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Technical Tips from Mark W. Persons
Repair of Moseley PCL-606 STL Receiver

Here is a Moseley PCL-606 STL (Studio-Transmitter-Link) receiver from the mid-1980's.  On the bench because it quit working, you can see one module pulled up for service.  Fortunately, the front-panel multi-meter switch led me to LO1, the first local oscillator.  The normal reading is mid-scale, but this one read zero upon arrival.   
The problem turned out to be C19, an 18 pf variable capacitor, used to tune the first frequency doubler on the 940-960 MHz 1st L.O. (local oscillator) module.  Moving this control slightly restored normal operation.  Well, that might work for a while, but certainly is not a permanent cure.  I have seen this happen before.  Replacing the controls was the right option.  Apparently the insulation/plastic dielectric between capacitor plates fails and shorts the capacitor. 

If you run into this, I recommend replacing all 18 pf capacitors on this board with the nearest current industry equivalent, 22 pf.  There are two similar capacitors in the IF on the 70-3 Double Converter board, which should also be replaced to prevent future problems.       

This is one of the original 18 pf capacitors.  As you can see, the control shaft is corroding.  The capacitor plates are suffering the same fate as well.      

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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