Model 148 Input - Keithley 148 Instruction Manual

MAINTENANCE
MODEL 148 NANOVOLTMETER
FIGURE 14.
Model 148 Input Compartment.
Note exact physical location of parts.
If replacing, duplicate location and order of leads on posts.
Figure 21 gives
circuit designations.
4-3. MECHANICAL CHOPPER REPWXMKNTS.
a. The mechanical chopper
is
designed for long life. However, since it is
mechanical, it will eventually wear and become noisy.
At this point, replace-
ment is necessary.
b.
Removal Procedures.
1.
Disconnect the chopper drive coil at connector 5302 (Figure 20).
Carefully remove the three thumb screws in the input compartment and the
three chopper lead lugs.
2.
Carefully slide the degaussing coil, L302 (Figure 20), from the chopper
body; remove the old chopper.
c.
Replacement Procedures.
1.
Slide the degaussing coil, L302, over the new chopper body from the bottom.
Orient so that the cut-out in the coil fits over the chopper drive lead.
2. Mount the chopper and coil in the input compartment.
Dress leads as
shown in Figure 14.
26
0365