[R-390] Mech filter REPAIR

[Tim Shoppa]

"Craig C. Heaton" <wd8kdg at worldnet.att.net> wrote:
> 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.

I'm not sure you'd have to do it destructively. Knowing the wire gauge
(I can't tell you that #38 is wrong, but I can't tell you it's
always right either) and the resistance of a good one,
you can get a pretty good grasp of the total length of wire. Again
using a good one either out of the magnet or far from resonance, you
can measure the inductance. At that point it may be best just to wind
something new from scratch rather than dink around with the old form
and potting agent. In my other, larger-scale coil/choke rewinding
projects, I've been using G-10 forms with complete success. Small
teflon, delrin, etc. rods are all readily available too, and ought to
be good for another half century. Foam replacement is something
I understand less, but there's lots of different kinds of foam
in the McMaster Carr catalog :-).

Is it likely that all the coils are the same between all the
bandwidths, or is it more likely that they are all the same?
Clearly the mechanical elements have to be different between
the different bandwidths but I cannot tell you whether this
implies that the coils have to be different or the same, I just
don't know.

Its just that all the wires and coils inside the can are so dinky.
Like, I've seen larger coils in electronic watches.

Tim.