[MRCA] Working on the R-392

[David Olean]

Hello again!

I have been restoring a few Collins R-392 receivers. The first two units 
were not much trouble. All required tweaking the PTO and alignments. One 
had a difficult problem that was traced to a gear cutting into the main 
harness and breaking wires!  R-392 number three, though, has taxed my 
brain to the point where I have totally confused myself and have turned 
into a babbling idiot, banging my head against the wall and drooling 
uncontrollably.

The basic problem was an inoperative AGC system. I also was hearing some 
crackling  noise coming from the RF amplifier circuits that indicated a 
failing part somewhere in the RF signal chain.  This particular unit was 
not employing 12AU7s, in V601 and V-602, but had small diodes stuck in 
the tube sockets instead.  At one point, I had the thing working, but it 
was short lived. So my problem was intermittent.  Measuring negative AGC 
voltages at J-614 (the black test point) showed very little change 
between weak or strong signals. strong AM BC stations sounded very 
distorted. I looked around the AGC amplifier and saw that it was 
amplifying just fine. I saw no leaky caps in the AGC buss that might 
bring down the AGC levels.  Being a closed loop system, it was difficult 
to figure out where the problem was.  I had better luck poking around 
trying to ferret out the intermittent problem. At one point I suspected 
the third IF stage. I dug into the IF strip and checked each capacitor. 
They were all Sprague Vitamin-Q hermetically sealed caps.  None of them 
checked bad. I saw one cap that indicated it had some leakage. When I 
removed it and tested it again, it checked fine. I replaced it anyway.  
My capacitor checker is new to me and cost $12. I am not sure that I 
trust it. It does test for ESR. I reverted to my old method of applying 
a variable high voltage to one lead and measuring any DC feedthru on the 
other lead with my Fluke high impedance DVM. All the Vitamin-Qs had 
absolutely no leakage.

One area to suspect would be the coupling caps from the plate of the AGC 
amplifier, C-630 and C-632 that connect to the rectifiers.  They were 
both fine.  Another suspect cap would be C-638 in the cathode side of 
the two AGC rectifiers. I had the correct voltages here depending on 
whether I used AGC ON or AGC OFF settings:  5 volts, or 25 volts on the 
diodes, so I know the cap was not shorted, but I placed another 0.01 cap 
across it and the AGC voltage climbed a bit. Maybe it had lost its value 
and was not directing the RF signal to ground?  I removed the part. It 
checked fine. (?) A new capacitor there helped slightly.

I also had a strange symptom going on with the RF level meter. As I 
would tune the KC dial through a strong signal, the meter would climb 
very high, then drop down as the signal was centered in the passband. It 
would climb again as I continued and the signal started to exit the 
passband. I would see a corresponding change to the AGC voltage in step 
with this.  For the life of me, I could not imagine what was going on to 
cause this.  At this point I was drooling and banging my head against 
the nearest hard surface. I also had a small pile of perfectly good 
Vitamin-Q caps on my bench.

At some point, as previously mentioned, I noticed some intermittent but 
horrendous noise appearing in the signal path. It was being generated 
inside the radio. It sounded like a noisy capacitor. I unplugged the IF 
input BNC connector and the noise went away. It was definitely in the RF 
stages.  After a short while, the noises stopped. It was only then that 
I noticed that the AGC problem and the RF noise was only on one band! I 
changed bands from 1-2 MHz and went up to 5 MHz and listened to WTTW on 
5.085. Lo and behold the audio was not distorted and the intermittent 
internal noise was gone too. Interestingly, I was seeing similar 
sensitivities and meter levels on both bands. Receiver gain had not 
changed much.  Further checking on 0.500-to 1.000 MHz also showed no 
noise, so my noise problem is confined to just the 1-2 MHz band coils. 
For some reason this is also affecting the AGC just on that band. AGC 
voltages at J-614 are much higher on all other bands than I am seeing on 
1-2 MHz.

I immediately stopped drooling and banging my head against hard 
objects.  Now I know where to look! I can isolate the offending part by 
using the E test points and a sensitive O-scope. I celebrated my good 
fortune and set things aside for awhile.

Incidentally, I wanted to paint my three cases with a nice gloss OD 
color as they were a bit scratched up and worn. One was really bad. 
Being a member of the US Army for a few years, I was aware of the army 
culture. One phrase often heard was..." If it moves, salute it!  If it 
does not move, paint it!"  This bad R-392 case had been painted multiple 
times with so many different OD hues that it was a mess.  The paint was 
just glommed on and very thick. I ended up stripping it and sanding it 
down to bare aluminum. I primed it with some gray primer and let it sit 
for a week. Then I used my new HVLP paint sprayer system and applied 
some special formula Sherwin Williams paint. The following is the 
formula that I ended up with. It is still a bit dark and needs to be 
lightened up to look  a bit more green.

Sherwin Williams INT EXT Architectural oil based. Gloss

Olive Green Equipment

BAC Colorant          32 OZ

W1 White                    5

B1 Black                    59

R2 Maroon                 3

Y3 Deep Gold          18

Y1 Yellow                    7

I really am liking the R-392. It is very sensitive and makes a great AM 
BC radio. I can hear a CW beatnote with a -140 dBm input signal!

I fixed up the last of the three  392 PTOs. I had built a PTO test 
fixture for the 75A4 and 51J4 type PTOs.  It has a nice vernier dial 
with gear reduction so that I can measure the linearity quite easily. I 
did rework my 51J and 75A4 PTOs and it worked just fine.  had to modify 
my test fixture again when I started working on R-390s and R-390As. The 
mounting arrangement was different and I needed two DC voltages.  Then I 
got to the R-392. Yikes! It had a very weird mounting arrangement. Look 
Ma, no screws at all!  There was no easy way to mount it in my fixture, 
so I winged it by removing the PTO from the radio, and placing a black 
dot on the oldham coupler with a small magic marker. Then I would crank 
it around for ten turns and see how close or far off I was. Of course 
there was all sorts of parallax in trying to line up the black dots. I 
would sit there and squint and guess at how well things were tracking. 
It was amazing that I got them pretty close and within about 300 Hz in 
all three cases. I also bought a $5 PTO on E Pay from K1ANX just to play 
with. It was advertised as being DOA, but repairable.   The RF cables 
were cut on it, but I was surprised to see that it actually worked when 
I applied voltage to it. I was also able to reset the end points so it 
is within 300 Hz as well. So now I have a spare!

Anyway, I am sure there are many stories about screwed up AGC circuits 
in the R-392.  I am also looking for a carrier meter. I see plenty of 
R-390 VU meters on EBay and elsewhere, , but never the 17 ohm 1 ma 
carrier meter. I sure could use one on one of the R-392s.

73

Dave K1WHS