TRANSMITTER INHIBIT CIRCUIT
TX Inhibit for the FT-847 and K3
One of the problems for running a station under control of a sequencer is controlling
the IF radio PTT for voice modes and Tx keying with CW. Many have tried to route PTT
lines from mics and CW keys thru contacts in a sequencer to prevent RF output until
the correct stage of the sequence (usually the last step going from Rx to Tx).
But those methods are not without issues. AC hum can often be picked up by the mic cable
modulating the radio and routing the CW key prevents using the radio in QSK mode for regular
operation. Much better to leave mic and CW key connections standard to the radio and find
a method to prevent RF output until the final stage in the sequence.
G0AFH published a method for providing Tx inhibit on the FT-847 by use of the TUNER interface
jack. I made some changes to suit my use of a station controller for both a FT-847 and
the Elecraft K3.
The FT-847 inhibit circuit can be enclosed inside the mini-DIN plug by use of a 78L05 regulator
and deriving +13.5 vdc form pin-1 of the TUNER jack. I decided that I wanted the circuit
mounted in my station control and powered by it.
The K3 inhibit simply used +5v derived via a dropping resistor from +13.5 vdc found on the
K3 ACC jack. I decided to provide the +5v for both radios from a LM7805 voltage regulator.
My radios will not have Tx inhibit when the power to the station control is turned off.
But this is acceptable because all the equipment that needs sequencing protection is disabled
in that case. The radios are either directly connected to antennas or the TR relays default
in the Tx position. Immediate RF transmission would not damage preamps. Transverters are driven
at mw level thru separate Tx connections so no damage would result to them.
I made the circuit to inhibit the FT-847 by default. Switchover to K3 inhibit is accomplished
by +5 vdc generated by the K3 band select lines at the ACC jack which triggers a relay on the
station controller board. K3 inhibit is enabled only for VHF+ transverters via the BAND-0 and
Band-1 output of the four digit BCD band control. DIGOUT-1 is added so inhibit may be enabled
for 6-meters. FT-847 inhibit is defeated when the K3 is inhibited.
Also the station control has a manual inhibit bypass switch.
Note: I added a diagram for K3 inhibit-only on VHF.
The diagrams, below, are self descriptive of the circuits:
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