[Milsurplus] H-250/U mic repair

[Robert Nickels]

If necessity is the mother of invention, then I suspect impatience is 
the father.  Having determined that the mic on my H-250/U handset was 
open circuit I figured I had nothing to lose by attempting a repair so I 
could try out my "new" PRC-74.   Other manufacturers may vary but this 
one has only a single dynamic mic element, which obtains the 
noise-canceling ability by having one aperture on the front side of the 
diaphragm that picks up noise plus your voice, and another on the back 
side that picks up mostly noise.  The backside noise tends to cancel out 
the frontside noise, leaving the voice as the primary force to move the 
diaphragm.  

Prying the mic element loose showed a broken wire, although the element 
itself also measured open circuit.  (Because the mic wires had been 
pulled tight and gooped in place with RTV, it would have been virtually 
impossible to remove the element without breaking the wires anyhow in my 
case).   So it became a "replace" mission rather than a "repair" one. 

A cheap replacement dynamic element turned out to be as near as my 
junkbox.  Everyone must have at least one set of the cheap headphones 
that come with with CD and MP3 players laying around, if not, the dollar 
stores sell them.  I found a suitable element in a pair of "Philco" 
(sorry for trashing that legacy brand!) over-the-ear types although even 
smaller ones could be found in earbuds.   Pictures tell the story:

http://smg.photobucket.com/albums/v652/ranickel/H-250%20mic%20repair/

The new mic fits perfectly and sounds good.  I did cover the rear-facing 
diaphragm to get a crisper, less "boomy" sound, and obviously there's no 
longer any noise cancellation going on.  This fix may not satisfy the 
purist but I thought it would be worth sharing, and maybe get a few 
handsets back in service.

73, Bob W9RAN