[R-390] The R-392 saga

Tue Dec 24 20:41:37 EST 2019

Dave, Thanks for letting us know about the humor and problems you found in
your 392's. I was not in the Army, but assume that 3 letter acronym was in
reference to it.  Are you going to leave it in there?

Nice work on the repairs.  I've heard about the liquid tape, but have not
tried it.  Where did you get yours?

My 1st encounter with low voltage plate tubes was when my Mother got a 1960
Chevy.  The factory radio in it had 12 volt plate tubes in it.  Before
this, I know about sub-miniature hearing aid tubes that would operate on
22.5 volts.  The lowest voltage miniature tubes were 45 volt.  I was just
learning about electronics and did not think it was possible.  Well, come
to find out that is why they short out easily.  Are the 25 volt plate tubes
more rugged?  I hope so.

As for overloading, I don't know if that is normal or not - I would think
not, but do not know.  Perhaps the agc is not working quite right.  Or the
IF gain is set too high.  Is the overloading causing audio distortion due
to the 5th or 6th IF from being over driven?

You sure did fix a lot of problems.  Good going.

Regards, Larry

On Mon, Dec 23, 2019 at 4:20 PM David Olean <k1whs at metrocast.net> wrote:

> 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
>
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