
TM 9-4935-601-14-7&P
Model 8640B
INSTRUMENT MODIFICATIONS
below. The fuse blows because of a momentary short
on the primary input that can occur on a switch terminal
used in wiring the pilot lamp. The modification is simple
and requires no special tools.
7-87.
Modify the switch as follows:
a. Remove the power line cord.
b. Remove instrument bottom cover and side
cover nearest line power assembly.
c, Remove nut that secures line switch to front
panel and lift line switch free of instrument.
d. Slip insulation tubing off line switch to expose
wiring.
e. Remove white-green-gray (958) wire from
switch and clip off near cable sleeve. This wire will no
longer be used.
f. Unsolder resistor lead from terminal where the
white-green-gray (958) wire was previously attached and
solder it to the terminal where the white-red-gray (928)
wire is attached. Switch wiring should now appear as in
Figure 7-12.
Figure 7-12. Modified Line Switch Wiring
g. Slip insulation tubing back over switch and
install switch into front panel.
h. Remove white-green-gray (958) wire from
terminal C of line power assembly and clip off near
cable sleeve.
i. Unsolder white-yellow-gray (948) wire from
terminal E and solder it to terminal D (where black-
green (05) wire is also attached).
j. Insert power cord and check instrument for
proper line switch operation.
k. Reinstall covers.
7-88.
Reduction in Popcorn Noise on Power
Supplies (Serial Number Prefixes 1406A and
Below)
7-89.
Excessive popcorn noise (i.e., random voltage
shifts on the order of 1 mV) can be reduced by changing
the IC regulator (A18U1, A20U1 and U2, A22U1 and U2)
to HP 1826-0177. The noise can cause random shifts in
frequency of the RF oscillator which is particularly
sensitive to noise on the ± 20V supplies. After
modification perform the Power Supply Adjustments
(Section V).
7-90.
Recommended Fuse Replacement for 220/
240V Operation (Serial Number Prefixes
1423A and Below)
7-91.
The recommended replacement for the power
line fuse Fl to 220 or 240V operation is 1.25A normal
blow, HP Part Number 2110-0094. The higher current
rating will prevent occasional blowing of the fuse at turn
on.
7-92.
To show the new fuse rating on the line power
module, an adhesive-backed label is available
(A14MP1, HP Part Number is 7120-4264).
7-93.
"REDUCE PEAK POWER" Lamp Drive
Modification (Serial Number Prefix 1526A
and Below)
7-94.
In some instruments, the REDUCE PEAK
POWER annunciator lamp will come on or glow dimly
when no error condition exists. This is the result of
leakage through A26A4Q8 turning on Q9 (see Service
Sheet 12).
7-89
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