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

[Craig C. Heaton]

Chuck,

I think the BFO is a multi-turn device. Hope you have a sig-gen or access to
one, it will be needed, check out Chuck Rippel's web site, link is provided
to his IF Deck Alignment page. At the bottom of the page is the correct way
to set the BFO. His method works, I've used it.

http://www.r390a.com/html/if_deck.html


later..........craig
73's
wd8kdg

-----Original Message-----
From: r-390-bounces at mailman.qth.net
[mailto:r-390-bounces at mailman.qth.net]On Behalf Of Chuck Curran
Sent: Saturday, February 11, 2006 5:05 PM
To: r-390 at mailman.qth.net
Subject: [R-390] BFO Problem/Question for a newly restored R-390A


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







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