[R-390] AGC Troubleshooting
David Wise
d44617665 at hotmail.com
Mon Mar 11 21:53:39 EDT 2024
Correlating a schematic to physical wiring is a critical mental skill. Get out your schematics, for example chapter 5 in Y2K-R3, and just walk along the AGC line. You can always ohm it out if you wonder if a point is part of that line. If you inject some voltage at AGC's origin in the IF Deck (with power off), you can probe various points on the far side of several resistors; if those points read the same as the near side, you eliminate that whole downstream sub-network at a stroke. Note that you must consider not only capacitors but several inductors, for example L503 and L507, crystal filter tuning and IF transformer respectively. And yes indeed, the mechanical filters. Regard C551 as bad unless proven good.
HTH,
Dave Wise
SWL (inactive)
________________________________
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of gary.biasini at shaw.ca <gary.biasini at shaw.ca>
Sent: Monday, March 11, 2024 3:44 PM
To: R-390 at mailman.qth.net <R-390 at mailman.qth.net>
Subject: [R-390] AGC Troubleshooting
I followed Charles Steinmetz's troubleshooting procedure for AGC issues and
the following resulted after tuning to a very strong BCB station:
Voltage Terminal 3 & 4: -0.37V - not good
Then step 2:
Resistance Terminal 3: 460k - maybe passable
Resistance Terminal 4: 24.6k - horribly off spec
So, my problem is most likely in "any of the several dozen bypass caps on
the AGC line in the IF and RF sections, or leakage to ground in one or more
of the mechanical filters".
Now my ignorance shows up. How do I identify what are bypass capacitors? I
assume that these are capacitors that go to ground?
I also note that, on the very strong BCB station, I get no sound output
until the RF gain is up to 8, if that helps narrow down where I should start
searching.
Thanks in advance for any help.
Regards,
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