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Model 427A
Table 5-3,
1% Resistors
RESISTOR (2195)
-hp- PART NO.
0757-0984
0757-0198
0757-0159
0757-0839
0757-0367
0698-3583
0698-3592
5-47.
DC CIRCUITS.
5-48.
А precision dc voltage
source
is required for
de calibration
and performance
checks,
Since
the
427A is only calibrated on the 1 volt, 0,3 volt and 0.1
volt ranges,
only three different voltages are needed.
5-49.
Amercury battery has good short term stability.
Connect a series voltage divider with a variable output
across the battery and monitor the output with a recently
calibrated dc voltmeter knownto be four times as accur-
ate as the Model 427A.
Use this output to calibrate
the 0.1, 0.3, and 1 volt ranges.
5-50.
AC REJECTION.
5-51.
To check AC Rejection on 0.1 volt range
an
oscillator
with output of 10 volt peak-to-peak into 10
M ohms may be used.
I the -hp- Model 738BR Volt-
meter Calibrator
is available a 400 Hz 10 volt peak-
іо-реак output
may also be used to check the AC Re-
jection of Model 427A.
5-52.
INPUT IMPEDANCE
CHECK,
5-53,
Measure the resistance betweenthe VOLTS ала
COM terminals directly with the Model 427A Function
Switchat -DCV or -DCV using an accurate ohmmeter.
Another
Model 427A could be used for this purpose.
If an L-C
meter or capacitance bridge is available,
measure
the input capacitance between
the
VOLTS
and COM terminalsdirectly.
Be sure the Model
427A
FUNCTION
switch is in the ACV position.
5-54.
BATTERY
REPLACEMENT.
5-55.
Figure 5-7 shows the battery holder and the bat-
tery connectlons. Turnthe twist-lock fastener 1/4 turn
counterclockwise, tiltthe battery and holder toward the
rear of the instrument,
and lift out. Replace the battery
with an Eveready 763 or RCA VS102, 22 1/2 volt dry cell
=
~
~
ЗА E
|
427A-B-05654
P
Figure 5-7,
Battery Replacement
01791-3
Section V
5-56.
TROUBLESHOOTING.
5-57.
Whenthe Model 427A operates improperly, first
adjust and calibrate it according to the procedures in
Paragraph 5-25.
If calibration is impossible,
then
proceed with the troubleshooting steps.
5-58,
Check
the
instrument
for obvious evidence of
trouble,
such
as loose
or broken
wires
or broken
connectors,
Check
the printed
circuit
boards
for
cracks and separations,
and ensure that all connectors
and pins are clean and tight.
5-59.
Isolatethetrouble to a particular circuit using
the troubleshooting table (Table 5-4) and the Theory
of Operation (Section IV). Then refer to the Trouble-
Shooting steps for that circuit in the following para-
graphs.
Table 5-4,
Troubleshooting
PROBABLE
TROUBLE
=
Battery ok, no re-
Power Supply.
(See
sponse to input.
Paragraph 5-60.)
DC offset circuit. Check
А2СВ1.
Then check
А106.
DC Amplifier.
Check
A1Q8 and associated
circuit.
Check value
of range resistors.
Check A1R14 and А199.
DC Amplifier.
(See
Paragraph 5-63.)
All +DCV ranges pegged
with no input.
ACV and
OHMS ok.
Nonlinear tracking on
voltage ranges.
Constant upscale deflec-
tion on lower +DCV
ranges; voltage at TP1
low.
Meter pegs on +DCV
and OHMS
ranges,
DC Amplifier; check
A1Q6, A1Q8, and A1Q9.
(See Paragraph 5-65.)
Range resistor (Ra in
Figure 4-3) open.
Check R19 through
R25 on 51.
Range resistors (Въ іп
Figure 4-3) shorted.
Check R12 throughR18
оп 81.
Check Power Suppl
Either one of R19
through R25 (Rg) shorted
or one of R12 through R18
OHMS
range always
Teads near zero, DCV
and ACV ok.
OHMS
always reads =.
DCV and ACV ok.
open.
Intermittent operation.
Power supply, check А1-
Q4 (See Paragraph 5-60. )
Zero-offset in ACV
AC Metering Circuit
as
greater than
2
Meter pegs full scale
on any AC range,
(See Paragraph 5-67.)
.
AC Metering Circuit,
check А2СЕЗ and A2CR4
(See Paragraph 5-67.)
5-7
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