[Milsurplus] The R-392 saga

[David Olean]

I have been working on a few R-392 receivers and found some interesting 
things.  I think I already mentioned finding the message inside of a 
disgruntled repair tech from the Vietnam era. That was pretty funny.  
That "F.T.A." unit had so many things wrong with it, I was wondering if 
I ever would get it finished. Well, today, I think it is done. The last 
problem was having the crystal calibrator being on all the time. 
Whenever I would dig into the radio, the problem would disappear only to 
come back as soon as I tried to put things back together.  This radio 
had some serious mechanical problems with tuning racks as well. Two of 
the racks would jam, while the 2nd variable IF rack was missing. I found 
replacement parts and fixed the 2nd variable IF (3-2 MHz) only to find 
that the 4-8 MHz RF coil rack jammed along with the 8-16 MHz rack. The 
4-8 problem was that the width of the channel that the rack slides up 
and down in was only 0.261" wide, while the sliding part of the rack was 
also 0.261" in diameter. It would get stuck. The 8-16 MHz guide was 
0.264 wide. All the others were 0.267 or so. I wonder if the chassis had 
been slightly bent due to rough usage.

My solution was to get some fine emery cloth and stretch it over a thin 
flat metal bar. Then I could slide the emery cloth up and down on the 
rack guide opening to enlarge it slightly while maintaining a flat and 
smooth surface for the guide to work with. The 4-8 rack started working 
great once I got the width to about 0.264 or 0.265". The next rack still 
was hanging up and I finally figured out (after enlarging it so the rack 
would slide freely, that someone had the wrong spring installed there. 
It looked OK, but had way too little force when stretched. Thank 
goodness I had a junker RF deck to pick new parts off.  A new spring 
fixed it right up.

Once the racks all worked fine, I did a final alignment of all the coils 
and noted that the calibrator still was always on! I suspected a bad 
bypass cap in the cathodes of the calibrate circuit, but everything 
checked out OK there.  When I looked back at the mode switch that turned 
on the calibrator, it worked fine. I then removed the upper deck assy 
with AF, IF and calibrator chassis and powered it up on the bench 
without the rest of the radio. I used some bench supplies to power 
filaments and the 28 VDC "high" voltage. It all worked fine. The 
ocillator came on only when I grounded the proper pin in the connector. 
I think it was Pin 4. When I put it back together, it quit and looked 
shorted out now all the time. I dug around and determined that the cable 
harness and 20 pin plug were at fault. I took the plug apart, but all 
looked great inside there. That left the harness all by itself!  Upon 
close examination, I found that a previous repairer had positioned the 
harness too close to the gear train at some point, and the gear teeth 
had chewed through the outer plastic wrap on the cable and actually had 
dug into two shielded wires, cutting the outer shield as well as the 
insulation inside to expose the center conductor of the shielded wire. 
It happened a long time ago, as I could see green copper oxides from 
corroded flakes of copper that stained the harness.  This was all hidden 
on the underside of the harness and was almost invisible. My solution 
was to carefully pick out the two damaged wires and remove any wisps of 
braid from around the bare wires. It took me a few attempts, but 
eventually, I got it so that any shorting action was gone. Then I sealed 
everything up again with liquid electrical tape. This is a black plastic 
coating that dries to a glossy rubbery consistency and will keep any 
wisps of braid away from the center conductors. From there, it was a 
simple matter to rebolt the harness down and put the front panel back on 
the radio.  I had no desire to change out the wires in the harness. I 
think the liquid trape idea will work out OK. I have used it before 
where harnesses have been burned by errant soldering irons. I can make 
my own insulation over the exposed wires!

I swapped out a bunch of GE 0.1 MFD caps in the audio section. They were 
all leaky and making noise. Now the R-392 IS HOT on all the bands. I can 
easily pick out a weak -140 dBm signal on CW, so it is very sensitive. I 
did notice that it gets overloaded by strong BC stations at night. If I 
listen on 1.8 MHz for ham signals, I can hear all sorts of BC band 
artifacts. It isn't too serious, but it does get overloaded. Still, it 
is a joy to listen to AM broadcasts in the 8 kHz bandwidth position.  
All three of my R-392s are early Collins Radio units from the first 
contracts.

73

Dave K1WHS