HP 740B Operating And Service Manual page 65

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Model 740B
Section V
a. Set Model 740B controls as follows:
5-88. Use the following procedure to evaluate the
selection of A2R19*.
Sensitivity. A2R19* is a 1/4 watt composition ±10%
resistor with a typical value of 68 kn. A2R19* may
have to be reselected if the Meter Loop Chopper
(A16)
is replaced or if repair work is performed on the
Meter Amplifier (p/o A2).
e. Move one of the ohmmeter leads to the middle
terminalonA17. Connecta jumper wire from
A17V1 (red wire) to A17V2 (white wire). Ohm-
meter should indicate < 30 kn (Modulator
parallel impedance).
f. Connect ohmmeter leads to A17V3 (black wire)
andA17V4 (white/red/blue wire). Ohmmeter
should indicate> 132 kn.
g. Leave ohmmeter leads connected to A17V3
andA17V4. Connecta 20 IlFcapacitoracross
A17V3 (middleterminalandblackwire). Ohm-
meter should indicate < 42 kn (Demodulator
output impedance).
FUNCTION . .
RANGE . . . .
SENSITIVITY
. . . VM
.1000 V
. . . Xl
L...
,...
L...
- - - - - N O T E - - - - -
If the instrument passes all of the
checks, A17 is working properly.
If one or more of the check results
are marginal (within 20% of passing)
and the Main Loop performance is
worse than marginal, A17 may not
be the source of trouble. Before
replacing A17, check the wave-
shape at A8TP20 to verify proper
operation of the Main Loop Neon
Driver. The correct wave shape is
shown in Figure 7-12. Components
inA17 are not separately replaceable.
5-82. REPAIR PROCEDURES.
5-83. The following paragraphs contain information on
special repair procedures and replacement parts
selection for the Model 740B.
I
WARNING
I
TO PROTECT THE OPERATOR FROM POS-
SIBLE HIGH VOLTAGE SHOCK WHEN FLOAT-

NG THE MODEL 740B, CHECK THE CON-
TROLKNOBS FOR PRESENCE OF INSULAT-
ING MATERIAL IN SET-SCREW HOLES.
IF MATERIAL IS NOT PRESENT, FILL
THE SET-SCREW HOLES WITH GENERAL
ELECTRIC TRANSLUCENT SILICON RUBBER
RTV -108 (-hp- PART NUMBER 0470-0304).
5-84. REPLACEMENT OF FACTORY SELECTED
COMPONENTS.
5-85. Certain components within the Model 740B are
individually selected in order to compensate for
slightly varying circuit parameters. These com-
ponents are identified by an asterisk (*) on the schematic
diagrams and a typical value is shown. The following
paragraphs describe the function of the factory selected
components and give replacement instructions.
5-86. A2R19*.
5-87. A2R19* is factory selected to set the response
time of the Meter Loop to 2 to 3 seconds on the X10 4
01794-1
b. Apply a + O. 1 V input voltage to the Inputtermi-
nals.
c. Depress X10 4 SENSITIVITY.
Meter should
deflect to
+
end scale in 2 to 3 seconds.
5-89. If response time is longer than 3 seconds, de-
crease the value of A2R19* in 5 kn steps until response
time is between 2 and 3 seconds. Do not use a value
less than 30 kn for A2R19*.
5-90.
If
response time is shorter than 2 seconds, in-
crease the value
of
A2R19* in 5 kn steps until response
time is between 2 and 3 seconds. Do not use a value
greater than 100 kn for A2R19*.
5-91. A3R9*.
5-92. A3R9* is factory selected to control the gain of
the Low Level Amplifier (A3). A3R9* is a 1/4 watt
composition ± 10% resistor with a typical value of
22 kn. A3R9* may have to be reselected if the Main
Loop Chopper (A17) is replaced or if repair work is
performed on A3. To determine if A3R19* is the
correct value, perform the Loop Gain Check (Para-
graph 5-76). If the instrument passes, A3R9* should
not be changed.
5-93. If the instrument has low gain (meter deflection
> 15% end-scale), increase the value
of
A3R9* in 5 kn
steps until the meter deflection is between 8% and 15%
of end-scale. Do not use a value greater than 47kn
for A3R9*.
5-94. If the instrument has too much gain (meter de-
flection <8%end-scale), decrease the value of A3R9*
in 5 kn steps until the meter deflection is between 8%
and 15% of end-scale. Do not use a value less than
6.8 kn for A3R9*.
5-95. A7C8*.
5-96. A7C8* is factory selected to compensate for
the effects of inter-element capacitance in the Power
Switch transistors (Q1 and Q2). A7C8* is a ± 20%
ceramic 1000 V capacitor with a typical value of
O. 00681lF.
A7C8* may have to be reselected if Q10r
Q2 is replaced or if repair work is performed on the
Pulse Width Converter (p/o A5).
5-97. To determine if A7C8*is the correct value,per-
form the following procedure.
5-29