[R-390] AGC voltage issue

[Larry H]

Hi Ryan,  Sounds like you solved 1 of your problems.  Great.

As for the overall gain, the pot r519 should not need to be at max - something wrong if so.  Check gain in the 1kc position (no filter).  If its better, look at the filters, check 2, 4, 8, and 16.  If all bandwidth positions are about the same, then the filters are probably ok.

In ref to c553, there are some .01's you should not use.  See the discussion in this list in August by Charles Steinmetz.  Allied and Mouser carry a good one 715P10356KD3 for a few $.  Or I could send you one.

Also, the test cable connectors are available at Fair Radio and other places - no need to cannibalize anything good.

Regards, Larry


On Sunday, September 14, 2014 11:46 AM, Ryan Scott <n7qj.rs at gmail.com> wrote:
 


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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