[Radioroslyn]

Yep
The ptt wiring is on pin 8 of the connector on the back and is connected to the tip of the mic connector on the front panel. The ring is the audio input. On my sets I've rewired it the other way round so the audio is on the tip and the ring is the ptt line. On the APS-50 there is no connection to pin 8. The reason for that is the power cable is shielded with lot of AC wiring which may induce 60cy hum into ptt/mic wiring.
http://www.flickr.com/photos/42774321@N08/3938290891/
Here's a pic of one I did about 20yrs ago. The small trans is to power the relay coil thought the mic switch with a small dc voltage. The contacts on the relay are used to open primary of the plate transformer on the APS-50. If your power transformer also supply's the heater voltage too you will want to use it to open the CT of the HV winding to kill the HV. When you build the p/s keep your HV filter cap 20MFD or less. Larger ones will cause the transmitter on lag on a bit before it completely dies causing terrific feedback. Other relay contacts can be used to mute the rx and ant changeover. You'll also want to change the transmit/stby switch to close the ptt circuit.
So what are you going to use for the rx??
Terry N3GTE