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Technical Tips from Mark W. Persons
Circuit Research Labs Audio Processing Repair

Audio processing of all types comes into the M. W. Persons repair facility frequently.  Here is a CRL Circuit Research Labs) SGC-800 Stereo Gain Controller on the bench.  We usually change all of the electrolytic capacitors on these because they age and cause problems.

If you look carefully at the oscilloscope, the waveform is a somewhat distorted sine wave.

A closer look at the scope confirms that. 
A scope probe is looking at one of the audio outputs.  The problem turned out to be a shorted 1N4742A (12 Volt/1 Watt) zener diode in one of the audio inputs at the rear of the processor.  In this case, there are a pair of Zener diodes from each input line to ground.  They are used to shunt voltage transients to ground.  Those voltage spikes can come from static electricity or lightning.  The diodes sometimes fail in the process of protecting integrated circuits in the processor.    
Removed from the processor and sitting on a solder-wik container, the Zener diodes are just 5/16 inches long plus the leads at each end.  The cathodes are tied together so they limit the audio input to 12.6 volts on each audio peak in both directions on each audio input and audio output line. 

The stories go on and on.  Stop in again sometime.  I'll leave the soldering iron on for you. 
Mark W. Persons   ham W0MH      

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