[Boatanchors] BC-342 thermal problem re-visited. (long post)

[Philip Atchley]

Hi All,
Well, after "false alarms" a couple times I believe that I've FINALLY repaired
the thermal problem in this BC-342.

Problem:  Outside of the normal drift the set would be fine and stay right
were I calibrated it.  It would hold WWV at 10 or 15MC for hours on end
OUTSIDE of the cabinet.  Put it in the cabinet and it would be fine for
awhile.  Once it got good and hot it would all of a sudden jump frequency
approximately 100 KC higher at 10MC  and the lower bands, especially the
1500-3000 KC band would go totally berserk.

False alarm fix #1:  I "assumed" the problem was in the LO as that is the
frequency determining area.  I'd already replaced C-83 and R41 and was
dreading pulling the entire LO module.  Then while testing the 6C5 oscillator
tube I found it WAS erratic in the tube tester.  Resoldering it's pins cured
that and the tube was good (not unusual in Octal tubes).  Installed the tube,
checked set alignment and it was good.  Ran it on the bench, heating the LO
compartment to VERY warm, no problems.  Put receiver back in it's cabinet and
installed it back into listening post #1.

The set ran for about an hour and went BELLY UP again, same problem!

Back on the bench I pulled it out of the cabinet and ran it for about 3 hours,
no problems.  It apparently doesn't get hot enough out of the cabinet.

THEN A LIGHTBULB LIT UP IN MY MIND.

I went and got the XYL's hair dryer.  Now, this isn't an ordinary hair dryer,
it's a behemoth that sounds like a B-52 taking off and blows about as much hot
air as one too! (She got it through the beauty supply store)  It brought the
LO compartment up to red hot status very quickly, no problems showed up!

I let the radio cool down again, turned the hair dryer on the RF deck with the
3 tubes (two RF amps and Mixer).  In about 3 seconds the radio went belly up
with the original frequency jumping problem.  Wobbling the mixer tube made it
come and go.  Aha, bad tube pins?  I resoldered them and turned the radio on.
Radio playing fine.  Wobbled tube.  Radio playing fine.  Gave it another shot
of hot air from the behemoth, in a couple seconds the radio went belly up
again.  Hmm.

At this point I pulled the 3 tubes, pulled the RF deck up where I could see
under it. The only mixer related parts there is capacitor C47, a 100 mmF mica
(pF weren't yet invented in 1943) and resistor R13, a 50K grid resistor.
Resistor checked good.  I replace both the capacitor and resistor just to be
on the safe side.  I then re-assemble the RF deck and tubes and test the
radio.

Voila!  It works!  I give it a breath of air from the behemoth, no effect, it
works!  Calibration is 'slightly' off, more than likely due to capacitor
tolerances in C47. No problem, I let the radio cook (helping it just a little
with the behemoth) covering it with a folded towel to help simulate a cabinet.
Then I did an alignment and put it back in it's cabinet.

It's now ensconced back in the Number 1 SWLing position and so far is holding
WWV perfectly!  It seems to have a little less overall drift now too!

Whatever failed in the mica capacitor it was a hard on/off thermal failure,
probably in the number of plates making contact affecting it's capacitance.

73 de Phil KO6BB
A genuine Antique SWLing from Central
California with Antique Radios.

RX 1:  Farnsworth BC-342N Military Surplus (circa 1943).
RX 2:  Hallicrafters SX-71 (circa 1955).
RX 3:  Grundig YB-300 digital (spotting set ONLY).
Antennas: 65' sloper, 1/2 size G5RV Inverted Vee,
                 15M Inverted Vee, Hustler 6BTV Vertical.