[Milsurplus] Notes and Tips: RCA AVR-20, AVT-112, AVA-126

[David Stinson]

Working with an friend of long acquaintance (blessed to
know someone smarter than me!) on ressurecting the 
popular WWII-era RCA AVR-20/AVT-112/AVA-126 
light aircraft set.

Copying some of my notes here for the one or two people
in the universe besides we two who might be interested ;-).
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On the little RCA sets:
First thing I recommend is to put the sets on the bench and
work-over every single ground point you can find. That
includes things like screws that hold tube sockets and terminal
strips, nuts that hold the IF cans, etc.
Usually, a little loosen/tighten "tweek" is all it takes. 
I'm finding this problem in just about every WWII era BA
I work with these days and suspect it will infect the 60s-70s
stuff before long. "Old age oxidation."
I had an ART-13 transmitter on the bench a few years 
ago. "Ground" from one end of the chassis to the other
measured 1300 Ohms. After going over all the ground
points, it was just a couple of Ohms.
Also had a Ranger (not Johnson) light aircraft transmitter
with no modulation. A terminal strip was grounded with 
a screw through the chassis. The screw head was pulled 
up tight against the chassis skirt and read completely 
OPEN to the chassis. One quarter turn back and
forth to break the oxidation and modulation was restored.
On the AVR-20:
I've reworked three of them. ALL the paper 
caps were bad and all the resistors over about about 
220K as well. The biggest pain is the "multi-cap" 
can right behind the tuning cap. There are a bunch
of individual caps in that can with one end tied together
to that black lead that comes out the middle to ground.
All of them inside that can are likely bad, which includes
cathode bypasses. They are potted in bee's wax.
I used a heat gun to soften it and pull out the gloop.
Restuffed minus the wax.

IF cans: Only thing in there besides the coils 
AFAIK are some 120 pFd caps. I've never had one
be bad and never had to remove one.

Pots and switches will need some De-Ox-It.
I've leached it into the front of the toggle switch around
the toggle and that got it working.

When you align the set, beware of those three "plunger" 
caps behind the 6K8 and in front of the 6S7:
C23, 24 and 25, the RF, MIXER and LO trimmers. 
They have this odd conical "compression nut" on one end 
which you must loosen to move the plungers in-and-out 
to adjust them. These are easily broken. 
Not as easily as the "egg shell" LO trimmer caps in the
ART-13, but fragile.
IMHO, I will trade a few DB of performance and/or 
a few KC of dial calibration for the sake of preservation. 
Unless the alignment is just beyond what you can tolerate, 
I'd leave them alone. If they must be tweeked, 
proceed with caution; don't force anything. 
I'm living with a cracked one now.
I foolishly tried a drop of penetrating oil on the 
threads of the remaining two, which of course
"penetrated" right into the caps, which I then had to
remove and clean. What a pain and just recompense 
for such nit-wittery.
Using the sets on 40:
The receiver in the BC-474 is, with a few changes, just
a re-packaged AVR-20. Although both the receiver and
transmitter are speced below 7 MC, by doubling in the 
transmitter PA I've worked many states on 40 CW with
the set. The receiver will cover the low end of 40 as-is, 
but I've never checked if it will go to 7160 KC. 
Also don't know if the AVT-112 will go there fundimental
or if it will require doubling as well. 
If nothing interferes (and something usually does),
I will break-out the set and see what works and what 
doesn't. If the receiver can't quit stretch, one can of 
course tweek the LO a bit. There's messing with 
those plunger caps again, LOL.

One oddity I've seen on all AVR-20s and on the 
receiver in the BC-474: When tuned all the way up to
40 meters, the output of the receiver develops a "ripple"
sound, as though the filter caps were dying in a line-
operated set. At first I wondered if this were some
sort of weird "common mode" thing, but operation from
batteries did not cure it. Makes me wonder if the LO
is "sputtering" at the high end or if the IF is trying to
break into oscillation. But then, is the ripple freq being
so close to 60 Hz coincidence? I haven't investigated
further as it's just a minor annoyance on CW. On phone,
it may be grating. I should look further, if I ever get the 
time to fix my busted scope, HI.
On the AVT-112:
Only thing beyond grounds and the obvious caps and resistors
I can think of at the moment is the fragility of the antenna
tuning assembly. I can't imagine how it would have lasted
if the set had to be retuned regularly. Be gentle.
When I get the set out, I'll make notes on tuning indications
for 50-ohm antennas and etc. The antenna switching relay
(and the ones in the power supply) 
just needed burnishing with some kraft paper soaked 
in De-Ox-it.

On the AVA-126:
Worked on three of them. Never found a single good
cap; they all were bad or started going bad shortly after
ressurection. I've also had to "kick-start"
the oxidized vibrator contacts with higher voltages.
Those took some patience and I did have to remove
one from its can to burnish the contacts, but all were
eventually recovered. I've found a couple of 0Z4 
rectifiers that were "hinky," the output swinging
in random amounts. Guess they're getting "long
in the tooth." Replaced with NOS and had no 
further problems with that.
Will let you know what I find with the set on 
the bench. All I can think of for now.

A QSO on 7160 with these sets on each end
would be cool indeed. 

73 Dave AB5S