[Milsurplus] BC-348 Conversions

[w7qho]

I wholeheartedly agree with Robert and others; if truly "mint" condition leave it that way.   As such it's a better investment than the stock market these days.  If you do decide to operate it, count on having to replace many (all is better) of the black, moulded paper caps inside the set.  Micas and the bathtub types can go bad too and in some cases the APC trimmers in the RF and LO boxes can fail (recently had to do a wholesale replacement of these in a P model).  But, after you fix all this stuff it won't really be "mint" any more.  As others have mentioned, better to do a rescue job on one of the many previously  "improved" examples floating around.

So, having said the above I would like to offer up a few BC-348 improvements I've found to be worthwhile.  First off, the dynamotors have a finite operating life and are hard to find so install an AC supply if the radio is to be used regularly.  Next, the 348 was designed for headphone listening and is somewhat short on audio gain for speaker use.  See ER #103, Nov. 1997 for a nifty two-diode second detector circuit that largely overcomes this shortcoming without drilling any holes in vintage aluminum.  Finally, the models using the double ended front-end tubes (6K7) were (oddly) designed to reduce the RF and IF gain when the BFO is turned on.  The original design also provides far too much BFO injection further desensitizing the radio.  A significant improvement in CW and SSB reception can be achieved by removing resistor 
43-4 (10K), adding  200K in series with resistor 47 (68K), and removing the gimmick cap between the BFO and the IF amp. sections of the 6F7 second IF stage. 

More detail info available on request.
--
Dennis D.  W7QHO
Glendale, CA
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