[Milsurplus] BC-348Q Modification identification

[Ray Fantini]

Never saw anything in CQ or QST published for a universal power option, think the issue may be a shorted filter capacitor being that’s about the highest failure rate item. If done correctly the B- of the HV is held above ground by a 50 Ohm resistor so if the short is out in the radio that resistor will be damaged, if the short is in the power supply the resistor will be good, so that’s a first thing to look at.  If the transformer was properly sized for the radio it won’t run hot and destroy itself but found from experience that many Hams back in the day used transformers that often were undersized and ran hot and that can always be a potential fail point. Today’s higher line voltages are also an issue but with proper sized transformer the issue is more along the lines of having too much B+ and filament voltage and that tends to stress everything in the radio. I tend to run all the old radios on around 105 volts input with the goal of the filaments being no more than six volts.
Don’t know if it will help you or not but I have a web page that has the CQ Surplus Conversion Manual version of the AC supply for the BC-348 and you can see that at:

http://staff.salisbury.edu/~rafantini/bc348modifications.htm

Also may want to consider installing the fuse holder if it’s still there in the primary AC line with a one Ampere fuse to prevent any other future disasters. Tube type power transformers are starting to get hard to find, epically ones that will fit into the small space of the original dynamotor deck.

Thanks for bringing up the subject of BC-348 modifications! The use of that receiver in the Ham and SWL world are a important factor in their history and often overlooked by people who tend to just concentrate on its military role.

Ray F/KA3EKH




From: Milsurplus [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of Matthew Paulonis
Sent: Monday, April 03, 2017 10:48 PM
To: Milsurplus at mailman.qth.net
Subject: [Milsurplus] BC-348Q Modification identification

After following this group of distinguished and highly knowledgeable individuals, I must draw upon your vast experience to help me identify where to begin with identifying the modifications effected to a BC-348Q that was given to me a few years ago.

I have been spending my spare time both acquiring new and replacing electrolytics in my vintage gear to keep them all in one piece and operational for anothet lifetime. While busy reforming capacitors to be reinstalled in a long dormant HP bench power supply, I was listening to my 348 on the top shelf of my bench. I don't think I was out of the garage for 15 minutes, but when I returned, the garage was nearly full of smoke. From the smell of burned varnish, I could tell immediately that it was the 348.

This radio was very carefully refurbished and modified by KE0GN (SK) with great care probably back in the 50's or 60's, and used fairly regularly for the past 25 years before being passed along to me. The power supply has 2 transformers, a 12BW4 rectifier tube, assorted caps, chokes, and a 12 volt vibratory all mounted on the original dynamo to plate. The power input to this is a 9-pin "octal type" plug on the back of the radio so it was capable of being powered by either 115vac or 12vdc. Before I go about the forensics to try and locate the fault causing the "meltdown" of the large power transformer, I would like to see if this was a standard modification for which I could get a schematic so that I can try and rebuilt to its original specs. Then from this I could study and try and determine what could possibly have failed to cause the meltdown: there are no other signs of excessive heat or bulged/popped capacitors within the entire chassis with the exception of one 5 watt ceramic resistor in which one end lead had lifted off of a terminal on the underside of the power supply plate. Any ideas? My initial thoughts of possible failure points are:
a) failure/internal short of the rectifier tube;
b) overheating of the power transformer that was probably designed for 110 - 115vac, not the modern day 120;
c) a possible short within the 12v vibrator???;
d) a downright mystery.
 I have a selection of pictures of the damage if any of you care to see (and perhaps help to identify the design). I guess then I could also use suggestions for removing all of the melted varnish covering parts of the chassis.

Thank you in advance.

Matt P.
W2NS
Houston, TX
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