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

[Mike A]

On Thu, Feb 26, 2015 at 01:56:20PM -0500, Bill Abate wrote:
> 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.

That's a lovely job of problem determination, OM. First the hub-to-gear joint,
and then the audio bug. I know not what course others may take, but as for me,
give me more stuff like this!

-- 
Mike Andrews, W5EGO
mikea at mikea.ath.cx
Tired old sysadmin