[R-390] Filters (Tisha, AA4HA)

[Tisha Hayes]

I understand where this is all coming from; as the filters get older we
will see an increasing rate of filter failures due to foam deterioration,
killer cap "events" and contamination. There are a couple of directions to
take this;

1. Retrofit in different mechanical, ceramic or LC filters into the same
physical space where the original filters fit. Deal with the impedance
matching and filter curve differences.

2. Design in a DSP based solution, ideally into the same dimensions of
where the filters are today (square can). Deal with the software design,
microcontroller code, etc...

3. Rebuild the existing filter. Take it apart, clean out the goo, wind new
coils, make new foam, assemble it and solder it back together.

All three approaches have merit; traditionalists will opt for option #3.
The more adventurous types would shoot for option #2.

Looking at the challenges involved in option #3 (filter rebuilds) the
biggest obstacles are in unsoldering the tube, removing the guts, cleaning
it all out, winding new voice coils, cutting new foam and reassembly. I
think that most of us are pretty adept at taking things apart and putting
them together again. As we all get older (more mature, yes, that sounds
better) our eyesight is not as good, our fingers do not work quite the same
for detail work or we do not have the patience to spend a day working on a
single filter.

To me, I am intimidated by winding new coils. If I was able to buy
pre-wound coils that fit the exact same dimension and had the same
electrical specification life would be so much easier. If there was a
rebuild kit where someone had already made new coils mounted on teflon
tubing with new cheek pieces that could be reattached to the mechanical
resonator and needle and soldered to the end-caps with new foam doughnuts
then it turns into a clean-out job and soldering the end caps back in place.

It seems that if someone set up a little cottage production line to make
nothing but coils they could sell rebuild kits for a reasonable price and
still make a nice little profit. Doing something once is expensive when you
look at the time investment. Doing it a few hundred times with a coil
winder and selling a set of coils and foam spacers for $20-$40 might be a
nice income supplement.

-- 
Ms. Tisha Hayes/ AA4HA


*"neque est alter hujus universi locus quam anima"*
*('For there is for this universe no other place than the soul or mind')*
*-Arthur Schopenhauer*