TM 9-4935-601-14-7&P
Model 8640B
INSTRUMENT MODIFICATIONS
7-64. A18, A20, and A22 Power Supply Regulator
Improvements (Serial Number Prefixes
1333A and Below)
7-65. Changing the power supply regulator driver
transistors will reduce the possibility of oscillation. If any
of the power supplies oscillate, change either A18Q2,
A20Q2, or A22Q2 and Q3 to a new transistor (HP 1854-
0232) with a lower cutoff frequency (15 MHz as opposed
to 200 MHz). After completion of the modification,
perform the Power Supply Adjustments and the
appropriate performance tests (see Post-Repair Tests
and Adjustments table in Section V).
7-66. Time Base Vernier and Pot Replacement
(Serial Prefix 1333A and Below)
7-67. On instruments with serial number prefix 1333A
and below, changing the TIME BASE VERN knob will
also require changing the TIME BASE VERN pot
A8A4R1. The old knob (HP 08640-40007), which is no
longer available, extended through a hole in the front
window and would occasionally bind. The new knob
stands in front of the window and requires a pot with a
longer shaft. Also, since the new knob has a larger skirt,
it may be desirable (but is not necessary) to replace the
front window (MP35) since the skirt will slightly cover
some of the markings near it. The window has the same
part number as before, but the silk-screening has been
revised to accommodate the new knob.
NOTE
Instruments with serial prefix 1339
have the old knob but the new pot
and window.
7-68. To make the modification, order the parts listed
below:
Qty. Description Part Number
1 Knob, TIME BASE 08640-40067
VERN
1 Resistor, Var, 5k 2100-0647
ohm/SPST SW
1 Window, Front 08640-40049
7-69. Replacement of the pot is straightforward. To
gain access to the part, remove the A8A2 assembly as
described in Service Sheet B.
7-70. A11 Variable Frequency Modulation
Oscillator Improvements (Serial Number
Prefixes 1339A and Below)
7-71. Distortion in the Variable Frequency Modulation
Oscillator (Option 001) at high frequencies can be
reduced by changing AllQ1. (Refer to Service Sheet
9A.) The new transistor has a higher frequency
response. Distortion will improve mainly on the x3k
band (60 to 600 kHz) and only for the signal at the audio
output jacks. The change will not affect the signal into
the AM and FM circuits.
7-72. To make the modification, order HP Part
Number 1853-0050 and replace A11Q1 on All Variable
Frequency Modulation Oscillator. Perform the Internal
Modulation Oscillator Test (check voltage levels only)
and the Internal Modulation Oscillator Distortion Test
(Option 001).
7-73. A12 Rectifier Assembly Input Crowbar
Failures (Serial Number Prefixes 1339A and
Below)
7-74. If the input crowbar SCR A12Q1 should fail, do
not replace it. Instead, the entire A12 Rectifier
Assembly should be replaced with a new assembly. HP
Part Number 08640-60190. The crowbar on the new
assembly has a built-in protection circuit and time delay.
As a temporary measure, the old A12Q1 and A12VR1
(refer to Service Sheet 22) should be removed until
replaced by the new assembly. The assembly is located
directly behind the power transformer.
CAUTION
With the input crowbar disabled, care
must be taken to ensure that the Line
Power Module is set to the correct
line voltage. Failure to do so could
result in severe damage to major
portions of the instrument. Also
check that the proper line fuse is in
place.
7-87
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