FT-857/897 Transceiver has ACC jack which can be used for keying the transmitter (Page 33 of FT-857 manual). This 3.5-mm 3-pin jack accepts external ALC (Automatic Level Control) voltage from a linear amplifier on the tip connection, and accepts a “Transmit Request” command on the ring connection. The main shaft is the ground return.
The “TX Request” connection, when shorted to the ground, puts the FT-857D into the transmit mode, and sends out a steady CW carrier, for linear amplifier or manual antenna tuner adjustment.
So it seems like a simple task: ground the ring and give some negative voltage (0V to -4V) and we can tune antenna tuner or amplifier with lower power (this is especially useful with automatic antenna tuners because they will not tune if full power is applied), if we ground just the ring we will tune with full power.
All this is not entirely true, when we connect ground and negative voltage at the same time we get short spike at which transmitter gives out full 100W and then drops to preset value, this happens because of delays in ALC circuits.
I tried to came up with schematic that would prevent this behavior.
When S2 is pressed negative ALC voltage is applied but keying because of C3 is delayed, finally transistor T1 opens and through diode shortens center ring to the ground.
The battery is 12v micro battery used for remote doorbells and it sells in every supermarket. When none of buttons are pushed no power is drawn from battery thanks to diode D1. I have been using this thing for about 10 years and battery still going strong.
I used old radio components I had in my junk box, transistor can be replaced with any n-p-n all purpose transistor like 2N2222, diode replaced with 1N4148 or similar.
There is another use of ACC Jack: it can be used to key amplifiers or switch Transmit/Receive antennas, if you have separate receiving antenna like 160M Beverage antenna. When opened top cover on your radio there is jumper J27, when it is moved to second position instead of transmit REQ it becomes TXINH so when your radio goes into transmit it grounds center ring, but in this case you cannot use this tuning aide anymore, if you want both the same TXINH is routed on CAT/TUNER Mini DIN so you can take it from there.
73 Jonas N0LY/LY3QA