HP 740B Operating And Service Manual page 66

Section V Model 740B
a. Set Model 740B controls as follows:
b. Observe waveshape at Ql - Q2 collectors.
(Waveshape is shownin Figure 7-11). Wave-
shape should be stable and noise-free.
FUNCTION .
RANGE .
VOLTAGE SET.
. . STD
IV
.00000
b. To remove a component, clip a heat sink
(long nose pliers, commercial heat sink
tweezers, etc.) on the component lead as
close to the component as possible. Place
the soldering iron directly on the component
lead, and pull up on the lead. If a component
is obviously damaged or faulty, clip the leads
close to the com ponent, and remove the leads
from the board.
5-98. If the waveshape is unstable or noisy, reselect
A7C8*. Change A7C8* in approximately 0.002 JlF
steps (either increasing or decreasing) until Ql - Q2
collector wave shape stabilizes. Value limits for
A 7C8* are 0 to 0.02 JlF.
5-99. A8R8*, A8R9*.
5-100. A8R8* and A8R9* control the range of adjust-
ment of the Meter Loop Chopper Frequency Adjust-
ment, A8R3. A8R8*andA8R9*are 1/4W composition
±
5% resistors with a typical value of 75 kn. A8R8*
and A8R9* may have to be reselected if repair work
is performed on the Meter Loop Neon Driver (p/oA8).
A8R8* and A8R9* should be reselected only if the
Meter Chopper Frequency Adjustment cannot be set
to 95 Hz (Paragraph 5-38).
5-101. To reselect A8R8* andA8R9*, use the following
procedure.
a. Set A8R3 (Meter Chopper Frequency Adjust-
ment) to mid-value.
b. Measure Meter Chopper Frequency (Para-
graph 5-38).
c. If measured frequency is greater than 100 Hz,
increase the value of A8R8* and A8R9*. If
frequency is less than 90 Hz, decrease the
values of A8R8* and A8R9*. A resistance
change of 10 kn affects the chopper frequency
approximately 20 Hz. The resistance values
for A8R8* and A8R9* must be equal and
should never be changed to less than 40 kn or
greater than 150 kn.
5-102. A13R13*, A13R14*.
5-103. A13R13* and A13R14* are factory selected to
match the voltage characteristics of A13CRI with the
output voltage requirements of the Reference Oven
Assembly (A13). Components within A13 are factory
matched and aged. For this reason, none of the com -
ponents within A13 are separately replaceable.
5-104. SERVICING ETCHED CIRCUIT BOARDS.
5-105. The Model 740B contains plated-through double-
sided, etched circuit boards. When working on these
boards, observe the following rules to prevent damage
to the circuit board or components:
a. Use a low-heat (25 to 50 watts) soldering iron
with a small tip.
5-30
EXCESSIVE OR PROLONGED HEAT
CAN LIFT THE CIRCUIT FOIL FROM
THE BOARD OR CAUSE DAMAGE TO
COMPONENTS.
c. Clean the com ponent lead holes by heating the
solder in the hole, quickly removing the
soldering iron, and inserting a pointed, non-
metallic object such as a toothpick.
d. To mount a new component, shape the leads
and insert them in the holes. Clip a heat sink
on the component, heat with the soldering
iron, and add solder as necessary to obtain a
good electrical connection.
5-106. SERVICING ROTARY SWITCHES.
5-107. The Model 740B contains several rotary
switches: SI RANGE/FUNCTION Switch, S4 - S8
VOLTAGE SET Switches, S9 STANDARD VERNIER
Switch, and Kl Output Rotary Switch (solenoid-actuated).
When replacing components on these switches, observe
the following rules:
a. Usea low heat (25 to 50 watts) soldering iron
with a small tip.
b. When replacing components, attempt to dress
them as nearly to their original alignment as
possible.
c. Clean excessive flux from the connection and
adjoining area.
d. After cleaning the switch, apply a light coat
of lubriplate to the switch detent balls. DO
NOT apply lubricant to switch contacts or
allow lubricant to contaminate components.
e. If switch knobs were removed and reinstalled
during the switch repair, the set screw holes
should be filled with General Electric Trans-
lucent Silicon Rubber RTV -108 (-hp- Part No.
0470 -0304).
5-108. The switches are self-cleaning in normal use;
but may, with time, build up a dust accumulation.
Generally, the effects of dust can be eliminated by
rotating the switches from stop-to-stop several times.
(K1 can be rotated by cycling the OUTPUT pushbutton
switch several times with the instrument turned on
and the Output Cable Assembly connected.)
If
dust
accumulation is severe, a small amount of distilled
water may be used as a cleaning agent. DO NOT use
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