[R-390] RE: Mechanical Filter Repair

[Laird Tom N]

>From the WC9M archives:

Hope this helps,

Tom Laird WC9M
Moline, IL.
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Success with the 75A-4 filter repair. The solder on the end opposite the pins
does not have to be removed. It is only for sealing the filter. I used a
small butane torch, and removed the shell. No foam, but 2 rubber supports.
Everything looked ok. Played with it, but could not get it to work. Both
coils at the ends checked ok at about 44 ohms. Gave up, and went to bed.
Next AM, I was looking throughout the KWS-1 manual, and looked at the drawing
of the mechanical filter. There is a small wire that comes from the coil
assembly, and is attached to the second disk at each end. On looking at the
filter again, found that one wire on the top of the filter (opposite the
pins) was missing. Tapped filter, and the wire came out of the coil and
touched the disk. I applied a small amount of super glue and let it sit for
the day. Works fine. No signal loss when switching to this filter now. Do
not know how the wire was originally attached, but the super glue seems to
be working. Ken, WW3KP, soon to be W3KMP
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I was thinking about a common topic on the net about the failure of the 4 Khz filter, and the problem with the failure of the dc blocking cap that feeds them. If I remember correectly, the common failure mode is that the input wire (around #36 in size) to the input transformer breaks near the solder connection. We used to just send the filter back to the line, they would open it up and resolder the wire. You have to carefully unsolder both ends of the tube, and withdraw the total assembly. I suspect that the failure mode would be the same for the shorted cap, and the metal fatigue type, a break near the solder termial. If you don't care to try, mail a couple to me, and I will see if my educated guess is correct. The address is: 

John Watkins  
302 Cheshire Circle  
Seguin, Tx. 78155  
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Rockwell Collins still makes mechanical filter modules using modern design.
You can buy them on small boards as plug ins for the S Line from Inrad. Or
you can buy the modules directly from Rockwell I believe. Also, I think
Longwave Products sells direct replacements for the 75A-4 and 390A that uses
the Rockwell modules mounted in the metal cans. The modules are small and
could easily fit inside an old mechanical filter can.