[Boatanchors] BC-348H Recap

Mon Oct 21 21:59:36 EDT 2013

Drew,

You have heard a lot of feedback from the purist element, so let me 
give you some thoughts from the im-purist folk:

There is VERY LITTLE demand for BC-348's in pristine condition 
compared to Collins or other high end gear, which negates any 
delusion about restoring a junker to museum condition. Given the 
missing major components (dynamotor, AF output transformer, etc) 
and the bent panel and mismatched case, if you wanted a museum 
piece, it would make sense to buy a BC-348 in complete, restorable 
condition. In your case, consider the BC-348H as a work in progress.

Based on the stated condition of the BC-348H, you have an excellent 
platform to learn restoration skills, and experiment with 
modification to improve the performance closer to a modern 
receiver. A lot has been written about BC-348 conversion and 
improvements that meet individual operators needs (ie: s-meter, 
noise limiter, improved audio, etc).

The  designers of the BC-348 family used the best of 1930's 
technology when designing a cost-cutter version of the BC-342. 
Hence, some of the oddities that you found. If you are concerned 
with bypassing the IF stages, then do it right at the edge of the 
IF can and use a series resistor to isolate the B+ from 
stage-to-stage. It won't hurt anything, and may reduce the 
possibility of instability. There are MANY examples in modern 
receiver design using this technique.

Getting rid of the B- side bias generation and opting for cathode 
bias in the AF stage simplifies the power supply wiring and 
eliminates the possibility of the AF stage running away when you 
accidentally short the B- to ground. Likewise, an external power 
supply would reduce the inherent drift in BC-348's. The BC-342 
designers had this figured out early on and used a neon lamp as a 
shunt regulator, as well as a separate oscillator tube (BC-348 cost 
cutting went to a osc/mxr design).

I have restored several BC-342's and BC-348's, as well as 
extensively modifying some, with very gratifying results. Both 
series make excellent AM or SWL receivers with some work in the RF 
stages, mixer, AVC and audio stages. I use disc ceramic capacitors 
in the RF and IF stages, and orange drop film caps in the audio 
stages, with excellent results. I usually remover the oil 
capacitors to make room inside the set for additional circuit 
components.

Best of fun in your endeavor,

73 Bill N4LG

At 02:57 AM 10/21/2013, you wrote:
>I have a question relating to a BC-348H which I have apart for 
>recapping.  Replacing the .01 uF Micamolds with something modern 
>is straightforward, replacing the .5 uF dual section metal box 
>capacitors is possibly less so.
>
>Right off the dynamotor supply high voltage output is a trio of 
>sections of these caps paralleled for a total of 1.5 uF.  This 
>part of the B+ line connects, among other places, directly to the 
>cold end of the plate windings of the 2nd and 3rd IF transformers. 
>There are no bypass caps inside the cans, and none denoted in the 
>assembly diagrams and schematic.  The only IF plate return, then, 
>for the 2nd and 3rd IF stages is via the aforementioned 1.5 uF 
>capacitance.  There is a shared lead length feeding B+ to the 2nd 
>and 3rd IF transformers of about 6 inches from these 
>capacitors.  The shared lead length plus the not inconsiderable 
>inductance of the 1.5 uF total of paralleled caps, and lack of any 
>decoupling resistors here must make for some amount of feedback 
>around the 3rd IF stage.
>
>Does anyone know if feedback via this path is a design intent?   I 
>believe that such a large capacitance (1.5 uF) was used to do 
>double duty of filtering out dynamotor ripple as well as attempt 
>to provide return for IF on the B+ line to the 2nd and 3rd IF stages.
>
>It will be used with an external, AC operated, well filtered B+ 
>and filament supply.  This would, from a power supply ripple 
>filtering perspective, permit eliminating most of the dozen .5 uF 
>metal box cap sections, and replacing them with compact, easy to 
>install .01 uF caps, for RF and IF bypassing, directly at the 
>applicable places. I'm also considering adding, for expediency, 
>.01 uF bypass caps at the 2nd and 3rd IF transformers' B+ 
>terminals, and adding decoupling resistors, then eliminating the 3 
>paralleled .5 uF cap sections there. (Also would add cathode bias 
>resistor and electrolytic bypass cap for audio output stage.
>
>Heretic disclaimer: This BC-348H is fairly ratty, with somewhat 
>hacked wiring, missing audio output transformer and B+ choke, 
>missing dynamotor, slightly bent front panel, slightly bent rear 
>chassis casting, some corrosion here and there, cabinet from a 
>BC-348M, and it will never be a museum quality piece.
>
>Drew
>______________________________________________________________
>Boatanchors mailing list
>Home: http://mailman.qth.net/mailman/listinfo/boatanchors
>Help: http://mailman.qth.net/mmfaq.htm
>Post: mailto:Boatanchors at mailman.qth.net
>
>List Administrator: Duane Fischer, W8DBF
>** For Assistance: dfischer at usol.com **
>
>
>This list hosted by: http://www.qsl.net
>Please help support this email list: http://www.qsl.net/donate.html