[R-390] Further AGC measurements

[Craig Heaton]

Alan,

A couple of questions, sort or wondering if the cart is in front of the
horse? Going back thru this thread on the R390 e-mail reflector & AM Forum,
have you replaced the BBOD's & electrolytic caps yet in the entire receiver?
Have all the switches been cleaned? There are so many of those little
got-ya's which should be addressed first. Have all the variable IF & RF
transformers been removed/cleaned so there is good pin contact? 

I'm rather new playing with these R-390/A's, it takes me about two weeks of
cleaning, replacing old caps, etc., before any mechanical or electrical
alignment. After which comes all the bugs to track down. (poor grounds, bad
pots, leaky caps, more out of speck resistors)

Should the module pull & mainframe breakdown come first?

Craig,
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-----Original Message-----
From: R-390 [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Alan Victor
Sent: Tuesday, October 07, 2014 6:54 PM
To: r-390 at mailman.qth.net
Subject: [R-390] Further AGC measurements

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
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