[R-390] R-390A intermittent loss of Diode Load voltage

[Jim Whartenby]

JordanHave you pulled and reseated all tubes in the RF, Crystal Oscillator and PTO?  Might be a bit of corrosion on the tube or socket pins.Regards,Jim 

Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy

-----Original Message-----
From: Jordan Arndt <Outposter30 at shaw.ca>
To: Collins R-390 HF Receiver List <r-390 at mailman.qth.net>
Sent: Thu, Feb 17, 2022 11:31 am
Subject: [R-390] R-390A intermittent loss of Diode Load voltage

Hi again group...

This one has me searching for clues and I hope someone might have 
encountered this problem, or help me narrow down what to look at next.

While this 1960 S-W 390A has had a lot of intermittent issues, I've been 
able to resolve all of them so far through testing and analysis.

This latest one however has me stumped so far.

While checking daily for intermittents by monitoring a very strong local BCB 
station, I'm seeing an occasional random drop in the Diode Load voltage and 
a drop in the Line Level Meter reading and the Local audio output, while the 
Carrier Level reading and AGC voltage level, do not change. The voltage drop 
can be from 2-3 volts to nearly 10 volts measured at the Diode Load jumper 
on the rear panel.

So far, I've replaced the IF deck, the audio deck and the power supply with 
known properly working modules, to no avail. I even replaced C101 with a new 
.22uF film cap which did not solve the problem.

What's left is the RF and crystal osc deck, and the interconnecting cables 
and thier connectors, which have all been cleaned and re-seated numerous 
times. I've also cleaned the fuses and the contact connections within the 
fuse holders because I had found contact issues therein.

What's puzzling to me is that if the problem was in the RF/xtal modules, I'd 
expect to notice a corresponding loss of gain which should appear on the 
Carrier Level meter and/or the AGC voltage, but I don't see that at all.

Any thoughts or insights as to what to check next would be greatly 
appreciated...

73...Jordan VE6ZT 

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