Note: This wire needs to be as short as practically possible, ie place the Ground Tuner side by side or on top of the Transmitter.

Attach a length of random wire to the binding post marked Output to random length of wire (Counterpoise Wire or Ground Connection Wire). Be careful not to reverse these wires.

The random wire should be a quarter-wave length or less at the operating frequency. Place the random length of wire on the floor and tape up the open end of the wire to avoid any RF burns.

To obtain maximum RF ground current for a low impedance RF ground, alternately adjust the two controls on the front panel, the Switch for Inductance and the Variable Capacitor. Turn the Inductance Switch first until the highest amount of current can be seen on the RF Ammeter. Then adjust the Variable Capacitor control to peak the amount of RF current.

Try several inductance setting for the highest readings.

What if the needle goes off the scale or doesn't move at all? Then the sensitivity of the Ammeter needs to be adjusted. Turn the Sensitivity pot control until the needle is in the middle of the scale. Then readjust the Switch for Inductance until you get the highest amount of RF current and use the Variable Capacitor to peak the RF current. If the needle is still off scale or zero, readjust the Sensitivity control and repeat the tuning process.

The Black Commercial ATU to Ground Tuner mod

The Commercial ATU an FC-125 consisted of an RF sensing circuit a large coil a 11 position ceramic switch prewired to the switch but not the coil.

I added a 180 Pf wide spaced Variable Capacitor the largest size I could fit into the ATU Case and altered the wiring to the circuit shown above.

As the coil was large I reduced it to 43 Mh inductance and tapped the coil as per the MFJ-931 which uses a similar sized coil/variable capacitor combination.

For the lucky builder who might have or be able to acquire an FC-125 the coil tappings are as follows: Turns 1 to 6 tap every turn,Turns 7 to 12 tap every second turn,tap turn 20 and the last turn turn 23 (ie 1,2,3,4,5,6,8,10,12,20,23).

Note the Variable Capacitor must be insulated from the FC-125 casing .

For those inquiring minds the meter trim pot is located on the RF sensing board and can be adjusted by drilling a small hole on the top case cover otherwise the top cover needs to be removed each time.

73! de ZS1JHG

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