Constant Zero Reading - HP 3456A Operating And Service Manual

Service
2 V
2 msec
Figure 8·B·3. AID Zero Waveform.
can be determined by checking for the correct signatures
at A20U402. If a Signature Analyzer is not available,
the faulty area can be determined by statically checking
the AID Converter's Digital Circuitry. This method is
in the AID Digital Circuitry Troubleshooting paragraph
(Paragraph 8-B-3I). The preferred method is as follows.
Do the following:
I . Turn the 3456A Off.
2. Connect and set-up the Signature Analyzer as
follows (Schematic 8):
Stan: A30J3I-SRT
Stop:
A30J3I-STP
Clock: A30J32-A
Gnd: A30J3I-GND
Hold:
Self-Test:
3. Move A30J33/J34 to the "2" (middle) position
(Schematic 8), as shown in Figure 8-B-2.
4. Turn the 3456A On.
5. Connect the DATA PROBE of the Signature
Analyzer to A20TP406. Connect one end of a
clip lead to A20TP407. Using the other end of
the lead, take the
(Schematic 4):
Signature
P08U
HAU7
6635
FHCC
A733
I. If any signatures are wrong, troubleshoot the AID
Digital Circuitry (go to Paragraph 8-B-27). If the
signatures are correct, the AID Analog Circuitry is at
fault. Continue with the next step.
m. To troubleshoot the AID Analog Circuitry, do
the following:
I . Turn the 3456A Off.
2. Carefully remove the A30 Assembly from the in­
strument. Set it aside.
8-8-4
Neg. Tfi9.
Out
Out
following signatures
Chetked at
A20U402 Pin 2
A20U402 Pin 12
A20U402 Pin 6
A20U402 Pin 10
A20U404 Pin 7
3.
Using a clip lead, carefully short across A2OC403
(Integrator Capacitor, Schematic 4).
4.
Make sure jumper A20J29 is still in the GND
position.
5 .
Turn the 3456A On.
6.
Using a high impedance Digital Voltmeter (like
the 3456A), measure for zero volts «
at A20TP402 (use ground #4 as low).
7.
I f the voltage is other than zero, the integrator is
at fault. Replace Q406 or U405.
8.
I f the voltage at TP402 is zero, caiefully apply
+ 5 V dc from an external power supply to pin 6
(gate) or Q406.
9.
Measure for approx.imately - .7 V at TP403.
10.
I f the voltage is wrong, the slope amp is defec-
tive. Replace U406 or diode CR402.
1 1 .
I f the voltage is good, measure for approximate-
ly zero vohs at TP405.
12.
If the voltage is not zero, replace U409.
13. If the voltage is good, remove the + 5 V from
Q406 and apply - 5 V to pin 6 of Q406.
14. Measure for approximately + .7 V at TP403.
15. I f the voltage is wrong, replace CR403.
i s
16. If the voltage good, measure for + 5 V at
TP40S.
17. I f the voltage is wrong, replace U409.
1 8 . If the voltage is good, replace C403 .
8·8·22. Conltlnt Zero Re.ding.
8-8-23. A Constant Zero Reading can be caused by the
analog or digital circuitry in the AID Converter. The
Digital Circuitry troubleshooting is in Paragraph
8-B-27. The procedure to isolate the Digital Circuitry
from the Analog Circuitry
troubleshooting information), is as follows. Do the
following:
a. Turn the 3456A Off.
b. Make sure the Integrator Capacitor A2OC403
(Schematic 4) is not shorted. Replace C403 , if shorted.
c. Make sure the FrontlRear Switch of the instru­
ment is in the FRONT position.
d. Make sure the cable from the Isolation Logic Con­
nector (A3J7) is unplugged (Schematic 12).
e. Make sure A30J35 and J36 are in the Non-Run (2
and 3) position (Schematic 8).
f. Make sure A30J331 J34 is in the " I " (Run) posi­
tion (Schematic 8).
g. Place the AID Test Jumper A20J29 (Schematic 4)
in the GND position, as shown in Figure 8-B- 1 .
h . Turn the 3456A On.
Model 3456A
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± 1 0 mY)
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(plus the analog
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