SECTION 7
hlaintmnce
7.7.6 input Offset Voltage Adjust
If the input amplifier op amp, or a component in the off-
set voltage adjust circuit is replaced, a course adjustment
to cancel input offset voltage will have to be performed.
Repladng any of the following components will require
the pot adjustment to be made: U142, R191, R195, R198,
RZO, and R215.
Perform the following steps to adjust offset voltage:
1.
2.
3.
4.
Turn on the instrument and allow it to warm up for
two hours.
Configure the Model 486/487 as follows: Zero check
disabled and the 2mA range selected.
Connect the Model 197 DMM across the input and
select the 2oOmV range.
Using a small bladed screwdriver, adjust pot R198
for a display reading of less than 5OpV on the Model
197.
7.7.7 Fusible Resistor Replacement
A fusible resistor (R203) is used to protect the analog out-
put circuitry.
CAUTION
To prevent damage to the instrument, ad-
here to the following precautions when
floating the input above chassis ground:
1. Do not connect ANALOG OUTPUT LO
to CHASSIS. If there is a ground link in-
stalled at the rear panel binding posts, re-
move it.
2. Make sure that any monitoring ins&u-
ment connected to the analog output is float-
ing.
Paragraph 3.16.8 explains the precautions for
floating measurements in more detail.
In the event that the fusible resistor WO3) becomes dam-
aged, replace it with the the following type:
7-19
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