For both the DX-70 & DX77 a 9 volt battery can be used
in a circuit with a 10K, 25K or 50K potentiometer. The pot
should have a on-off switch so that the battery can be
disconnected when not in use.
The - lead of the battery is connected to one side of
the switch, the other side of the switch is connected to
a resistor of the same value as the potentiometer. The
other end of the resistor is connected to one end of the
potentiometer. The resistor is simply a safety part
to cut the -9 volts down to -4.5 volts. The wiper of the
potentiometer is then routed through a signal diode with
the cathode (banded end) connected to the wiper. The anode
end is connected through a shielded cable to the ALC phono
plug on the back of the rig. The + end of the battery
and the ground (shield) of the cable is connected
to the third side of potentiometer.
This detail is as I remember it. I built my bias box
about 3 years ago & since I'm on a business trip I can't
verify all the details. The idea is to allow the negative
bias to adjust the power down to the desired level. Since
it is an open "loop" its normal to see some power level
change during the 1st part of a transmission. This is
apparently due to the parts heating up in the xmtr.
My bias box is built into a small Radio Shack
enclosure that I place on top of the rig.
72, Tom Howey WB1FPA