[R-390] Re: [Collins] R-390A Filters

[John KA1XC]

>  If this failure is not peculiar to my filter the future
> could hold time bombs.  To correct this situation would take a lot more
> patience than I have.

I beleive you are right Bill, this is a not uncommon failure, and as the
decades progress so will the deterioration of the foam mounts. It's almost
certain that at some future time the mechanical filter failures will
approach the frequency of molded capacitor failures, and the very thorough
R-390A restorer will then be faced with re-filtering in addition to
re-capping :^(

> If I wanted to test the rest of my filters (?) I would remove the
> filters and gently turn it upside down to see if I can feel any movement
> (without causing a failure).  If the filter needed new foam I would open
> the filter, remove the old stuff and put in new, reseal the filter and
> hope it still worked (connections are about #40 wire that is hard to
> see, let alone solder).  Definitely not something I would want to try or
> recommend.

Too bad these filters weren't plug-in, then the above efforts might be
worthwhile.
>From my experience the filter loss starts to increase rapidly just before
failure. One R-390A that I brought home from a flea worked quite well when I
got it, but as it was worked on and it got moved around and over on the
workbench I noticed that it was loosing sensitivity only in the 4 KC
bandwidth. first a few db, then a few more, then 20 dB, then dead. The
filter was replaced but I haven't done a post mortem on it.

So I'd say that the state of the filter innards in any 390A is indeed a crap
shoot, even in the multi-kilobuck "'museum quality" radios that pop up now
and then for sale. The only thing I do different now is to try minimize any
mechanical shocks or vibration when moving or transporting a 390A, for
example putting it on a cushy mat or blanket when transporting one in the
back of a car or truck, not for the sake of the tubes but for the filters.
It's ironic that although the R-390 is a much older beast than the average
R-390A it is not as fragile in this respect.

73,
John