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144 MHz TX inhibit for FT-847



This weekend I finally got around to putting a "144 MHz TX inhibit" circuit in my FT-847.  The purpose is to prevent accidental transmission into my 2400 MHz downconverter.  Accidental transmission can happen in a number of ways.  For instance, I could switch from AO40 to FO29 and forget to switch out the downconverter.  Or I could accidentally press the wrong key on the radio and bump into the PTT.  My modification automatically disables the 144 MHz transmitter when I energize a coaxial relay that connects the downconverter to the 144 MHz antenna port.

The mod is very simple.  It does not require soldering of tiny surface mount components, and only one wire needs to be clipped inside the FT-847.  I don't have a service manual, but looking at the schematic I found a signal called "VHF T8" that goes to the VHF PA section on J4003, pin 1.  There is no voltage on this line in RX mode.  In 144 MHz TX mode this line applies 7.8V to the 144 MHz T/R switch and to the 144 MHz PA bias circuits.  I chose to remotely open/close this line using a 12V relay.  The relay gives excellent isolation between the PA bias circuits and the external TX inhibit signal.  I used a miniature 12V relay from Radio Shack, part number 275-0241.  I made a "TX inhibit" circuit by connecting the "VHF T8" line through the normally-closed relay contacts.  I could just as easily have made a "TX enable" circuit by using the normally-open relay contacts.  I prefer "TX inhibit" so the radio is still completely functional for portable non-satellite operation.

Performing the mod: Remove the top cover and the top shield.  J4003 is directly in front of the 144 MHz antenna connector.  Pin 1 (VHF T8) is the furthest pin from the antenna connector, marked by a white semi-circle on the PCB.  Clip the wire to Pin 1 about 4 cm from the connector.  Glue the relay upside-down to an empty area on the PCB in front of J4003.  Vertical clearance is tight, so I shortened the relay leads to ensure that they don't touch the cover plate.  I soldered the clipped "VHF T8" wire to the normally-closed relay contacts.  I soldered new wires from the relay coil contacts to a phono jack mounted to the closest rear panel ventilation slot in the receiver section.  A phono jack doesn't quite fit in the ventilation slot, so I had to drill out the slot very slightly with a 1/4 inch drill bit.

External connections: On the 144 MHz antenna port I have a coaxial relay that switches between my 144 MHz antenna and the downconverter.  I run the same 12V control signal to both the coaxial relay and the 144 MHz TX inhibit connector.  144 MHz TX is automatically disabled whenever the relay connects the downconverter to the 144 MHz antenna port.  The control switch could be mounted in any number of places, possibly in your 12V power supply or in your rotor controller.  I have a homebrew switch box for 4 remotely switched relays.

Operation:  Because I'm merely removing bias voltages, the radio "thinks" it can transmit on 144 MHz when 144 MHz TX is disabled.  That is, when I key the disabled transmitter the TX icon lights up and sidetone is generated.  But the internal power meter doesn't indicate any power output.  I checked the RF output with an HF SWR/wattmeter which is VERY sensitive at 144 MHz.   I could not detect any RF power when "transmitting" at max-power with the 144 MHz transmit inhibited.  This is to be expected when the PA is biased off and the T/R switch is connecting the 144 MHz antenna port to the receiver.

This is a very easy modification, but I do have digital photos of the modification in case anybody wants to see what it looks like.  I don't have a web site, but I can email the photos to anybody who asks for it.  Please contact me if you are willing to host the photos and text on your web site.

Wayne Estes W9AE
Mundelein, IL, USA

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