{Collins} Re: Mechanical filter shorts



Robert, Here is a thought. A mechanical filter, especially if it is working,
is a pretty mechanically sound structure inside. If it is still "good" after
your little experiment, it probably is mechanically sound. There is a
growing problem with old filters though. To increase the mechanical
stability of the suspended mechanical filter, they wrapped the filter in a
foam material before "canning" it. This foam has been found to destabilize
with age and turn into a gelatinous material that can be pretty fluid. Now,
I do not know what the conductivity of that gelatinous stuff is. You may
have gotten some across the input and established a conductive path which
you then burned or blew away. I am just guessing based upon what I know and
have seen of the construction. The filters have also been found to go up in
loss as they age and this material could well be the problem. I would just
keep using it as long as the performance is acceptable.
Bill
N7OTQ
K0CXX, Trustee, Mogollon Rim Collins Collectors Club
CCA # 94-0009


----- Original Message ----- 
From: "Robert Nickels" <w9ran@xxxxxxxxxxxx>
To: "Collins" <collins@xxxxxxxxxxxxx>
Sent: Monday, December 17, 2007 3:31 PM
Subject: {Collins} Mechanical filter shorts


> I was contemplating yanking the intermittent 354A-1 mechanical filter
> adapter from my upgraded 51J-3 anyhow, but after it lost almost all
> sensitivity for no apparent reason, it became  a necessity.   A quick
> check showed that I had 500 khz IF signals going into the adapter, but
> nothing coming out.  Removing the adapter wasn't as bad as I feared, but
> the Collins manual basically says "no user serviceable parts inside" -
> saying it was like a tube and shouldn't be opened.  Of course I ignored
> that advice, but they're pretty much right - it's a very packed,
> difficult-to-service module.  I found a short from the common input side
> of the filters to ground, and first suspected the shorting contact on
> the selector switch, but instead it turned out to be a short to ground
> inside one of the filters itself.  (I could have found this without
> removing the adapter if I'd been smart enough to remove and check them!)
>
> So I start wondering what might cause a 3.1 khz filter to develop a
> short of <5 ohms to the case even while  both coils measure around 20
> ohms.  Figuring I don't have much to lose I apply 10 volts from a
> current limited power supply from the shorted terminals to case, and
> voila, no more short.  The filter (FA series 3.1 kc) now works, although
> it first sounded "lossier" than the 6 kc filter.   the filter
> connections seem to be well insulated by glass-metal feedthroughs, so
> I'd be curious if anyone has any thoughts about what might be going on
> inside this mechanical filter, and how it managed to heal itself.
>
> 73, Bob W9RAN
> .
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.
SHARE THE FUN: JOIN THE COLLINS COLLECTORS ASSOCIATION TODAY !
      http://www.collinsradio.org/membership/    -paypal accepted-
.
 NETS: Tues & Thur: 3.805 MHz-2000 Central + Fri: 3.895 MHz-2000 Pacific
  SUNDAYS: 14.263 MHz-2000 UTC  +  AM Net: 29.050 MHz-1200 Central
       1st Wednesday AM Net 3.880 MHz-2000 local (ET, CT, MT, PT)





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