[R-390] This may be of help to someone

[Bill Abate]

After repairing and aligning (I dislike the word 'restoration') four 
other R-390A's I thought another would be no biggie.  I bought my fifth 
a few years ago at a great price and then put it on the shelf for a 
future project after doing some cleaning and degreasing.  Recently I ran 
out of 'round to's and remembered the 390A.  After a power up with no 
smoke I put in the missing tubes and connected a speaker.  Zipo! 
Nothing!  A close inspection showed that the megacycle knob did not turn 
all the related gears.  A gear clamp was broken.  Darn!  Now I had to 
mess with the gear train to get at that clamp.  Even worse, a magnifying 
glass and flashlight showed six more broken clamps!

After hours of machine work I fabricated some replacement clamps.  I 
replaced the clamps and reassembled the gear train.  Darn, the gears 
were still slipping.  Turns out the gear (#48) was slipping but the 
clamp was tight.  Removed the gear and found the hub to gear connection 
was a cold solder joint.  Yes, its called out in the military manual as 
a soldered gear (its not tin/lead solder).  Did not expect that!  I was 
worried about the heat damaging the gear if I tried to resolder it since 
it is tempered steel but I could not come up with a better idea.  I dug 
out my low temperature silver solder and oxyacetylene torch. The gear 
repair turned out well.  I did not give it a complete fillet as original 
since I wanted to keep the heating time to a minimum. The gear did warp 
slightly despite being tacked in just two places but it still was 
usable.  Evidently the gorilla who worked on the gear train before me 
kept tightening the clamps until they broke when he could not stop the 
gears from slipping.  Brawn instead of brains!

Next problem was to synchronize the gear train.  When the broken clamps 
allowed some of the gears to slip on their shafts it became a giant 
Rubik's Cube.  I never felt the need to disassemble a gear train before 
so this was all new to me.  I set the cams where they belonged at 7 +000 
but then found out the bandswitch was not synchronized.  I found the 
section on synchronizing the 6 position RF bandswitch that showed you 
get a 56K resistance between two points (page 110 of TM 11-5820-358-35) 
when at 7 +000 setting.  So I loosened the gear on the band switch shaft 
and rotated the switch shaft until I got the 56K reading and then an 
infinite reading when switched to 8 MHZ.  You must satisfy both 
requirements. When that got satisfied I reassembled the rig but I still 
could not get the radio to align.  Turns out there is more than one 
shaft position that satisfies the test.  After studying the schematic it 
turns out the easiest way to synchronize the band switch is to look at 
the rear wafer and verify B+ to the 17 MHz oscillator when in the 00 to 
07 positions.  Finally got it right. After dropping the front panel and 
removing the RF subchassis more times than I care to admit, I found I 
still could not align the radio.  I neglected to align the switch gear 
assembly (#87). I could not find explicit alignment instructions in the 
manual other than to use figure 68.  I carefully positioned the bottom 
cogs with the vertical centerline of the gear and finally got the radio 
to align properly!  It took me over a week to diagnose and fix these 
problems.  Yeah, I'm slow.

Thought I was done at this point but the audio did not sound right.  It 
passed the tests for the AF module but it just wasn't right.  Playing 
around with various settings I discovered that turning off the noise 
limiter made the audio louder and better.  HUH?  Found no B+ getting to 
the noise limiter tube plates.  Pulled the AF module and the switched 
RF-IF B+ was there but the line to the noise limiter switch was 
grounded.  Could not find anything bad in the module so I disconnected 
the multiconnector plug and the ground disappeared.  So it was in the 
wiring harness.  Figured the switch on the noise limiter was bad.  But 
it was fine.  YUK!  How do I find a short in the fully laced harness? 
Well dumb luck prevailed.  When I moved the harness in a certain 
location, the ground disappeared. Turns out it was right next to the 
PTO.  Now this might help someone.  Somebody put an extra long screw 
through the frame that holds the RF assembly in place at that location. 
  The harness is on the other side of that screw.  Sure enough the screw 
pierced the wire insulation at that point and shorted that wire to 
ground!  Replaced the screw with a shorter one and added some electrical 
tape and all is well.

Sorry this got to be so long but it might help someone else.  I could 
not find something similar on synchronization but if this has been 
covered before, my apologies.

Bill, K3PGB