TM 9-4935-601-14-7&P
Model 8640B
INSTRUMENT MODIFICATIONS
capacitor (C27, HP 0160-3877) from the output
of U13D (pin 11) to ground.
7-16.
Improved Audio Output Level Accuracy
(Serial Prefix 1229A)
7-17.
On instruments with serial number prefix 1229A,
the level of the internal modulation oscillator into the FM
system can be increased so that it can turn on the
REDUCE FM VERNIER annunciator (to verify its
operation) as follows:
1. For All Fixed-Frequency Modulation Oscillator
Assembly (with part number 08640-60020)
refer to Service Sheet 9. Change AllR7 to 2870
ohms (HP 0698-3151) and change AllR14 to
2610 ohms (HP 0698-0085). Perform
Fixed-Frequency Modulation Oscillator
Adjustment in Section V.
2. For All Variable-Frequency Modulation
Oscillator Assembly (Option 001) (with part
number 08640-60019) refer to Service Sheet
9A. Change AllR36 to 6190 ohms (HP 0757-
0290) and change AllR39 to 2370 ohms (HP
0698-3150). Perform Variable Frequency
Modulation Oscillator Adjustment in Section V.
7-18.
A26A1 Power Amplifier and AGC Detector
Assembly Improvement (Serial Prefix 1229A)
7-19.
On instruments with serial number prefix 1229A
and an A26A1 Assembly with part number 08640-60017,
level accuracy for narrow pulse widths in AM PULSE
mode can be improved by changing Q1 to HP 1853-
0015; refer to Service Sheet 13.
7-20.
Reduction of RFI Leakage from Front Panel
(Serial Prefix 1244A and Below)
7-21.
On instruments with serial number prefix 1244A
and below, RFI leakage from the front panel can be
reduced by adding RFI barriers to the main and fine
tune shafts and the output attenuator shaft. The
following parts are required:
Qty
Description
HP Part No.
2
4-40x1/8 Setscrew
3030-0007
1*
Braid Ring (Main Tune)
8160-0238
1
Braid Ring (Fine Tune)
8160-0239
1
Retaining Collar
08640-20228
1*
Attenuator Shield
08640-00065
*Order only for serial prefixes 1229A and below
7-22.
Remove A3 RF Oscillator Assembly (see
Service Sheet F for procedure) and Al Output Level
Assembly (see Service Sheet A for procedure). Modify
and re-install as follows:
A1 Output Level Assembly
1. Install gold Attenuator Shield on output
attenuator's cursor drive gear bushing. The
shield should be cupped toward front panel, and
the locating tabs should seat in the cursor
locating slots.
2. Before re-installing attenuator, apply a small
amount of grease (such as PLM grease) to
portion of subpanel where Attenuator Shield will
rub.. This will smooth rotation of vernier and
prevent squeaking.
3. Re-install attenuator. If cursor drive gear has
disengaged, follow the Output Level Knob
Adjustment procedure in Section V.
A3 RF Oscillator Assembly
1. Install Main Tune Braid Ring on oscillator's
FREQUENCY TUNE shaft. If braid appears to
be unraveling, apply solder to braid end. With
the braid on the shaft, make braid thinner by
pinching with pliers; this will ease installation of
oscillator.
2. Slip Retaining Collar and Fine Tune RFI Braid
Ring on FINE TUNE shaft. The braid may also
need soldering.
3. Re-install RF Oscillator. Check that the green
counter time base cable is not pinched under
the Oscillator.
7-80
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