[R-390] filters

[Roger Ruszkowski]

John,

You ask,
Would any of the knowledgeable folk on the list care to comment about the
 donut material and whether the old material was chosen as an acoustic
isolator to prevent microphonics?

So some time in 2005 / 2006 one of the fellows at Fair Radio Sales
in Ohio was rebuilding the filters.

I my self had opened a few failed filters in 73 74 and even back then
found the "packing" was goo.

We can believe today that every filter is now in a goo condition
yet these filters are still working well. Thus it may be logical to
jump to the conclusion that the packing provides no part in the
filtering activity.

You can use a hot hollow tube to melt a round core plug from
a sheet of foam to make up new packing. Also use a hot knife 
to put in some slots to let the wires pass through the packing plugs.

If you wrap the barrel of the filter in a wet rag you can re solder
the end caps with out melting the new plugs to goo right off the 
start. Invest in a spool of low temp solder. A few degrees could
make the difference between success and failure.

Maybe a plug cut from great stuff could be used.
The stuff does not hold up in sun light but
we at least know what it looks like as it decomposes.

I am thinking styrene would have a long life inside the filters.

There are a number of ceramic filters about in the 455 range.
We may need to re stuff the cans with a set of these items
selected to provide a narrow band width. Much like building
a crystal filter up from crystals.

Roger