FT212 & FT712
-------------
I have made the following connections/modifications to the FT212/FT712
transceivers:

For receive, I connected a 10K resistor to pin 11 of Q01 in the IF-unit.
The other end of the 10K resistor is the output.  The resistor is there
to protect the transceiver when you make an error at the external connection...

For transmit, I connected a 10K resistor to the top end of VR03 in the main
unit (near pin 1 of the MIC unit connection).
This point is fed rather directly to the vco varactor.

I have made the external connection with a shielded cable that can run
through a hole near the antenna coax.  There is a small plate on the heatsink
that you can remove and a suitable hole appears.

When you connect the transmit line, it is possible to switchoff the mike
amplifier (at least when you don't have the voice recording unit).

Use 2 diodes, with the kathodes connected together to the external "PTT"
switching lead.  The anode of one diode is connected to the normal PTT point
on the mike connector, the anode of the other diode is connected to pin 10
of Q02 in the MIC unit.  When you now pull the external lead to 0V, the
set will transmit but the microphone does not work.  You can leave it
connected!  Very convenient when you want to use it for fone...

Summary:

       IF Q01 pin 11 -----|=====|------ > audio out
                            10k
       MAIN VR03 top -----|=====|------ < audio in

       PTT ----------------->|---|
                          1n4148 |
                                 ------ PTT
                                 |
                          1n4148 |
       MIC Q02 pin 10 ------>|---|

(this is a text I sent to someone else before)

I have heard both horror- and success-stories about the TM531 too...
The problem is, that different people refer to the same results as "It works
perfectly, just a few bit errors", or "It's a complete disaster, every
3rd frame I have to send a re-try, killing the throughput".
We are operating a succesful 4800bps link using the TM531 with the HAPN
modem, but that one is a bit less demanding.


73,
Rob  PE1CHL