[Boatanchors] BC-348H Recap

[Drew P.]

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