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Technical Tips from Mark W. Persons
Hum/Buzz in RF Preamplifier

An Advanced Receiver Research P900VDG 960 MHz STL RF Preamplifier system that needed maintenance.  These are excellent amplifiers with GaAsFET transistors for very low noise figures in the 0.7 dB range.  They are probably better than any STL receiver front end made today.  It makes sense to use one of these between an STL receive antenna and an STL receiver whenever the signal level is low. 

This one exhibited a rather interesting problem.  It introduced a hum and buzz in the receiver's audio.  Since the STL receiver detects a frequency modulated signal, you have to wonder how it could happen.

The STL transmitter and receiver were temporarily replaced as the first part of the troubleshooting exercise, but they were found to be OK.  That is when it was discovered the preamplifier was the real problem.  It was turning on and off at a 120 Hz rate.       

The culprit turned out to be the wall-wart power supply, which had high 120 Hz ripple.  The power supply had near-zero ripple on the 12 VDC output when it was first installed.  An internal electrolytic capacitor, used for filtering, failed with time allowing huge variations in voltage at a 120 Hz rate. 

An oscilloscope confirmed the problem. 

Repair consisted of replacing the power supply with a new one and adding a 1000 mfd electrolytic capacitor across the preamplifier power input terminals for added insurance.

All is well that ends 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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