S-COM 7K, Manual, Page: 357 / 400

Application Notes
B-3
Positive Voltage TX Keying Circuit
Some transmitters require a positive voltage to be sourced into their PTT inputs
to key; removing the voltage unkeys the transmitter. This contrasts with the
style of transmitter that requires the PTT line to be grounded to key, opened to
unkey.
Transmitters that require positive voltage keying include models made by
Hamtronics, Maggiore, and the RCA 500 and 700-series repeaters.
A simple outboard circuit can be placed between the 7K controller and the
transmitter PTT. A PNP power transistor allows current to flow to the
transmitter to key, and opens the current path to unkey. Be sure to use a
sufficiently large PNP transistor for Q for Q1, as these transmitters or exciters
can draw considerable current. The TX DIP switch in the 7K controller should
be ON.
Q1 can be a 2N2904 if the transmitter/exciter draws 500 mA or less;
recommend TIP30 for loads of 1 A or less.
Q1
2K
4.7K
+13.6VDC
to TX PTT
(+13.6V = key)
(0V = unkey)
S-COM
PTT