[Milsurplus] BC-779 Question

[Al Klase]

Hi Joe,

60-year-old mica caps are prone to failure, perhaps not to the point of 
shot-gun replacement, but replace if you're suspicious.  The mica grid 
couplers on the RF stages in the Super-pros will screw up the AVC if 
they get leaky, and the do get leaky.  I think I replace at least one in 
the "good" set you have.  82pF is probably close enough.  Alignment 
should take up the slack.

The bath-tubs are a mixed bag, and I find them a pain to replace.  
(Install terminal strips.)   The one's with oozing red-rubber seals are 
especially troublesome.  I'd, at least, replace any that sit on the AVC 
bus.  Also, you might encounter bath-tub caps that have decided to be 
batteries, maybe a volt or so.

While the 6K6's will probably kinda work in that circuit, I don't think 
they're a good bet for peak performance.  Grid bias is probably too 
high, partially cutting them off, and they're not designed for the plate 
voltage involved.  ^F6's are easy enough to come by, get 'em out.

Regards,
Al

Carole White-Connor wrote:
> I'm working on a BC-779B. The radio plays well and has decent sensitivity.
> The audio is not as good as it should be and it's noisier than it should
> be. I sense some distortion and the AVC seems sluggish. (I'm comparing it
> to the civilian SP-210 that I have).  I've already replaced the paper caps
> and about a half-dozen out-of-spec resistors. All voltages are pretty close
> to spec.
>
> I'm looking at C-13, which is a 91 mmf grip-coupling cap. The part list
> specifies that it is a silver mica. It looks like a standard paper cap.
>
> Here are my questions:
>
> 1. Is this type of silver mica of this vintage prone to failure? It looks
> crappy but that may just be the moisture-proofing compound sprayed rather
> liberally on the underside of this set.
>
> 2. The closest replacement I have is 82 mmf. Is that good enough?
>
> 3. Are the .05 mfd bathtub caps along the side of the set prone to failure?
>
> 4. The manual specifies 6F6s for the audio section. The prior owner had
> 6K6s there. Is this good, bad or indifferent?
>
>   

-- 
Al Klase - N3FRQ
Flemington, NJ
http://www.skywaves.ar88.net/