[R-390] BFO Problem/Question for a newly restored R-390A

[Chuck Curran]

Hello All:

 

In April of 2005 I received a R-390A from an Uncle who had decided to hang
up his radio activities at the age of 85.  I had given him a 75A-3 about 18
years ago, which he traded for this particular R-390A.  Upon deciding to
clean out his basement, I became first on the list to get the Capehart 1961
vintage R-390A.  My lucky day!

 

This particular radio came complete , all covers present and in very, very
good physical condition.  I started by gathering all information, printing
out full sized schematics and then diving in.  It had never been touched
since it's departure from the military, as near as I could tell.  Seven
tubes were so bad, my Hickok almost threw them back at me.  I re-capped the
unit, carefully cleaned all gears with a 100% disassembly.  Yea, I messed
up, even pulled out the Geneva, and that took a bit to get re-synced.  After
picking up a HP606A signal generator, I was able to properly re-align the
electrical end of the radio, and all seemed pretty good, except that darn
BFO!  That Heath IG-102 just couldn't handle the task.

 

The main problem now is that the BFO would kick in, and then weaken and drop
out within 10-20 seconds.  Sometimes it did not work at all, and I had to
flip the switch repeatedly to get it to kick in.  O.K., today I decided to
determine what had happened with the BFO circuit.

 

I first checked the resistances on V505 comparing all to the manual values,
most were "close" to nominal values, but off enough to cause a strong
interest, especially on pin 5.  I then removed the IF deck and started to
dig in a bit deeper.  Upon checking individual resistances I quickly found
that R530, the 22K plate resistor for this Hartley oscillator, had changed
from a correct value of 22K to 33K.  That did not seem to be good, so I
installed a new  22K resistor.  OK, many of you know how crowded things are
in that part of the 390A.  I needed to remove the flexible shaft coupling to
access this area around the V505 tube socket.  I successfully replaced this
resistor, using many chunks of solder wick, and then re-installed the IF
deck for a test.

 

I fired up the receiver, and then tried to zero out the calibration signal
at 3.000 MHz- no audio tone at all was heard, but a strong Calibration
signal was still present, at least displayed on the meter.  I putzed around,
a bit puzzled, since I had not even heard the normal 10 second long "weak"
BFO signal.  Where was it?  I spun the dial with the BFO still on and got
hammered at 2.987 MHz with a very strong signal.  I turned off the BFO and
it disappeared.  Tried the Off/On routine and was able to convince myself I
was actually controlling an off frequency BFO signal.

 

Prior to removing the flexible coupling, I had marked the BFO sealed unit
shaft with a black magic marker dot.  I "assumed" I could run that puppy
back to the same spot  and all would be well.  Wrong, I believe now that I
failed..  During the flexible coupler removal, I realized there were
actually four Bristol set screws, two on either end spaced about 110 degrees
apart - gee, why not 90, was this a manufacturing error for this coupling?
Ah, minor detail.  I ended up twisting and cussing, and removed the coupler,
without being sure how much the shaft had rotated.  I cleverly told myself
that it was surely like a variable cap, with a 360 degree rotation pattern
that would just repeat itself.  Maybe not?

 

Here is my question, sorry for the long diatribe.  Should I start turning
the BFO shaft until I achieve a tone at the expected points?  Is this shaft
on a threaded adjustment, with 3, 4, or 5 turns possible so I can achieve
this?  

 

As an alternative, I thought I could try to sample this BFO oscillator
output with my frequency counter, and see where it is.  I think I can do
this using a tube test socket and just sample the signal off the available
exposed V505 plate contact point.  If I am actually off by 13 KHz, I would
want to "try" to turn the shaft to come back to the correct 455 KHz
frequency for the center point of the BFO oscillator.   If this shaft just
has a 360 degree adjustment, then I guess I might have a problem, possibly
with the inductor and three caps controlling the oscillator base frequency.
Not likely, since it was on frequency prior to my messing with it.

 

Being totally and 100% ignorant of the internals of that sealed BFO can, I
hope someone may have learned the construction features and can offer some
advice.  Is it a multi-turn adjustment, or just a 360 degree affair?

 

In advance, thanks for any comments on this situation,

 

Chuck WA9POU

Cedarburg, WI