Can someone describe the intricacies of Mode C?  I didn't install my
original panel, but I have done many upgrades (I did not install the Mode
C).  I have had it fail before and the avionics shop repaired it.  It
failed again a few months ago and I removed the King box and replaced
it.  I heard something rattling around in it, so I went ahead and replaced
it.  I figured at 150 bucks from Spruce it would be worth a shot to see if
that was the problem as opposed to having to fly it to my closest avionics
shop an hour flight away and having to leave the airplane, get a ride back,
etc etc.  Well needless to say, that's not the problem and I was wondering
if you guys had any suggestions.  I haven't had it tested with any ground
equipment, but ATC does not receive my Mode C.  It failed during a trip a
few months ago.  Thanks for any input.

Steve,

 

It is either the transmitter, antenna or the connection between the two.  An avionics shop should be able to quickly locate which component is at fault.  You can leave the transponder at the avionics shop (not the airplane) and, if you are in some non TFR controlled airspace, (B-veil or C) you should be able to call up ATC (phone or radio) and get non-transponder transit through the area.