Assuming the 940 has a 12 V PTT output on the xvtr jack as does the 930,
you can use that to directly drive the gate of a MOSFET. (Use a
resistor and a zener diode to protect the MOSFET gate, though.) The
MOSFET can key a DC relay, as in the SB-220. Better yet, modify the
amplifier to use a MOSFET to directly switch the bias, install a switch
to lock the amplifier relay in the transmit position, and install a vacuum
relay to switch the antenna from transmit to receive. The vacuum
relay is driven by another MOSFET, and can be in a separate box, if necessary.
This system gives you QSK, as well as VOX operation without damaging any
The 930 12 V PTT output comes up 2 ms before the transmitter output, which
allows time for the vacuum relay to close. By driving a 24 V relay coil
from a 50 volt power supply, with the appropriate series resistor, I was
able to get the relay to switch reliably within 1.3 ms. The system has
been working for several years. If anyone is interested, I'll be glad
to provide further information.
For amplifiers with a 120 VAC transfer relay, you could probably use a
triac opto isolator, driven by the 12 V PTT output. No QSK, though.