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144 MHz TX inhibit on FT-847

This weekend I opened up my FT-847 to do a mod to mute microphone audio when the radio is keyed from the data port.  As long as the radio was open, I looked at the schematic and probed around the 144 MHz PA a little bit.

J4003 is a connector on the VHF/UHF PA board with VHF control signals.  The pins that looked interesting are:

Pin 1:  VHF T8
Pin 5:  VHF AMP
Pin 2:  VHF ALC

"VHF T8" appears to be a "VHF Transmit 8V bias supply".  This pin supplies a DC bias to the 144 MHz PA, drive circuit, and antenna switch.  I measured 0.0V in RX mode, 7.78V in TX mode.  I suspect that the 144 MHz power amplifier could be disabled by opening this line.

"VHF AMP" didn't appear to do anything.  This signal goes to the base of a switching transistor in the 144 MHz PA area to control something, but I never could find a condition where this signal was used.  0.0V on RX, 0.0V on TX (FM and SSB mode).

"VHF ALC" appears to be a "power detect" OUTPUT from the PA board.  I measured 0.0V in SSB TX mode with no modulation and Max. RF power setting.  0.8V in FM TX mode at minimum power.  Approx. 6V in FM TX mode at maximum power.

Like I said in a previous message, I don't have a 2400 MHz downconverter or antenna.  So there is no urgency for ME to do this modification.  I was in a hurry to get the radio back together in time to work J6/N9UUR on FO-29, so I didn't spend the time to find out for sure if breaking the "VHF T8" signal would disable the 144 MHz transmitter. Here's what I should have done:

1. Clip the "VHF T8" wire.
2. Key the 144 MHz transmitter at minimum power in FM mode, verifying that the current drain is not excessive.
3. Slowly increase the RF power control to maximum, checking for undesired smoke.
4. Assuming there is no smoke, measure the power on the 144 MHz antenna port with the RF power control set to maximum in FM mode.  Hopefully I won't see any detectable power.

If the TX inhibit test is successful, the next step is to implement a method to break the "VHF T8" line to disable the 144 MHz transmitter. I would be wary of pulling this line low with an external device because I didn't see any pullup resistors or series resistors on this line in the schematic.  I might damage something if I force this line low while the bias supply is trying to pull it high. I would add a small 12V relay to open/close the "VHF T8" line using the same 12V signal that controls a coaxial relay to connect the 2400 MHz downconverter to the 144 MHz antenna port:

1. Re-connect the clipped "VHF T8" signal through the normally-closed contacts of a small 12V relay.
2. Connect the relay coil to a connector added to the back panel of the FT-847.
3. Label the connector "144 MHz TX inhibit".

Or if you're more paranoid, make a "144 MHz TX ENABLE" instead:

1. Connect the relay's normally OPEN contacts in series with the "VHF T8" line.
2. Label the new connector "VHF TX enable".
3. Use a SPDT switch to remove 12V from the "VHF TX enable" relay at the same time that you energize a coaxial relay that connects the 2400 MHz downconverter to the 144 MHz antenna connector.

All of this assumes that opening the "VHF T8" line will actually disable the 144 MHz PA.  Maybe I'll open up the radio again soon to verify it, but I wouldn't mind if somebody beat me to it and reported the result on amsat-bb.   ; )

Wayne Estes W9AE
Mundelein, IL, USA

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