IET Labs, Inc. 1864-1644, User And Service Manual

Page: 35 / 46

1864-1644 Positive Polarity Megohmmeter
25
SERVICE AND MAINTENANCE
If all the voltages are out of tolerance in the same
direction, they can be set within the tolerance by
adjusting R140 located on etched-circuit board P/N
1864-2701 (common to both the 1863 and 1864 Meg-
ohmmeters and shown in both Figures 6.2 and 6.4).
The adjustment can be made as soon as the instru-
ment is removed from the cabinet (paragraph 6.3). It
is not necessary to move either of the printed-circuit
boards, since the adjustment is on the top printed-
circuit board. This adjustment affects all voltages by
the same amount, but adjustment at 200 V minimizes
possible errors due to resistance tolerances.
If all the voltages are correct but all meter readings
are in error in the same direction, the meter sensitiv-
ity can be reset. Adjust R243 (Figures 6-2 and 6-4),
located on the same etched-circuit board as R 140,
to correct the meter readings. This adjustment affects
ail measurements but on the 1863 is most sensitive
at 200-V and 250-V and least sensitive at 100 V.
In the 1864, it is most sensitive at the lower set-
tings of the
fi rst digit of the test voltage adjustment,
i.e. 100 V, 200 V, etc.
6.5.4 Range-Resistor Accuracy
The range resistors can be checked by performing
procedure steps in of section 6.2.
6.5.5 Coarse
∞
Adjustment
If it is impossible to set the in fi nity controls on the
front panel, set both controls at their center positions
and adjust R244 (Figures 6-2 and 6-4), located on
the etched-circuit board with R140, for a reading as
close to
∞ as possible. Make the fi nal adjustments
with the front-panel controls.