SB-035

Making the MC-45 Microphone work on the TR-7400A                5-22-79


     The MC-45 Touch Tone Microphone is supplied with a 5 pin connector
installed on it.  This allows it to be used with the TR-7600 and tr-7625 2M
FM transceivers.  The MC-45 may also be used with the TR-7400 if the
microphone plug is changed to the 4 pin type used on the "TR-7400" (E07-
0403-05) and the microphone socket ont he TR-7400A is modified to supply
power to the microphone.

     The 5 pin microphone connector is changed so the PTT ground and
microphone ground wires are connected together on the 4 pin plug.  This
means the wires on pins 4 and 5 of the 5 pin plug will be connected to pin
3 of the 4 pin plug.  The other PTT lead ( on pin 2) would go to pin 2.
The microphone hot lead goes from pin 1 to pin 1.  The power lead ( on pin.
3) would be connected to pin 4 of the 4 pin plug.

     The  TR-7400A modification requires removing the front panel and
Display Unit (X54-1210-10) so you may get to the microphone socket.  The
switch board may not be removed.

     The front panel is removed by taking off all the knobs first.  The
"MHz" switch knob is push-on type.  All the other knobs are held on by set
screws.  (Be sure to note the position they are in so they will be
reinstalled correctly.)  The front panel is then removed by taking out the
4 screws (2 on each side) that hld the front panel on.  You should remove
the volume/squelch control at this time by loosening the nut and sliding
the control up from the sub-panel.

     Unplug all 6 display digit boards from the Display Unit (X54-1210-10).
Be sure to keep the digit boards in order, as there are 4 different types
used.  The display unit is secured by 4 screws.  Three of these screws are
visible now.  The screw that is nearest the S-meter also holds a bracket
for 2 LED's.  Remove this screw and carefully lift out the bracket.  This
will reveal the fourth screw.  Remove the three screws that are left
holding the Display Unit in place.  Thie display  unit may now be lifted
out by the back of the circuit board to give you enough room to work on the
microphone connector.

     There is a jumper wire between pins 3 and 4 on the microphone socket.
Remove this jumper.  Pin 4 is the lowerof these two pins.  Install a wire
from pin 4 to terminal C9 on the TX unit (X56-1210-10).

     Reinstall the indicator unit (X54-1210-10) and replace the digit
boards.   Put the volume/squelch potentiometer back in place, next mount
the front panel and knobs.  This completes the modification.

     The modified MC-45 microphone may now be connected to the TR-7400A.
Depressing any key on the keypad will activate the PTT circuit and the tone
circuit.  There is also a short delay of 1/2 second that the PTT circuit
will remain closed after the key has been released.  This will allow you to
key up several numbers without dropping the carrier.

JRJ/yn

     
(c) 52279TKC