[R-390] capacitor analysis

[David Wise]

> suspect the problem is here.  When the radio flakes out, the 
> carrier level
> drops.  But it doesn't happen very often.  And it doesn't 
> last very long.  I
> hate these intermittant kind of problems, they're almost 
> impossible to find.
> It just feels like a capacitor somewhere to me.  I could be 
> wrong of course.
> The IF deck is an old Collins.  There are no adjustable 
> tuning capacitors
> near the mechanical filters, and one of the filters had been 
> replaced with a
> Dittmore Freimuth filter.  When I looked at C553, and also at 

Does it drop on all bandwidths, or only one?  On my 1954 Motorola,
the level was low on 8KC.  I traced it to the mica cap tuning
that mechanical filter.  In my 30 years (on and off) of restoring
vintage radios, this was maybe my third bad one.  Micas are pretty
good, but you should not trust them blindly.

Those of you with newer decks with trimmers: what's the
temperature coefficient?  NPO, N750, something else,
or unmarked?  (How about the other trimmers?  The manual
doesn't say.  It may not be important.  I'll take a peek
tonight.)  Mouser carries a bunch of trimmers, with various
TCs.  If I switch over sometime, I'd rather not figure it
out by hand...

73,
Dave Wise