TB 9-6625-1188-24
e. Set POWER switch to OFF and wait one minute. If meter does not indicate zero,
adjust meter zero screw cw until pointer moves up-scale and then starts to move down-
scale toward zero. Continue adjusting cw until pointer is directly over zero mark.
NOTE
Numerical values, control settings, and adjustments pertaining
to model 428B are shown in parenthesis when they differ from
model 428A.
NOTE
When indications specified in paragraphs 8 and 9 are not
within tolerance, perform the power supply check prior to
making adjustments. After adjustments are made, repeat
paragraphs 8 and 9. Do not perform power supply check if all
other parameters are within tolerance.
8. Electrical Zero and Stability
a. Performance Check
(1) Set POWER switch to ON and RANGE switch to 3 MA (1 MA).
(2) Adjust TI meter to 2.7 (0.9), using ZERO (ZERO and VERNIER) control.
(3) Vary autotransformer between 105 and 125 V ac and then back to 115 V. If
meter drift does not remain within 0.5 mA of value set in (2) above, perform b (1) through
(4) below, and if necessary, b (5) through (9) below.
(4) Adjust ZERO (ZERO and VERNIER) control until meter indicates zero.
b. Adjustments
(1) Set POWER switch to OFF.
(2) Squeeze probe head flanges together and clean mating surfaces with brush
furnished with TI. Repeat a (1) through (3) above. If this method is insufficient, probe will
have to be disassembled as in (3) and (4) below.
CAUTION
Do not twist cable during disassembly and reassembly.
(3) Unscrew probe center section and clean surfaces with pencil eraser.
(4) Reassemble probe so that polarity arrow points toward side of terminal strip
with heavy red lead of probe cable. Repeat a (1) through (4) above.
NOTE
If zeroing difficulties still remain after cleaning, demagnetize
probe head as outlined in (5) through (8) below.
(5) Set POWER switch to ON.