10
RS-925D
10
Maintenance
5.4 adjustments
If one or more resistances fall outside the limits, the
associated resistor should be trimmed. This applies
for 1
Ω
steps and over. There is a trimming network
provided for each resistance in these ranges. These
may be accessed by removing the housing and access-
ing the particular decade PC board.
If the minimum 10 mΩ needs to be adjusted, consult
IET Labs. For other resistors, 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 trimmed, that all
the resistances of that decade be tested and trimmed
as required.
To adjust any of the resistances, the following should
be observed:
•
Trimming considerations
•
Trimming procedure
5.4.1 Trimming Considerations
Before trimming any resistances, observe the
following:
The equipment measuring the unit should conform to
the guidelines provided in Section 5.3.
CaUTIOn
The RS-925D front panel should be
connected to the test instrument’s
guard point.
5.4.2 Trimming procedure
To trim any resistances,
proceed as follows
:
1. Stabilize RS-925D at laboratory temperature
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 RS-925D to a resistance
meter.
4. Rotate all knobs approximately 10 times.
Breaking in the switches helps elimi
-
nate any residual contact corrosion
resistance.
5.
Set all dials to 0, except for the 10 mΩ
decade.
10 mΩ decade has a minimum stop at
position 1.
6.
Measure the 10 mΩ setting
If an adjustment is needed, consult IET
technical support.
7.
Measure 1 mΩ, 10 mΩ, and 100 mΩ steps.
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
remeasuring.
