Name: Steve Kennedy
Email: steve@pacair.com
Subject: RD112-100 - PCB GD-2A Problem
Thread: 18
Time: Wed, 24 Jan 2001 05:09:48 GMT
*** These repair and troubleshooting should be left to professional technicians and repair personnel. Hazardous Voltages are found inside Music Man amplifiers!! ***
My first guess would be a bad or "leaky" CMOS switch (4053 or 4016 chip) used to select the distortion mode. Does the settings of the secondary ("Lead") Gain and Volume knobs effect the "leakage" you year when attempting to play cleanly at low levels? If so, it is probably IC-8 (4053)that switches the Gain controls or IC-9 (4016) that switches the Volume controls.
My second guess would be dried-out filter capacitors. It is possible that sympathetic distortions are being generated in the bad filter caps and passed along to every stage of the amplifier through the power supply connections. By using an oscilloscope you can see if there is excessive ripple (hum) greater than a few hundred milivolts.
Is the problem any different between low and high power?
My third guess would be a crystallized solder joint or cracked resistor. By making the problem audible (radio or CD music playing through the amp at low volume), you can use a small wooden stick (NOT a pencil) to push, probe and poke at the internal circuitry elements to see if any physical contact, thunping or pressure makes the problem worse or makes it disappear.
My fourth choice would be a bad coupling capacitor. By putting a substitute cap in parallel across a bad cap the problem will be lowered or eliminated.
My fifth guess would be microphonic output tubes. This is the least-likely cause but it can happen. With the volume cranked up but no input, does gently tapping the tubes cause any distorted noises in the speaker.
Of course, I must EMPHATICALLY WARN YOU that the Music Man amplifiers can have voltages exceeding 750 volts inside!!! This can easily KILL YOU!
If you are not trained or qualified to do your own repairs, I would highly suggest taking it to a professional amp repair person.
As always, I would suggest tht you get some tuner cleaner (to clean all the pots) and some good contact enhancer/cleaner (like Cramolin Red or De-Oxit from Caig Laboratories) to treat all your input and output jacks to make sure that oxidized and corroded contacts are not a part of the problem. Do this FIRST.
Steve