[Boatanchors] BC-348H Recap

[Mike Hanz]

On 10/21/2013 2:57 AM, Drew P. 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.

At frequencies significantly above the 915kHz IF, perhaps...but the IF 
tubes aren't exactly VHF performers, either.  Six inches of lead length 
at less than a megahertz is next to nothing...there is no feedback path 
intended in the design.  If there was, it would be far more overt.  The 
Fourth Edition Radiotron Designer's Handbook has a lot of good info on 
that sort of thing.  I would agree with AB5S. Check for oil leakage, and 
if none, check for capacitor current leakage at the B+ you plan to use.  
If it's low, then try them without replacement.  Then put a scope across 
them and see how much signal you get at 915kHz.  If it's substantial, 
tack a mica or two across the existing caps.  You've already said the 
set is ratty and hacked.  That's a lot easier than replacing them 
(unless they are actually bad. )

           73,
  - Mike  KC4TOS