[R-390] Collins Mechanical Filters

[ToddRoberts2001 at aol.com]

Hi Craig, thanks also for your website address and pictures of how you opened 
up an R-390A mechanical filter. I have repaired one of these in the past. 
Luckily the problem was one of the fine wires from the input coil broke right at 
the filter terminal inside and I was able to resolder it to the terminal and 
that fixed the filter. One trick I have used to remove the enamel insulation 
from ultra-fine magnet wire is to dip the wire end into a little bottlecap with 
a small amount of Klean Strip KS-3 Premuim Stripper paint remover. If that is 
not practical then just paint the end of the wire with a little of the paint 
remover and let it sit for 15-20 minutes. The paint remover will dissolve the 
enamel without hurting the wire. Wipe the end of the wire clean with a soft 
cloth moistened with alcohol. Scraping or heating the fine wire to remove the 
insulation will likely destroy the wire. Also I have found you need to be very 
careful when soldering ultra-fine wire. Too much heat and solder and the wire 
will literally dissolve. Use a fine-tip soldering pencil and solder as quickly 
as possible with minimum heat. 
       If the coil has an internal break and it is impossible to unwind the 
coil without destroying it then a possibility would be to measure the 
inductance of the other good coil with an inductance meter, then re-wind another coil 
with similar size wire on a similar-size coil form and by trial and error find 
the number of turns that gives the same inductance. Collins allowed for a 
little variation in the coils by using fixed capacitors + trimmer capacitors 
across the filters to peak resonate the coils at the filter frequency, although 
some earlier R-390As used fixed capacitors only across the filters. The number 39 
wire will probably break easily but with some practice one should be able to 
handle it carefully and wind a coil without breaking the fine wire. 73 Todd 
WD4NGG