Turn off audio input to the modem by turning the receiver-
transmitter AUDIO GAIN fully counterclockwise.
Check meter for direct current (20 or 60 mA)
flowing through DC LOOP NO. 1
To maintain loop current flow
set AUTO MARK HOLD switch
to ON position.
Meter will register in the right scale (upscale) when
loop battery module INTERNAL-EXTERNAL switch
is in INTERNAL position. Meter will read in the left
scale (downscale) when in EXTERNAL position.