
TM 9-4935-601-14-7&P
Model 8640B
INSTRUMENT MODIFICATIONS
7-75.
Reliability Improvements in AM and Pulse
Circuits (Serial Number Prefixes 1345A and
Below)
7-76.
The reliability of some of the switching logic in
the AM and pulse circuits (especially at elevated
temperatures) can be improved by changing certain
germanium diodes to hot carrier diodes. The new
diodes will prevent the possibility of the RF detector
filter capacitors (A26A1C5, C6 or C7) being switched in
when not selected. This can occur if the reverse
leakage current through the diodes (which flows from
the +20V to the +5.2V supply) is sufficient to turn on
transistorsA26A2Q8 or Q9 (refer to Service Sheet 13).
7-77.
To make the modification, order HP Part
Number 1901-0539. On A26A2 AM/Offset and Pulse
Switching Assembly, replace A26A2CR9 and CR13.
7-78.
All Variable Frequency Modulation Oscillator
(Option 001) Improvement (Serial Number
Prefixes 1350A and Below)
7-79.
Possibility of failure of the Variable Frequency
Modulation Oscillator (Option 001) can be reduced by
adding insulator A11MP17 between spur gear AllAIMP3
and the variable capacitor A11C1. The insulator
prevents the shaft or metal insert in the gear from
shorting the capacitor to ground. This modification is
recommended at time of oscillator service or repair.
7-80.
Install insulator as follows:
a. Order HP part number 08640-00087 (insulator),
b. Remove All Variable Frequency Modulation
Oscillator (see Service Sheet D).
c. Remove two 4-40 pan-head screws and two
hex-nuts which secure the Audio Oscillator Cover.
Remove cover.
d. Remove adhesive backing from insulator and
apply to the tuning capacitor, behind the plastic gear
A11A1MP3 and small spur gear A11AMP2.
NOTE
If there is insufficient clearance for
the insulator between gears and
capacitor, perform step e.
e. Remove back cover from the oscillator.
Then loosen three nylon screws which secure
the tuning capacitor to the PC board. This will allow a
small amount of shift in the capacitor's position and
afford clearance for the insulator. Tighten nylon screws
and reassemble back cover.
f. Replace Audio Oscillator Cover and reinstall All
assembly (see Service Sheet D).
g. Perform Internal Modulation Oscillator Test in
Section IV. If out of specification, perform Variable-
Frequency Modulation Oscillator Adjustment (Option
001) in Section V.
7-81.
Improvement in Lock Acquisition Capability
(Serial Number Prefixes 1350A and Below)
7-82.
The counter may fail to lock on random
occasions. This may be caused by the time base signal
present at A8A2TP6 which might instantaneously
exceed the phase lock error detector threshold when
going into the lock mode. Should this occur, connect a
1000 pF capacitor A8A2C31, HP Part Number 0160-
3456, between the line connecting to A8A2TP6 and
ground.
7-83.
+44.6V Regulator Improvement (Serial
Number Prefixes 1405A and Below)
7-84.
Changing A20Q4 regulator transistor to one with
a lower cutoff frequency will reduce the possibility of
oscillation. The recommended transistor is HP 1853-
0224. After modification perform the Power Supply
Adjustment for the +44.6V supply (Section V).
7-85.
Line Switch Modification (Serial Number
Prefixes 1406A and Below)
7-86.
If the line fuse blows as the instrument is
switched off the line switch and line power assembly
wiring (W16) should be modified as explained
7-88
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com