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

[Barry]

----- Original Message -----
From: "Chuck Curran" <ccurran at wi.rr.com>
To: <r-390 at mailman.qth.net>
Sent: Saturday, February 11, 2006 7:05 PM
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
>
Chuck,

I think there are ways to do this without a counter, but I've used a counter
for mine.  You can pick up a signal with a loop around the oscillator tube
or tie to the plate circuit through a cap.  If you think you might be off by
as much as 13kc, this would get you a *lot* nearer to where you should be
with it.

I think Roger has a "radio-only" procedure to set this, but I don't know it
offhand.

Good luck,

Barry - N4BUQ