[R-390] R390A AGC Troubleshooting technique.

[djed1--- via R-390]

I would think the source impedance of the AGC voltage would make a big difference.  A fairly low level of leakage in an AGC capacitor would pull down the normal AGC line, but would have no effect on a regular power supply.  Maybe if you fed the PS voltage through a 1 meg resistor you'd get a better idea of any problems. (I believe the RF stage AGC line is grounded through a 1 meg resistor, so you would expect some voltage drop with the setup I've suggested.
I haven't tried this, but I slept in a Holiday Inn Express last night.

Ed W2EMN

 

 

 

-----Original Message-----
From: Alan Victor <amvictor at ncsu.edu>
To: r-390 <r-390 at mailman.qth.net>
Sent: Thu, Oct 2, 2014 8:00 pm
Subject: [R-390] R390A AGC Troubleshooting technique.


Currently in the process of troubleshooting the AGC in the
R390A. The sensitivity on all bands is about 5 uV some bands
better than 1 uV. The unit needs alignment. However, the AGC
is clearly absent. The largest negative AGC voltage seen is
about -5 V at 3-4 terminal. I will be looking at all the known documented
causes from mechanical filters to leaky caps, open xmfr's,
bad tubes and so on.
However, my question is about a troubleshooting technique
documented in the Army TM. There they suggest opening the
AGC loop and provide a negative supply to the AGC line,
terminal 4, from -10 to 0 V. If all the AGC lines to
the first RF AMP and all 3 mixers track this forced AGC line,
then those stages are probably ok.Further, they suggest without
details, that should assist in tracking down the issue
of the AGC circuits on the IF subchassis. I believe I would
monitor termnal 3 as this would be the feedback AGC voltage.
Is this correct? I see in the Y2K book, their was a request
for evaluation of a good AGC line current. That would be helpful.
Was that ever documented? Thank you reflector in advance!

Alan
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