[R-390] AGC Troubleshooting

[gary.biasini at shaw.ca]

David,

 

Thanks for the advice.  Unfortunately, work took over my time and today I
leave the location of the radio in question for 3 - 4 weeks so any further
troubleshooting will have to await my return.  Too bad I just can't take the
radio on the plane with me as carry on luggage!

 

Thanks again.

 

From: David Wise <d44617665 at hotmail.com> 
Sent: Monday, March 11, 2024 7:54 PM
To: gary.biasini at shaw.ca; R-390 at mailman.qth.net
Subject: Re: [R-390] AGC Troubleshooting

 

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 <mailto:r-390-bounces at mailman.qth.net>
<r-390-bounces at mailman.qth.net <mailto:r-390-bounces at mailman.qth.net> > on
behalf of gary.biasini at shaw.ca <mailto:gary.biasini at shaw.ca>
<gary.biasini at shaw.ca <mailto:gary.biasini at shaw.ca> >
Sent: Monday, March 11, 2024 3:44 PM
To: R-390 at mailman.qth.net <mailto:R-390 at mailman.qth.net>
<R-390 at mailman.qth.net <mailto: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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