[Racal] Bandpass filters

[Pete Williams]

 G;day... Interesting problem with my RA-117... suddenly decided not to
exhale  any audio.
Problem traced to the 37.5MHz  BPF and while rf coming in , nil output.
Thinking that the parallel micas may be  at fault, they were  systematically
checked and 'indicated' OK on a capacitance  checker.
Re - installation  of the first strip showed no improvement,- why I wondered
-  and sniffing out the point where RF disappeared seemed a job  for a CRO.
Not having one  handy ( HP one needs service) I figured that the expected
levels from the manual would allow a counter to provide indication.
Level at the input was such that holding a short wire probe at the end of a
coax line  waved in close proximity gave good  reading--- ( the counter has
better than 10 mV sensitivity).
Checking subsequent stages  but with a  capacitor at the end of the  cable
and touching the rotor of the  variables soon showed which one was not
resonating. It so happened I had a 33pf and replacing it, restored operation
for a short while until the NEXT one also went.--- then found I'm fresh out
of  micas at that value 
So.... It  appears that ALL the micas labeled JMCYB should be replaced

Question ..... 1. why do 350V micas suddenly  go queer with a bit of RF ?

               2.  finding a  source of micas in OZ a job for  Agatha
Christie, but may be I'm not looking in the  right place. Any o'seas source
appreciated for 15, 33, and 39 pf 350/500V micas.
 

Maybe my story is 'oldhat' to more experienced in this Racal gear--- however
!
Comments /advice appreciated.
Cheers
Pete

 Pete D.Williams
jupete at bigpond.net.au
METUNG 3904 AUSTRALIA