TM 9-4935-601-14-7&P
Model 8640B
INSTRUMENT MODIFICATIONS
4. Press Fine Tune RFI Braid Ring against the
subpanel with the Retaining Collar and secure
collar with Setsscrews. Verify operation of the
Oscillator as shown in Operator's Checks in
Section III.
7-23. A26A4 AGC Amplifier Assembly
Improvement (Serial Prefix 1245A and
Below)
7-24. On instruments with serial number prefix 1245A
and below, the AM bandwidth in the 0.5 to 2 MHz ranges
can be improved by modifying the A26A4 AGC
Amplifier Assembly (part number 08640-60015).
Change C10 (see Service Sheet 12) from 0.027 µF to
0.018 pF (HP 0160-0302).
7-25. On the same instruments, adding R54 825 ohms
(HP 0757-0421) between the collector of Q8 and +5.2V
will improve the transistor's reliability. Unsolder and lift
the collector lead from the board and add the resistor
between the collector lead and the circuit board hole.
7-26. Reduction of RFI Leakage From RF
Oscillator (Serial Prefix 1246A and Below)
7-27. On instruments with serial number prefix 1246A
and below that have excessive RFI leakage from the A3
RF Oscillator Assembly, RFI leakage can be reduced by
replacing the RF Oscillator Transistor cap. The new cap
is shorter and includes two RFI braid gaskets. The old
transistor cap should be replaced with the following
parts:
Qty Description HP Part No.
1 Cap Transistor 08640-20224
1 Braid Plug 8160-0203
1 Gasket Plug 8160-0233
NOTE
The cap is located on the bottom
side of the cylindrical part of the
oscillator.
7-28. Replacement A7 FM Shaping Assembly
(Serial Prefix 1246A and Below)
7-29. On instruments with serial number prefix 1246A
and below, a new (replacement) A7 Assembly will
require modification to be compatible with
an A9 Peak Deviation and Range Switch Assembly that
does not have coaxial cable A9W1 (see paragraph 7-
32). The new A7 Assembly will have coaxial jack A7J1
(see Service Sheets 6 and 7), the original assembly
does not.
7-30. Refer to Figure 7-9 and Service Sheet 7.
Modify the board as follows:
1. Add a wire jumper between the two pads above
pins 12 and 13 on board. Coaxial jack A7J1 will
not be used.
2. The A7 FM Shaping Assembly characteristics
must be matched to the RF Oscillator. Perform
the Preliminary FM, FM Sensitivity and FM
Linearity Adjustments in Section V.
7-31. Replacement A9 Peak Deviation and Range
Switch Assembly (Serial Prefix 1246A and
Below)
7-32. On instruments with serial number prefix 1246A
and below, a new (replacement) A9 Assembly will
require modification to be compatible with an A7 FM
Shaping Assembly with a 08640-60030 part number
(see paragraph 7-28). The new A9 Assembly will have
coaxial cable A9W1 (see Service Sheets 6 and 7), the
original assembly does not. The modification consists
of replacing A9W1 with an insulated wire and adding
three capacitors. The following parts are required:
Qty Description HP Part No.
1 A9C6 150 pF 0140-0196
1 A9C7 750 pF 0160-3538
1 A9C8 1500 pF 0160-2222
7-33. Refer to Figure 7-10 and Service Sheet 6.
Modify the switch as follows:
1. Remove the coaxial cable leads from switch
wafer S3C rear and S3B front lug 11. Strip the
ends of a 81/2-inch length of insulated wire, #24
AWG, color code white-yellow-gray (948), and
connect it between wafer S3B front lug 11 and
pin 24 of connector A9P1. Solder both ends of
wire.
7-81
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