ESD SYSTEMS 41273 Calibration Procedure

C C A A L L I I B B R R A A T T I I O O N N P P R R O O C C E E D D U U R R E E
Item #41273 LCD Megohmmeter
This procedure must be followed exactly as
outlined in order to achieve accurate
readings from the meter. Start by installing a
fresh battery to ensure the meter's power
supply has the correct operating voltages it
needs to give accurate results. The room in
which this calibration takes place should be
about 70°F and the humidity should not be
above 40% RH.
In order to proceed you will need to collect a few
1% precision resistors of the following values: 1
Meg Ohm, 22 Kilo Ohm, and a 1 Kilo Ohm
resistor. Each of the three resistors must be at
least 1% tolerance or better and 50ppm or
better. In addition you will need a pair of external
probe leads to connect the calibration resistors
to the meter. These should take the form of two
short lengths of wire, (about 6" @ 24 AWG) with
the proper size mono, male plug soldered on
one end and an alligator clip on the other end.
Begin by disassembling the meter case. Do not
remove the front label if there is one. Open
the meter by disassembling the two case
halves. Removing the front label will destroy
the meter.
To disassemble the meter, begin by removing
the three small threaded input jack retaining
rings on the front of the meter. Next turn the
meter over and remove the four screws. (One in
each corner of the meter.) Carefully turn the
meter back over keeping the two case halves
together. Next carefully lift the meter case front
off and set it on edge to the left side of the
meter. You will see two wires attached to the
front power switch. Be careful not to break
these wires or the meter will cease to
function.
Looking down onto the circuit board you will see
the three small, blue calibration adjustment
controls in the lower right corner of the board.
From left to right these controls are as follows.
The first one on the left is R11 or the Zero adjust-
ment. The second one to the right or the middle
one is R10 or the Gain adjustment. The last one
on the right is R9 or the Linearity control.
Zero Pot
BATTERY
Fig. 1. Front view with cover removed.
LCD
R11 R10 R9
Linearity Pot
Gain Pot