[Collins] R-390A Filters

[Bill Abate]

Hi all,

I recently decided to restore my 390A PARTS RADIO!  It seems to work OK 
after installing the tubes and a few missing parts.  I have not done a 
full alignment yet.  I want to do the physical restoration first.  The 
only problem so far is the 4 KC mechanical filter.  THe insertion loss 
is very high, about 30 db or more.  THe bandpass seems to have shifted a 
few kc also.  Is this to be expected?  I have not replaced the mica 
resonating capacitors but I doubt if this could be the cause.  The input 
and output pins show the same resistance so the transducer has not 
opened.  Any ideas out there besides replacing it?  Has anyone opened 
one of these up?  I fixed a bathtub type one once.  These are soldered 
cans, as you know.  Are they potted inside or what?

Thanks,

Bill, K3PGB