HP 740B Operating And Service Manual page 67

Model 740B
Freon, Carbon Tetrachloride or any chemical solvent
on the switches.
5-109. MAIN LOOP FEEDBACK DIVIDER REPLACE-
MENT (A9R4 through A9R9).
5-110. The Main Loop Feedback Divider consists of
AVRIf
flifbugn~-A9Rt2".-
-A9R4-thfough-':A.9R9·'al'e-a
matched set of precision wirewound resistors. A9R10,
A9R11 and A9R12 are adjustments that calibrate the
divider and are adjusted for meter indication values
printed on the guard shield cover when performing the
Internal Alignment Procedure (Paragraph 5-57). The
meter indication values are derived at the factory for
each resistor set/instrument combination and should
be valid for the life of the resistors. If A9R4-A9R9 are
ever replaced, new meter indication values for adjust-
ments A9R10, A9R11 and A9R12 must be derived.
After installing ~he new resistors, the following pro-
cedure should be performed.
a. Perform steps a through i of Paragraph 5-62.
b. Set INTERNAL ALIGNMENT switch to positions
10, 11 and 12 adjusting A9R10, A9R11 and
A9R12 for a meter indication of zero at each
position. Return INTERNAL ALIGNMENT
Switch to Operate position.
c. Connect Model 740B to a Precision Divider,
1 V Reference Supply and Null Detector set-
up in the configuration shown in Figure 5-1.
- - - - - N O T E - - - - - -
1. The Precision Divider must be
very accurate -- preferably
better than ± 5 ppm division ac-
curacy at the 10:1, 100:1 and
1000:1 taps. Construction of
the divider shown in Figure 5-1
is described in Appendix C.
2. Calibration ofthe 1 V Reference
Supply is not highly important for
this procedure, but the Supply
must have good short-term sta-
bility characteristics.
Section V
h. Adjust Model 740B REFERENCE COARSE and
FINE Adjustments (A1R40 and A1R41) until
o
V ± 1 Jl V is indicated by the Null Detector.
(Periodically check Null Detector Zero setting).
The Model 740B output now matches the 1 V
Reference Supply output.
i. Return Model 740B VOLTAGE SET and
STAL~DARD VERNIER Co t ols to 0-0-0-0-0-0.
Set Sl·to position 1.
j. Connect Model 740B + SENSE and + OUTPUT
leads to point K (10: 1 division tap) on the
divider.
k. Set Range to 10 V and adjust ZERO Control
for a null indication (± 1 Jl V) on the Null
Detector.
1.
Set Sl to position 2; set Model 740B controls
for a 10 V output on the 10 V Range. Do not
vary ZERO Control.
m. Adjust A9R10 until Null Detector indicates
null(±l JlV). This calibrates the 10:1 division
accuracy of the new Main Loop Feedback
Divider resistors.
n. Return 740B controls to 0 V; set Sl to position
1; connect + SENSE and +OUTPUT to point L
(100:1 division tap) on the divider.
o. Set RANGE to 100 V and ZERO control for a
null indication (± 1
Jl
V) on the Null Detector.
p. Set Sl to position 2; set Model 740B controls
for a 100 V output on the 100 V Range. Do
not vary ZERO Control.
q. Adjust A9R11 until Null Detector indicates
null (± 1
Jl
V). This calibrates the 100: 1
division accuracy of the new resistors.
r. Return 740B controls to 0 V; set Sl to position
1; connect + SENSE and +OUTPUT to pointM
(1000: 1 division tap) on the divider.
d. Set Model 740B controls as follows:
FUNCTION .
RANGE .
VOLTAGE SET.
STANDARD VERNIER
OUTPUT .
. STD
1V
.00000
o
(fully ccw)
. . . . . On
s. Set RANGE to 1000 V and adjust ZERO Con-
trol for a null indication (± 1 Jl V) on the Null
Detector.
t. Set Sl to position 2; set Model 740B controls
for a 1000 Voutput. Do not vary ZEROCon-
trol.
e. Set Sl (in test setup) to position 1.
f. Adjust Model 740B ZERO Control for a null
indication (± 1 Jl V) on the Null Detector.
g. Set Sl to Position 2. SetModel 740B VOLT-
AGE SET and STANDARD VERNIER Controls
to 9-9-9-9-9-10 for a 1 V output on the 1 V
Range. Do not change ZERO Control Setting.
01794-1
u. Wait 60 seconds for the effects of self -heating
to stabilize and then adjust A9R12 until Null
Detector indicates null (± 1
Jl
V). This cali-
brates the 1000: 1 division accuracy of the
new resistors.
v. Return VOLTAGE SET and STANDARD
VERNIER Controls to 0-0-0-0-0-0. Discon-
nect test setup.
5-31