TM 9-4935-601-14-7&P
Model 8640B
INSTRUMENT MODIFICATIONS
7-50. Power Supply Improvement (Serial Prefix
1313A and Below)
7-51. On instruments with serial number prefix 1313A
and below, the reliability of the voltage regulators in the
power supply can be improved by adding pre-loading
resistors as follows (refer to Service Sheets 22 and 23):
1. On A18 -5.2V Regulator and Fan Driver
Assembly (with part number 08640-60004), add
R19 2610 ohms (HP 0698-7246) between pins 5
and 6 of U1.
2. On A20 +5.2V and +44.6V Regulator Assembly
(with part number 08640-60005), add R27 2610
ohms (HP 0698-7246) between pins 5 and 6 of
U1.
3. On A22 +20V and -20V Regulator Assembly
(with part number 08640-60006), add R26 10
kΩ and R27 to 10 kΩ (HP 0698-7260), between
pins 5 and 6 of U2 and U1 respectively.
NOTE
The resistors are most easily
installed by carefully lifting the leads
(pins 5 and 6) of the integrated
circuits out of the holes, inserting a
wire or test-point type of pin in the
holes, and then soldering both the
resistor and integrated circuit leads
to the pins.
7-52. A10A2 RF Divider Assembly Improvement
(Serial Nos. 1316A00464 and Below)
7-53. On instruments with serial numbers
1316A00464 and below and an A10OA2 Assembly with
an 08640-60023 part number, C5 1000 pF (from
collector of Q3 to ground) can cause spurious signals
from 500-1000 MHz. To prevent this, remove C5 (see
Service Sheet 11).
7-54. Improved Audio Output Level Flatness
(Serial Prefixes 1244A through 1316A)
7-55. On instruments with serial number prefixes
1244A through 1316A,changing R1 (see Service Sheet
9 or 9A) requires that the AUDIO OUTPUT LEVEL knob
be changed (MP9, Standard, MP6 Option 001); order
part number HP 0370-2387 (1V knob for standard
oscillator) or HP 0370-2381 (3V knob for Option 001
oscillator). Perform the modulation oscillator adjustment
in Section V.
7-56. A8A4 Counter Display Assembly
Improvement (Serial Prefixes 1323A and
Below)
7-57. On instruments with serial number prefix 1323A
and below, and an A8A4 Assembly with an 08640-60025
part number, if the time base UNCAL lamp burns out,
the time base will not be calibrated. To prevent this,
refer to Service Sheet 20 and add R5, 46.4 kS2 (HP
0698-7276) across the leads of UNCAL lamp DS1. To
accommodate R5, change R3 to a smaller size 51.1 k2l
resistor (HP 0698-7277).
7-58. Improved FM Bandwidth on 5 kHz Deviation
Range (Serial Number Prefixes 1323A and
1327A)
7-59. A slight peaking in FM deviation at
approximately 200 kHz rates may be reduced by adding
A9C8. See paragraph 5-21 for selection of the proper
value.
7-60. Improved Noise Performance (Serial Prefixes
1327A and Below)
7-61. On instruments with serial number prefix 1327A
and below, the SSB noise floor and residual AM may be
improved by increasing the breakdown voltage of
A26A4VR2 to 16.2V (see Service Sheet 12). With the
generator in a non-pulse modulation mode, the old
breakdown diode (14.7V) may conduct slightly and turn
on A26A4Q7. The noise generated by the low level
diode breakdown is amplified by A26A4Q7 and Q5, and
appears on the output signal as AM noise. To make the
modification, order HP Part Number 1902-0184 and
replace A26A4VR2 on A26A4 AGC Amplifier Assembly.
Check the operation of the generator's pulse modulation
and check either residual AM or the SSB noise floor.
7-62. A10A2 RF Divider Assembly Improvement
(Serial Number Prefixes 1333A and Below)
7-63. Spurious response and second harmonic level
can be reduced by adding a 1000 pF capacitor (HP
0160-3456) between the heat sink (-5.2V line) of
A10A2U14 and ground. Some RF Divider boards have
holes to accommodate the new capacitor. If the board
does not, solder the capacitor (C54) directly to the heat
sink lead (-5.2V) and to the ground plane.
7-86
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