NOTE: Crossposted to rec.radio.amateur.equipment and rec...homebrew.

For some years, I've had a Kenwood TM-251A 2-meter xcvr (with 440 rcv)
hanging from the ceiling of my pickup truck, and recently the TouchTone
pad (in the microphone) quit working and, while PTT causes plenty of RF
output, there's no AUDIO output.

I'm pretty sure the problem is in the microphone electronics OR its
8-wire cord. I've "beeped" the cord from the telephone/data-like (RJ-45?)
connector to solder joints on the mike PC board (it's quite a full little
board!) and don't find anything wrong (but it's hard to hold a probe on
each end and wiggle everything in the middle!). I'd replace the cord in
a flash except I can't handle that %$#@! RJ connector.

The only microphone detail in the xcvr Service Manual other than the
connector pinout:
UP
+8 volts (I measued +7.25)
GROUND
PTT
ME (Mike Earth?)
MIKE
RD (Read Detect? An output, perhaps for use with a TNC?)
DOWN
is the fact that PTT, UP, DOWN, CALL, VFO, MR, and PF are connected to
three analog-inputs (and ground), thus "using up" four of the seven
input wires and leaving just two wires (MIKE and ME) to handle the
remaining 16 TouchTone keys AND audio!

PTT, UP, DOWN, CALL, VFO, MR, and PF do work as they are supposed to.

With "Remote Control Mode" ON, letter "A" on the TT pad is supposed
to start frequency input from the keypad, but it does NOT. With an
oscilloscope probe on the MIKE and ME wires, pushing "A" gives no
visible change in the trace, but my jury-rig setup would probably miss
a single tone-burst, so that is probably an inconclusive test other
than the xcvr CPU didn't respond, either.

Any suggestions?

--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)