HP 740B Operating And Service Manual page 38

Model 740B
SECTION V
MAINTENANCE
Section V
5-1. INTRODUCTION.
5-2. This section contains maintenance instructions
for theModel 740B. Included are performance tests,
adjustment and calibration procedures, and trouble-
shooting and repair procedures.
5-3. REQUIRED TEST EQUIPMENT.
5-4. Table 5-1 lists the required test equipment
for maintenance ofthe Model 740B. Includedfor each
piece of test equipment are required characteristics
and a recommended commercial model. If the recom-
mended model is not available, select another model
with similar characteristics.
5-5. PERFORMANCE TESTS.
5-6. The performance tests presented in the following
paragraphs are front-panel procedures designed to
com pare the Model 740B with its specifications (Table
1-1). These tests maybe used for incoming inspection,
periodic maintenance and for specification tests after
repair or adjustment. The tests may also be useful
as a first step in troubleshooting (Paragraph 5- 69).
5-7. A Performance Check Test Card is included in
this section on pages 5-32a and 5-32b. The card lists
the test limits and provides spaces for recording the
test readings. The card can be removed from the
manual and used in conjunction with incoming inspection
to provide a permanent record of the instrument per-
formance.
5-8. In order to verify the accuracy specifications
with a high degree of certainty, itis necessary to use
a calibration system that is at least three times as
accurate as the specification being checked. The
most accurate mode of the Model 740B is Standard
where accuracy is within ± 24 ppm at maximum output
voltage on any range. This requires using a calibra-
tion system accurate to within ± 8 ppm to verify this
specification.
5-9. A calibration system consisting of a precision
divider, a 1 V reference supply and a null detector
is used (in various forms) in the accuracy tests which
follow. The divider is a string of adjustable transfer
resistors which have been accurately ratio matched.
Overall ratio accuracy is within ±2 ppm at all taps,
with no need to use correction factors. The 1 V re-
ference supply is a dc transfer standard, calibrated
to 1 V ± 2 ppm. The null detector is a sensitive dc
null voltmeter capab Ie of resolving off -null conditions
of less than 0.25 jlV. Using the technique described,
the overall accuracy of the system is within about
± 5 ppm -- well within the required ±8 ppm.
01794-1
5-10. Appendix C gives details on calibrating the
transfer standard and building the divider. Fixed
value standard resistors may be used in the divider
providing the correction factors for each resistor
are known (or derived) and applied.
5-11. The Meter Mechanical Zero Adjustment (Para-
graph 5-36) and the Internal Alignment Procedure
(Paragraph 5-57) should be performed before pro-
ceeding with the Performance Tests. These adjust-
ments calibrate the instrument for the temperature
existing at the time of adjustment.
If
the instrument is
mounted in a rack or otherwise relocated to an area of
different ambient temperature, the allowable accuracy
deviations (given in Tables 5-2, 5-3, and 5-4)must
be increased according to the following Temperature
Coefficient specifications:
a. DC standard mode: Less than (2 ppm of setting
or 1 ppm of setting, whichever is greater) per
°C, lOoC to 40 o C.
Example: Normal allowable accuracy deviation
for a 1 V output on the 1 V range is ± 24 /l V
obtained from the accuracy specification:
± (0. 002% of setting + 0.0004% of range). If
the temperature is changed 5
0
C, the allowable
accuracy deviation becomes ± 34
jl
V for the
'2
r,~
1 V output (original deviation of ± 24
jl
V plus
I
L
the temperature correction factor of 10
jlV).
b. Differential Voltmeter mode: (2 ppm of
reading + 1
jl
V) per °C, lOoC to 40 o C.
Example: Normal allowable accuracy deviation
for a 1 V measurement on the 1 V range is
± 55 jlV obtained from the accuracy specifi-
cation: ± (0. 005% of reading + 0.0004% of
range + 1 jlV). If the temperature is changed
5
0
C, the allowable accuracy deviation be-
comes ± 70 jl V for the same measurement
(original deviation of ± 55
jlV
plus the tem-
perature correction factor of 15
jl
V).
- - - - NOTE----
1. Ambient calibration tem-
perature and ambient oper-
ating temperature must be
between lOoC and 40 o C. Re-
lative humidity must be less
than 70%.
2. Allow a one hour warm -up
period before proceeding
with the performance tests.
5-1