[Collins] 75A-4 AGC

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Besides the RF gain control, an IF tube, especially the S-meter driver,
with grid emission will kill off positive going s-meter indication and
counteract AGC. Often the meter will go negative. Leaky AGC bypass
capacitors contribute to lack of AGC action. A failing S-meter zero pot
will also kill the S-meter indication.

Question might be, "is it AGC or indicator?" Do strong signals sound
clipped? Then its a loss of AGC, like leaky AGC time constant and bypass
capacitors, or a bad AGC detector tube. If strong signals don't distort,
then the S-meter circuit is suspect. The S-meter uses an IF tube as a DC
VTVM to show AGC voltage as signal strength. The zero adjust, the IF
gain adjust, and grid emission from that tube can affect the indication.

73, Jerry, K0CQ, Technical Adviser to the CRA

Contents copyright 2005, Dr. Gerald N. Johnson