[R-390] Re: recent experiences (slightly OT)

[W. Li]

Things have been a bit slow, so I thought I would
share my recent experiences with a predecessor to the
R390 series: a BC-348.

Many of us cut our teeth on this aircraft command
receiver in the 60's, when they came onto the market.
It was an eight tube single conversion set with an IF
of 915KC, maybe 50,000 units manufactured, and
installed in B-24's, B-25's, and B-29's. Even the
Russians had their version.

Articles in CQ and QST at the time only alluded
obliquely to a unique perculiarity in the power
supply: that the B-minus was grounded through a choke
incorporated in the audio output transformer. Most of
the conversion schematics for an AC supply erroneously
showed B-minus directly grounded. As noted by James
Moorer, and Mike Dinolfo and others, this error led to
excessive current drain by the audio output tube, and
no negative bias voltage to the mixer.

I failed to satisfactorly rehabilitate my first BC-348
in the 60's, and since then, have been on the lookout
for a second unit. Most were either very beat up or
heavily modified by multiple previous owners. Imagine
my surprise to find a cosmetically perfect unit in our
local thrift store (among household goods!). 
Wells-Gardner BC348Q, serial number 7541, order
11415-WF-43.

The interior was exceptionally clean, with that nice
phenolic odor. 

All the tubes were JAN originals, and had
transconductances exceeding specs. Upon tracing down
the *upgrades* done by its previous owner(s), I found
the following:

1_ all the heaters been rewired correctly in parallel
and the 5 and 7 ohm series dropping resistors removed.

2_ pins 1 and 5 on SO-104 were soldered together,
shorting the output transformer secondary.

3_ the heater string was soldered to the B-minus choke
(pin 6) (!)

4_ all the leads to the 350K audio pot were cut and
transferred to another pot in the dial light dimmer
position

5_ all the resistors looked brand new, yet their
soldered joints to the boards were clearly old and
wrapped as they were at the factory.

6_ all the caps were metal-jacketed with styrene ends.
There were no waxed paper caps or plastic caps
anywhere.

7_ the huge dual oil-filled cap under the chassis
showed less than 10uA leakage at its rated voltages.

Once all of the above were dismantled and corrected, I
fired it up with a metered bench supply, and it worked
beautifully, drawing just 50mA at 200V.

My conclusion is that this particular unit never
worked as designed, and sat around for 60 some-years
in a depot, or someone's shop.

Compared to our R390's, this piece of history won't
compare in performance, but it makes a nice addition
to one's vintage collection along my ARC-5's.

W. Li
Mercer Island, WA


      ____________________________________________________________________________________Fussy? Opinionated? Impossible to please? Perfect.  Join Yahoo!'s user panel and lay it on us. http://surveylink.yahoo.com/gmrs/yahoo_panel_invite.asp?a=7