[Collins] Need help -- 30K-1 downward modulation

[Dr.Gerald Johnson]

Some possibilities:

Not enough RF drive to fill out the peaks. Remember they are at 4 times 
the carrier power.

Not enough emission in the PA tube(s) for the PA to fill out the peaks. PA 
plate current doubles at positive modulation peaks.

PA loaded too heavily.

Asymmetrical modulation tubes you already checked by swapping 
them. I've seen a week "up" tube lead to zero positive modulation and 
lots of negative modulation. The BC transmitter sounded horrible since 
the operator was monitoring positive modulation on the modulation 
monitor. And was running about 300% negative modulation.

Asymmetrical audio drive probably from low emission from the audio 
driver tube or upset bias from a leaking coupling capacitor. Could 
come from any stage in the audio side. Replace ALL those black 
beauties!

73, Jerry, K0CQ, Technical Advisor to the CRA

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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.