[R-390] AGC voltage issue

[Craig Heaton]

Ryan,

Could it be the big 2uF cap on the IF deck? YES is my answer. If you have a
capacitor analyzer, test that cap at rated voltage! My wager says the
insulation resistance is about zip, nada, zero. I haven't found a good one
yet. One replacement is a NTE mylar cap 2uF MLR205K630. 

Question: Does the IF gain change (-7VDC @ 150 microvolts;455KC) with this
drop in audio????

Craig,

-----Original Message-----
From: R-390 [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Ryan Scott
Sent: Sunday, September 14, 2014 11:46 AM
To: R-390 at mailman.qth.net
Subject: [R-390] AGC voltage issue

Greetings to the group.

Having been a lurker for the last 7 years I am finally getting around to
working on the 12 or so R-390a's I have.  I have one R-390a that I use as a
test bed for testing the rebuilt AF and IF modules I have worked on.

The original IF module in my test bed works fine, (EAC '64 which I did
replace all brown beauty caps).  I recently completed working on an Amelco
IF module which I replaced all the capacitors (brown beauty) but also found
many of the resistors had increased in value well beyond the 10% tolerance
so I replaced those.  So after all that work, I place the module in the test
bed and it seems to work fine for about 10 min.  Then the audio drops out.
Still have reception, gain and RF gain make some difference in audio, and
BFO works.

Subbing several tubes have not solved the problem.

Here's what odd.  From a cold start, the IF deck works good.  If you move
the function switch to standby and back to AGC, the audio drops out (again
still have reception) and the AGC voltage drops to ~200mV.  When the radio
is cold, the AGC voltage is 2V..So why does moving the function switch kill
the audio??  Could it be the big 2uF cap on the IF deck?  Slow/med/fast AGC
do not change anything, same with MGC.

I'm certain it's the IF module since swapping the test bed IF module back
into the radio the problem goes away.

I have triple checked the parts I installed and R5's tube sockets and
tightened some of the grounding points.  The IF deck is using a 12BH7 in
place of RT-150 but since that is for the BFO and PTO I don't see how that
can be an issue.

I'm going to start looking in the archives and the Y2K R3 manual (Thanks to
all who wrote that!) for clues, but in the meantime has anyone seen this
when moving function switch from AGC to STBY and back to AGC?


Thanks and Regards,
Ryan
N7QJ

--
Ryan P. Scott
n7qj at arrl.net
503-880-1214
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