause is the electrolytic filter cap for the high voltage has failed. The cap should be obvious in the unit. It will be near rectifier diodes and have a voltage rating above 50 volts. To test, bridge the one in there with another cap observing the correct polarity. If hum changes, replace the original cap.

Posted on May 25, 2011


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