HP 740B Operating And Service Manual page 64

Section V
5-76. LOOP GAIN CHECK.
5-77. The following steps outline a method of evaluating
open loop gain of the Main Loop. If gain is too high
or too low, reselection of A3R9* (Paragraph 5-91) may
be helpful. Generally, insufficient gain causes slow
instrument response to an input voltage and may cause
inaccuracy in AVM or STD modes. Excessive gain may
cause output noise and instability. To perform the
Loop Gain Check, proceed as follows.
Model 740B
e. Move one of the ohmmeter leads to the rear-
middle terminal. Connect a jumper wire from
A16V1 (terminalnearestA13)toA16V2 (termi-
nal nearest T2). Ohmmeter should indicate
< 38 kU (Modulator parallel impedance).
f. Connect ohmmeter leads to A16V3 (black wire)
and A16V4 (white/black/yellow wire). Ohm-
meter should indicate > 78 kU (Demodulator
series impedance).
a. Set Model 740B controls as follows:
FUNCTION . .
RANGE . . . .
SENSITIVITY.
INPUT . . . .
. . VM
10 mV
. . X10
Shorted
g. Leave ohmmeter leads connected toA16V3
andA16V4. Connecta 20 MF capacitor across
A16V3 (black wire and white/yellow/blue wire).
Ohmmeter should indicate
<
64 kU (Demod-
ulator output impedance).
b. Depress Gain Check Switch, S10 (on bottom of
instrument, behind Guard Shield).
c. Meter should deflect to between + 8% of end-
scale and + 15% of end -scale. If meter deflects
to more than + 15% end-scale, gain is too low;
if deflection is less than + 8%, gain is too high.
If meter does not deflect, check AllBT1 (1. 3 V
minimum).
5-78. METER LOOP CHOPPER CHECK.
5-79. The following procedure can be used to evaluate
the performance of the Meter Loop Chopper, A16.
The chopper photocells change impedance somewhat
during instrument warmup; some effects of the imped-
ance change do not disappear for at least 24 hours. A
meaningful check of the chopper operation can be per-
formed only if the instrument has been turned off for
24 hours (or longer) prior to performing the check.
The tests should be performed in a controlled envi-
ronment with the ambient temperature remaining at
24
0
C ± 2
0
C (75
0
F ± 4
0
F). Since the chopper impedance
changes during warmup, all of the checks should be
performed during the period of 1 minute to 5 minutes
after instrument turn-on. Steps d and e check the
Chopper Modulator; steps f and g check the Demod-
ulator. A high input impedance ohmmeter (-hp-
Model 427A), a 1 MF capacitor (-hp- Part No. 0160-
3033) and a 20 MF capacitor (-hp- Part No. 0180-
0300) are required for the check. Do not turn on the
instrument until step d.
a. Connecta 1 MFcapacitorbetweenthe Common
and Ohms leads of the ohmmeter. Leave the
capacitor connected while making the imped-
ance measurements.
b. Remove A2 and DC Fuse. Set RANGE to 1 V.
c. Connect ohmmeter leads to A16V1 (terminal
nearest A13) and A16V2 (terminal nearest
T2). Carefully turn FUNCTION Switch to a
position between AVM and VM. Ohmmeter
reading will increase when switch is positioned
correctly. This isolates the Modulator from
any input voltage.
d. Turn on the instrument; wait at least one
minute. Ohmmeter should indicate >346 kU
(Modulator series impedance).
5-28
- - - - - N O T E - - - - -
If the instrument passes all of the
checks, A16 is working properly.
If one or more of the check results
are marginal (within 20%of passing)
and the Meter Loop performance is
worse than marginal, A!6maynot be
the source of trouble. Before re-
placing A16, check the waveshape at
A8TP21 to verify proper operation
of the Meter Loop Neon Driver. The
correct wave shape is shown in Fig-
ure 7-12. Components inA16arenot
separately replaceable.
5-80. MAIN LOOP CHOPPER CHECK.
5-81. The following procedure canbe used to evaluate
the performance of the Main Loop Chopper, A17. The
chopper photocells change impedance somewhat during
instrument warmup; some effects of the impedance
change do not disappear for at least 24 hours. A
meaningful check ofthe chopper operation can be per-
formed only if the instrument has been turned off for
24 hours (or longer) prior to performing the check.
The tests should be performed in a controlled envi-
ronment with the ambient temperature remaining at
24
0
C ± 2
0
C (75
0
F ±4
0
F). Since the chopper impedances
change during warmup, all of the checks should be
performed during the period of 1 minute to 5 minutes
after instrument turn-on. Steps d and e check the
Modulator; steps f and g check the Demodulator. A
high impedance ohmmeter (-hp- Model 427A), a 1 MF
capacitor (-hp- Part No. 0160-3033) and a 20 MF
capacitor (-hp- Part No. 0180 -0300) are required for
the check. Do not turn on the instrument until step d.
a. Connect a 1 MF capacitor between the Common
and Ohms leads of the ohmmeter. Leave the
capacitor connected while making the imped-
ance measurements.
b. Remove A3, A4, A5 and rear panel de fuse.
Set FUNCTION to VM; RANGE to 1 V.
c. Connect ohmmeter to A17V1 (red wire) and
A17V2 (white wire).
d. Turn on instrument; wait at least 1 minute.
Ohmmeter should indicate> 160 kU (Modulator
series impedance).
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