[Boatanchors] Failing Mica Caps.

[Rodger]

Mica caps with significant DC voltage across them do seem to be pretty 
failure prone over time and silver migration may explain a lot of these 
failures.  I have 3 SX-62 series Hallicrafters, an  SX-42, and a  S-36A 
and the S-36A is the only one that did not require mica cap replacement 
in the discriminator for proper FM operation; a large 1940's era all 
wave console with added FM tuner had the same leaky mica in the 
discriminator problem.  Adding a high value blocking cap to take the DC 
off of the precision micas in the phase shift networks of aging Central 
Electronics gear generally gets rid of the crackling noises and 
intermittent loss of suppression due to deterioration of those caps.  I 
have had to replace a few mica caps in other gear but nothing that fit a 
pattern like the discriminator cans in the Hallis and they certainly 
aren't as failure prone as black beauties but I am a lot more suspicious 
of mica caps now.  The only preventative replacement I do is if the caps 
are in a really difficult to access location; otherwise I replace ones 
that I find to have failed during testing and subsequent alignment.

The alternative theory is that I have a silver mica curse.  I did a 
complete restoration of my Pierson KP-81 a couple of years ago and after 
22 months of flawless operation the sensitivity dropped to the point 
that the receiver was no longer usable.  The culprit was a shorted 
silver mica in one of the IF sub-chassis; the cap in question was a 
brand new modern dipped mica unit purchased from Mouser.  My only real 
fear is that I replaced several other silver micas in the horrible to 
access front end (makes recapping an SP-600 or SX-28 look like child's 
play by comparison) with the same run of silver micas.  I feel fairly 
safe that the one was an oddball QC issue but it is nightmare material 
since it takes around 4 hours just to get the front end section out and 
open before you even begin working on it and re installation is equally 
painful.

Rodger WQ9E