[R-390] AGC Troubleshooting

[gary.biasini at shaw.ca]

Cecil,

Thanks for your message.  The radio is new to me but most of the "paper" caps appear to have already been replaced with mylar caps - the yellow "IC" (I think Illinois capacitor) caps.  The ones that weren't replaced on the IF module also look like mylar caps.  They are "Jaro MEF" (C504, C505, C517, C521, C533, C534), 1 Aerovox (C528)  and a metal jacketed C538 with no brand name.  A couple appear to have been replaced by brown drops or dipped Mica (C528 and C545).

I think that the 3 paper caps in the RF module are also Jaro MEF but I haven't confirmed, just by memory.

I guess I'll have to test some.

Thanks again.

-----Original Message-----
From: Cecil acuff <chacuff at gmail.com> 
Sent: Tuesday, March 12, 2024 10:55 AM
To: gary.biasini at shaw.ca
Cc: r-390 at mailman.qth.net
Subject: Re: [R-390] AGC Troubleshooting

It’s been my standard practice to replace all paper caps in the “A” model radio.  You may be able to find the single culprit but next week, or month there will be another. I would just bite the. Bullet and replace them all!

Cecil

Sent from my iPhone

> On Mar 11, 2024, at 5:44 PM, gary.biasini at shaw.ca wrote:
> 
> 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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