
TM 9-4935-601-14-7&P
Model 8640B
INSTRUMENT MODIFICATIONS
and MODULATION FREQUENCY (except
Option 001) knobs.
3. Remove A4 Meter/Annunciator Drive Assembly.
4. Remove A8 Counter/Lock Assembly by
removing four corner screws and three RF
connectors on bottom of assembly. Use the
special wrench provided with instrument to
remove the two screw-on connectors.
5. Remove A6 Annunciator Assembly by
unplugging miniature 8-pin connector A6P1 and
removing two screws on front of meter support
molding.
6. Remove meter by gently pressing frontward on
support molding and lifting meter up and out.
Remove the two meter leads.
7. Connect the meter leads to new meter. (The
white lead goes to the "+" terminal.) Insert
meter, taking care not to scratch meter face.
Check that the solder lugs on the meter
terminals do not short to each other or to the
chassis.
8. Replace A6 Annunciator Assembly, A8
Counter/Lock Assembly, and A4 Meter/
Annunciator Drive Assembly.
9. For the Option 001 modulation oscillator,
carefully install the dial and check the
oscillator's output with a counter to make sure
that the dial cursor disk gear is meshed in the
correct cogs. Install the modulation oscillator
RANGE and vernier knobs; tighten the vernier
knob first.
10. Install the AUDIO OUTPUT LEVEL knob and
check its scale accuracy with an external meter
by comparing the AM OUTPUT level at 1 kHz
into 600
Ω
with the scale indication. Adjust the
knob position until the scale is accurate to within
± 20% for all markings.
11. Perform the Meter Adjustment procedure in
Section V. The offset adjustments may be
omitted.
12. Perform the Output Level Knob Adjustment
procedure in Section V; the gears, however,
should not need adjustment.
13. Install the covers, trim strip, and front window.
7-44.
A26 AM/AGC and RF Amplifier Assembly
Improvement (Serial Prefix 1303A and
Below)
7-45.
On instruments with serial number prefix 1303A
and below, operation of the AM and pulse modulation
circuits can be improved by making the following
changes. The changes are recommended at time of
service or repair. Refer to Service Sheets 12 and 13.
1. To improve AM bandwidth on an A26A4 AGC
Amplifier Assembly with part number 08640-
60014), change R13 and R15 to 14.7 k
Ω
(HP
0698-3156) and change R22 to 1.78 k
Ω
(HP
0757-0278).
2. To improve RF pulse shape (specifically, to
reduce the 90-100% rise time) on an A26A4
AGC Amplifier Assembly with part number
08640-60014, change R41 and R42 to 100 k
Ω
(HP 0757-0465).
3. To improve general circuit reliability on an
A26A2 AM Offset and Pulse Switching
Assembly with part number 08640-60015,
change CR9 to HP 1901-0539.
7-46.
A26A4 AGC Amplifier Assembly
Improvement (Serial Prefix 1310A and
Below)
7-47.
On instruments with serial number prefix 1310A
and below, and with an A26A4 Assembly that has an
08640-60014 or 08640-60015 part number, AM noise
performance will be improved by changing C14 from
220 pF to 0.1 pF (see Service Sheet 12). The correct
part to install is HP 0160-0576. The new capacitor has
0.1-inch lead separation, and the leads should be
carefully formed to fit into the present 0.2-inch spacing
of the printed circuit board. This change is
recommended at time of service or repair.
7-84
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