[R-390] Mech filter REPAIR

[Craig C. Heaton]

Tim & All,

Someone with the correct tooling could make this wish happen. From the two
websites recently shown on this e-mail reflector, it seems there are
different styles of mechanical filters.

The only one I've disassembled, the interior is one unit and has to be
removed as one piece. Then it could be farther taken apart. Trying to pull
only one end off would of destroyed the filter.

The coil is the sticky wicket on this one. I'll guess the potting substance
covering the #38 enameled copper wire seems to be the same material as the
spool!! Once again a guess, it looks and behaves like delrin. This is the
same material that was used for the gears in the HP 8640B sig-gens. It
shrinks with age, snap goes the wire!

With a small lathe, I could trim off the potting material then count the
turns of wire. Note, there are two separate windings on the same spool.

At this point, most common solvents have been tried. The potting substance
is very hard, just like the spool. So, my limited resources are close to an
end. Don't have enough old, broken, failed filters for destructive testing.

Craig,

-----Original Message-----
From: r-390-bounces at mailman.qth.net
[mailto:r-390-bounces at mailman.qth.net]On Behalf Of Tim Shoppa
Sent: Saturday, June 02, 2007 5:08 AM
To: r-390 at mailman.qth.net
Subject: [R-390] Mech filter REPAIR


Several here and elsewhere have disassembled the mechanical filter
cans and made some nice pictures of the contents. Wonderful work.

Some have done a good job of reattaching single wires that had broken
off. Wonderful work.

Has anyone actually, say, completely disassembled a filter, put new
foam in, rewound a burnt in the middle (not just a reattachment)
coil, and reassembled the whole shebang to make a working filter?

Right now I'm with exactly the right number of working filters to
match all my radios, but I have several "bad" ones that have
accumulated over the years.

I compare the situation today with where Dallas Lankford was writing
about PTO's in the 80's. He clearly understood that there were
problems with PTO's that had gone beyond the endpoint adjustment or
needed an overall resetting at the corrector stack, but hadn't
quite gone as far as doing that. And now today there's a couple
websites that have really pristine pretty pictures of the true
innards of a PTO, where daring souls have gone in and completely
cleaned up and recalibrated from scratch. Those pictures and
details let me do the same a year or two ago when I rebuilt my
sludged-up PTO. Similarly there are websites showing how to
really do a complete teardown and clean of a RF deck today.

TMaybe I should stop being frustrated and open up a mechanical
filter repair shop, doing all the experimentation that I want others
to do myself and showing the results!

Tim
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