[R-390] Mech filter REPAIR

[ToddRoberts2001 at aol.com]

In a message dated 6/2/2007 8:08:34 AM Eastern Standard Time, 
shoppa_r390a at trailing-edge.com writes:
TMaybe I should stop being frustrated and open up a mechanical
filter repair shop, doing all the experimentation that I want others
to do myself and showing the results!

Tim
 
 
Hi Tim,
I think the next stage of R-390A filter repair would be to have the ability 
to rewind the input or output transducer coils if needed. I am sure there are a 
number of the coils with an open winding somewhere in the middle, not just at 
the ends. 
       The way I would do it I would guess the first step would be to measure 
the inductance of a good filter coil with an LCR meter as they apparently 
resonate with the mica or a mica plus trimmer cap at 455KHz.        Next step 
would be to determine the wire size and possibly count the number of turns if the 
coil could be unwound from the bobbin. 
      If the coil is varnished or epoxied to the bobbin determine if it can 
be freed up by a soak in paint remover or some kind of solvent. It may ruin the 
wire insulation but at least you could still count the number of turns in the 
coil. If it is impossible to count the turns then knowing the inductance of a 
good coil can lead to a target figure by trial and error of number of turns 
of a rewind.  One of the old MoReCo Morris Hand Coil Winders would be nice to 
have for rewinding the small bobbins.
      I believe the disc resonators should probably last forever unless the 
filter was dropped hard or physically damaged. There is really nothing there 
that can wear out except the wire coil or connections inside the filter and of  
course the original foam insulation that eventually turns into a sticky 
corrosive mess.
       73 Todd WD4NGG



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