Posted for "Ron Galbraith" <cfi@instructor.net>:

 This is a very basic expanation, but in short, a transponder works with
 15 pulses.  2 framing pulses for timing, and 13 others between the
 framing pulses.   If you have mode C, then these pulses are shared
 between the code being displayed such as 1200, and the altitude.  To get
 that altitude information to the transponder, you must have a separate
 Mode C encoder.  These are either a blind encode, or encoding altimeter
 where the actual altimeter sends the info to the transponder.  If you
 aren't having a problem with the transponder code getting to ATC, then
 the problem probably isn't the transponder, but rather the encoder or the
 wiring between the transponder and the encoder.  Since the problem is
 intermittant, then the best guess problem is the encoder itself.
 
 Ron Galbraith
 
 LNCE - N5ES
 

  > Can someone describe the intricacies of Mode C?