12
HARS-LX Series
12
Maintenance
5.4 Adjustments
If one or more resistances fall outside the limits, the
associated resistor should be adjusted. There is a
trimming network provided for each resistance in
these ranges. These may be accessed by removing the
housing and accessing the particular decade PC board.
Adjust the resistors from lowest to highest. The order
of decades does not matter.
Whereas it is possible to adjust any one resistance
step, note that the
n
th
step of a decade is the sum of
resistances 1 through
n
, so that errors are cumula-
tive. It is therefore recommended that whenever any
resistance of a particular decade is adjusted, that all
the resistances of that decade be tested and adjusted
as required.
To adjust any of the resistances, the following should
be observed:
•
Adjustment considerations
•
Adjustment procedure
5.4.1 Adjustment Considerations
Before adjusting or trimming any resistances, observe
the following:
The equipment measuring the unit should conform to
the guidelines provided in Section 5.3.
CAUTION
The HARS-LX front panel GND
binding post should be connected to
the test instrument’s ground point.
5.4.2 Adjustment/Trimming
Procedure
To trim or adjust any resistances,
proceed as follows
:
1. Stabilize HARS-LX at laboratory tempera-
ture of 23 °C for at least 8 hours.
2. Remove the 4 housing screws from the sides
of the instrument and remove the housing.
3. Using the binding posts in the 4-terminal
connection, connect HARS-LX to a resis-
tance meter.
4. Rotate all knobs approximately 10 times and
be careful not to rotate the minimum setting
decade beyond the stops.
Breaking in the switches helps elimi-
nate any residual contact corrosion
resistance.
5. Set all dials to 0, except for the minimum
setting decade.
The minimum setting decade has a mini-
mum stop at position 1.
6. Measure the minimum resistance setting
If an adjustment is needed, consult IET
technical support.
7.
Measure 1 mΩ, 10 mΩ, and 100 mΩ steps as
applicable to the HARS-LX being trimmed.
If adjustment is needed, proceed as follows:
•
If the resistance is
high
, reduce it by add-
ing a small bit of solder to an exposed
portion of the wire
•
If the resistance is
low
, increase it by
gently filing the wire in one spot.
8. Measure the higher decades. If an adjust-
ments is needed, proceed as follows:
Starting with the lowest setting, adjust the
associated trim pots.
See Table 5-1 and Figure 5-1 for
reference
9. Replace the housing and reinstall the 4 hous-
ing screws.
CAUTION
Make these steps carefully and slowly,
allowing the wire to cool down before
measuring again.