[R-390] Further AGC measurements

[Alan Victor]

I completed some more AGC measurements and have some clues.
However, bottom line, ready for a module pull and mainframe
breakdown. For the record though, I found the following:

Monitor the AGC TP3-4 points with VTVM. That AGC voltage tracks
AGC feed to the RF amp, and the three mixer grid control AGC voltages
within 0.2 V. Still
the AGC value always low even on large RF input of 150 uV/30%
modulation.

First a careful Z measurement again on TP 3 shows ~ 380K. Clearly
less than the desired 500 K. Possible issue here.

Next, look at all the series grid R values from RF amp and
all mixers. The values are all on target, except for the RF
amp, its 50 K to large! Measured over 500 K. Possible issue here.

Finally, the most interesting measurement, with the AGC loop
open. Function switch is set for the AGC ON, MEDIUM time constant.
The AGC control line is fed via an external power supply with series 10K
R to pin4. The RF input level is 150 uV/30% AM mod, now I can
easily control the AGC line so that -7 V (AGAIN AGC OPEN LOOP)
is generated at TP3 and the carrier meter comes alive.
Larger input RF level can readily drive the AGC line pin3
to over -12 V.

So the AGC circuit (OPEN LOOP) is functioning. Clearly, with the AGC loop
closed it is not. Either the AGC source Z is not nearly low enough to
source the required current (includes leakage) or the leakage which is
present is larger than desired for proper AGC action.

I think it would be helpful to measure the actual power supply current
when this experiment is run. That would confirm that the current required
by the AGC loop closed is just to high to support a functioning AGC.

Comments?


Alan