[R-390] 4kc filter repair?

[ToddRoberts2001@aol.com]

 A good subject on ways to rebuild or re-work the mechanical filters used in 
the R-390A. My current thinking is to look into the new Rockwell/Collins 
mechanical torsional filters and adapt them for use in the R-390A, along the lines 
of what Curry Longwave Products are doing. The new Rockwell/Collins mechanical 
filters are small enough to fit inside a 3/4" diameter metal tube that could 
be cut to similar dimensions of  the original filters for mounting inside the 
filter box cover. They currently list symmetrical filters in 3.80KHz bandwidth 
that would be close to the original 4KHz wide units. The newer filters have a 
much lower input and output Z of 2K ohms so a matching network will need to 
be mounted on the input and output sides for step-down and step-up to the HI-Z 
used in the R-390A circuits. The whole thing could be mounted on a circuit 
card small enough to slip inside a 3/4" diameter metal tube with end plugs. I 
wonder if anybody has looked inside one of the Curry filters to see what kind of 
matching network was used? Might be a fun project to try to produce a line of 
replacement filters for the R-390A. 73 Todd Roberts WD4NGG.