TM 9-4935-601-14-7&P
Model 8640B
INSTRUMENT MODIFICATIONS
2. Connect A9C8, 1500 pF, between switch wafer
S3B front lug 10½ /2 and wafer S3C front/rear
lug with the white-yellow (94) wire, bare wire, a
1500-ohm and a 300-ohm resistor. Solder the
end of C8 at wafer S3B front lug 101/2.
3. Connect A9C7, 750 pF, across switch wafer
S3B front/rear lug 10 and wafer S3C at the
same lug as A9C8. Solder both ends of A9C7.
4. Connect A9C6, 150 pF, across switch wafer
S3B front/rear lug 91/2 and wafer S3C front/
rear lug with two 3000-ohm resistors and a bare
wire to the lug on either side. Solder both ends
of A9C7.
5. Perform the Peak Deviation and Range Switch
Adjustment and the Range Switch Adjustment in
Section V.
7-34.
A10A1 RF Filter Assembly Switch
Improvement (Serial Prefix 1246A and
Below)
7-35.
On instruments with serial number prefix 1246A
and below, the reliability of the slide-bar switches can be
improved by modifying the AlOA1 Assembly. The
modification consists of replacing the plastic slide-bar
switch clamp with one made of aluminum.
Replacement requires the use of special fixtures and
tools and should be performed by Hewlett-Packard.
Contact your local HP office for more information.
NOTE
The plastic slide-bar switch clamp is
not available for replacement. If
found to be defective, it must be
replaced by an aluminum clamp.
7-36.
A8A1 RF Scaler Assembly Improvement
(Serial Prefix 1249A)
7-37.
On instruments with serial number prefix 1249A
and an A8A1 Assembly with an 08640-60041 part
number, low level triggering in EXT mode can be
improved by increasing isolation between the internal
and external signal paths. Refer to Service Sheet 18
and Figure 7-7 and add C7 0.01 /F (HP 0160-3879) from
the junction of R2 and CR5 to circuit board ground.
7-38.
A5 FM Amplifier Assembly Improvement
(Serial Prefix 1251A and Below)
7-39.
On instruments with serial number prefix 1251A
and below and an A5 Assembly with an 08640-60029
part number, spurious oscillations may occur in the FM
system in the FM OFF mode. To prevent this, refer to
Service Sheet 6 and add C9 430 pF (HP 0160-0939)
from pin 7 on the circuit board connector to circuit board
ground.
7-40.
A8A2 Counter/Lock Board Assembly
Improvement (Serial Prefix 1251A and
Below)
7-41.
On instruments with serial number prefix 1251A
and below and an A8A2 Assembly with an 08640-60027
part number, the pulse width into the Stall Counter may
be wide enough to overlap the clock input. The Stall
Counter will then count for 10 clock pulses instead of 9
and the instrument will phase lock to a frequency that is
one count higher than the indicated count. To prevent
this, refer to Service Sheet 20, change C12 to 100 pF
(HP 0160-3466) and remove R48 (196 ohms) and in its
place add L2, 15 pH (HP 9100-1620).
7-42.
Front Panel Meter M1 Replacement (Serial
Prefix 1251A and Below)
7-43.
When replacing the panel meter for instruments
with serial number prefix 1251A and below, the front
panel will also have to be changed to one with a hole
which accommodates the front panel zeroing. The hole
already exists in the sub-panel.
To replace the meter and front panel, order a current
panel meter (HP 1120-0539) and front panel (HP 08640-
00063) and proceed as follows:
1. Remove top and bottom covers, top trim strip,
front window, and all knobs, nuts, and washers
that secure lower half of front panel. The RF
OUTPUT connector is most easily removed
from behind the sub-panel. Avoid forcing
modulation oscillator's frequency dial cursor disk
(Option 001 only); gently work it along the shaft.
2. Replace front panel with new one. Replace the
nuts and washers that secure the connectors
and switches. Replace the FINE TUNE,
FREQUENCY TUNE, RANGE, PEAK
DEVIATION,
7-83
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com