[R-390] Re: Caps and more caps

[David Wise]

> From: Dallas Lankford [mailto:dallas at bayou.com]

snip

> inspected and rebuilt well over 100.  Neither of us have ever 
> found a bad
> (cracked or otherwise) brown encapsulated paper capacitor in 
> an R-390A.

I did.  For what it's worth, my used-but-not-abused '56 Motorola,
which contained browns*, had one that was visibly cracked and leaking
oil (or whatever it is.  The Cost Reduction Report calls them
"Prokars", which I seem to remember had a plastic-impregnated
paper dielectric**).  I can't remember the details (they might
be in the archive), but there are at least a few nets in the
IF deck that you can charge with a HV power supply and directly
or indirectly measure the aggregate leakage of several caps.
I did so, with the deck in a low (120F) oven, and it averaged a
few hundred uA per cap at their rated voltage of 400V.  One cap
shorted overnight.  I replaced it, the cracked one, and those
in high-impedance locations.  Left the others.  After a couple
years of frequent use, I haven't seen any problems.

* Except for a Black Beauty in the RF deck, which I replaced on sight.
** If you know this is wrong, please say so and stamp out bad data.

> period.  We have found bad capacitors in R-390A's, but they 
> were all (red or
> green) silver micas, and most of those were in oscillator decks.

You probably mean molded micas.  I replaced one mech. filter-tuning
one that was intermittent.

73,
Dave Wise