[R-390] Gain drift

[hamfish at comcast.net]

Dave,

Several years ago I chased a similar ghost(s) in my "Too Loud Amelco". Never
have found the audio issue, it has to be in the wiring harness. Your 'gain
drift' could be in multiple IF & RF transformers. Take the metal cover off
of every IF and RF transformer. Look for wire touching the cover! I found
several. In Z503 a single strand of litz wire was touching, real hard to
see. Make sure all the other ailments of R390/A's have been addressed,
replace caps, no leaky AGC line caps, etc. Proper alignment, then chase bugs
& ghost.

This receiver was almost unused, no green screws had been touched, original
tubes, no mods, etc. Not impressed with the quality of Amelco, but any
R390/A can be fixed given enough time, money, effort. My thought are this
was a receiver to be used in a military training class. It had lots of bugs
that would not have passed any inspection. 

Best of Luck,
Craig

-----Original Message-----
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> On
Behalf Of dog
Sent: Saturday, October 13, 2018 7:29 AM
To: r-390 at mailman.qth.net
Subject: [R-390] Gain drift

Not sure how to explain it except 'gain drift'. I thought it was dirty 2nd
Osc. bandswitch, but don't think so now. When I turn on the 390A with no
antenna on it, let it warm up so I can get some audio noise out of the diode
load to a small SS amp, then let it sit for a while listening to the white
noise, after about 5 minutes or less, the audio drops rather rapidly way
down and I mean to the point where I have to turn up the SS amp volume a
whole lot. Then if I do just about anything to interrupt the receiver, like
turn to STBY, or even sometimes MGC, or rock the Bandswitch, the volume pops
right back up. But none of these things are definitive to correct the
attenuation, it can be any of them. 
It's almost like the gain through the RX had been attenuated somewhere and
any interruption or change in the signal makes the attenuation go away.
Tried switching tubes, RF and IF, haven't tried the PTO. Even just hooking
up the antenna will do the same thing. Now if I have the antenna hooked up,
it never seems to do this. I've tried poking around the different modules,
tubes, etc when the volume drops and I just don't seem to be able to figure
out where this attenuation is occurring or why. Any help here? I'm kinda
shooting in the dark.

Dave N3DT
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