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

[Dan Merz]

Hi,  my first  complete military radio was a 348P Stromberg Carlson that I
bought at a garage sale around 1994 for a few bucks.  It had been converted
to use with an external pwr supply and I used it with a National doghouse
and was impressed with its performance after I replaced the umpteen  black
caps that were very leaky in most cases.  I later found a dynamotor and
restored it to its original configuration.  I can't say it performed better
after the restoration,  as the dynamotor is a little noisy in itself but the
experience was 90% of the reward.  I still have it but it's mostly a
"viewing" radio.  Your experience kindled my interest in listening to it
once again.   I seem to recall that the dynamotor cost me more than the
radio.  I vaguely recall that there was one version of the BC-348 according
to a knowledgeable friend that had superior caps and not the black type,
either the Q or R model but I am unable to find notes on which one it was.
It's amazing that these radios still show up in the junk category at thrift
stores. However,  the general population has little knowledge of this kind
of stuff. My notes,  which include a complete manual, also include reference
to the biasing/B- ground location and indication that  cathode bias
resistors were appropriate if B- were attached directly to ground.   Thanks
for mentioning your experience,  Dan.   

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of W. Li
Sent: Thursday, May 24, 2007 8:37 AM
To: R-390 at mailman.qth.net
Subject: [R-390] Re: recent experiences (slightly OT)

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 

_____________________________________________________________
R-390 mailing list
Home: http://mailman.qth.net/mailman/listinfo/r-390
Help: http://mailman.qth.net/faq.htm
Post: mailto:R-390 at mailman.qth.net
Unsubscribe: http://mailman.qth.net/mailman/options/r-390