May 192013

The FT847 is a great little radio but (in my opinion) has one major flaw. VOX is always on. If the radio is left in CW mode and you touch the key, it will transmit.

If you are running the rig in a system with a mast head preamp or a high power amplifier this can cause problems, and in many cases may cause damage.

The solution adopted by many FT847 owners is to pass both the microphone PTT and the key signals through an external relay. This is not entirely foolproof as the front panel MOX switch is still active. However, there is another, more elegant solution.

On the tuner interface socket there is a signal marked TX INH. Pulling this signal high prevents the radio from generating any RF. Keying the microphone, pushing the MOX, or touching the key will still put the radio into TX but no RF will be produced. Usefully the PTT lines from the STANDBY socket still work which make interfacing to an external sequencer even easier.

Below is the circuit that I use to allow the FT847 to be sequenced. It all fits neatly into the 8 pin connector shell.


U1 is a 78L05 5V regulator. R1 is shown as 1K but can almost vertainly be higher, 10K will probably be fine. D1 is a schottky diode. The type is not critical, I used a BAS16 that I had to hand.

An alternative to this circuit would be to add a pull up resistor to the TX-INH line inside the radio but of course that means opening the radio.


Thanks to PA4EME for suggesting using the TX INH signal.


This works on my FT847, and at least two others that I know of. It may not work on yours. I accept no responsibility whatsoever for any consequences of using the method described above.

73, Ian

Copyright G0AFH 2013.

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