[ARC5] Notes and Tips: RCA AVR-20, AVT-112, AVA-126

Sun Oct 18 12:38:51 EDT 2015

I believe the AVR-20 is essentially the same as the receiver in the BC-474.
I have had AVR-20s playing in the past, but a long long time ago. I will
attempt again to get the whole combination going, maybe in about 2 years
when I intend to retire. Of course, I have a lot of other projects lined up
as well, HI. 73 - Mike 

Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731
732-886-5960 

-----Original Message-----
From: ARC5 [mailto:arc5-bounces at mailman.qth.net] On Behalf Of David Stinson
Sent: Sunday, October 18, 2015 12:34 PM
To: ARC-5
Subject: [ARC5] Notes and Tips: RCA AVR-20, AVT-112, AVA-126

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 ;-).
-----------------------------------

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
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