Another RCA CT-100 discovery B8001599

[Penthode]

Originally Posted by dtvmcdonald

2. AGC problem.

I had the exact same thing. Simple cure: since you are running on a
local modulator, signal is too strong. Install a pad right at the set terminals. I have a cheapie balun followed by a balanced 300 ohm pi-pad made from 1/2 watt resistors. That instantly cured this infuriating problem. Signal to noise is still essentially perfect. I recall that my uncle, in 1954, had the exact
same problem ...

That was it! I attenuated the RF by using a 6 way splitter and that fixed the problem. I remain curious why the AGC latches? Obviously the heavy overload which prevents the keyer from generating a negative voltage but is fine once recovered. I see that an additional 0.1 ufd following a 680kohm resistor feeds AGC beyond the IF line. This means upon the sudden application of higher than normal RF, the IF AGC line will react faster than the RF AGC because of the additional low pass filter. Meaning the RF amplifier may go into heavy overload and not recover.

Removing the 4.7 Mohm AGC delay resistor alleviates the problem at the expense of lowered S/N. (As if that is a problem nowadays). Perhaps lifting the 0.1 ufd would fix this?

I am not going to bother since simply attenuating the signal was the fix. Very appreciative of this tidbit of information.

And than to both of you for the 15GP22 socket tip.