Inaccuracy Or Noise - HP 3456A Operating And Service Manual

Model 3456A
i. Connect the vertical input of an oscilloscope to
A20TP405 (use ground 114 as low). With the scope in the
external trigger mode, connect the trigger input to
TP404. Using the scope set-up information in Figure
8-8-3, check the waveform shown in the Figure
(Schematic 4).
j. If the waveform is good, the AID Digital Circuitry
is good. The analog circuitry may be defective. 00 to
step m.
k. If the waveform is wrong or not present, the AID
Analog or Digital Circuitry is the cause. The faulty area
can be determined by checking for the correct signatures
at A20U402. I f 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-D-3 1). The preferred method is as
follows. Do the following:
I . Turn the 3456A Ofr.
2. Connect and set-up the Signature Analyzer as
follows (Schematic 8):
A30J3I-SRT
Start:
Stop: A30J3I-STP
Clock: A30J32-A
Ond: A30J31-0ND
Hold:
Self-Test:
3.
Move A30J33/J34 to the "2" (middle) position
(Schematic 8), as shown in Figure 8-8-2.
4. Turn the 3456A On.
Connect the DATA PROBE of the Signature
S.
Analyzer to A20TP406. Connect one end of a
clip lead to A20TP407. Using the other end of
the lead, take
(Schematic 4):
POSU
HAU7
663S
FHCC
A711
I. If any signatures are wrong, troubleshoot the AID
Digital Circuitry (go to Paragraph 8-8-27). If the
signatures are good, 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.
3 . Connect pin 1 0 o f A20U408 (or pin 6 of P30) to
ground (use ground #4).
4. Turn the 3456A On.
5. Using a logic pulser, toggle pin 9 of U401
(momentarily short pin 9 of U401 to ground).
'-
( )
'-
( )
'-
( )
Out
Out
the following signalUres
Checked at
A20U402 Pin 2
A20U402 Pin 12
A20U402 Pin 6
A20U402 Pin 1 0
A20U404 7
6. Measure for approximately - 1 2 V at the gates of
Q404 and Q405. If the vohages are wrong, check
for a defective U40I, U407, or U408. A high
(zero volts) should be noted at pin 2 and 3 of
U40 I , and pin 1 3 of U408. Replace the defective
IC, if any of the voltages are wrong. I f the
voltages are good, make sure Q404 and Q405 do
not have a source to drain short (use an ohm­
meter). Replace, if shorted or very leaky. I f the
FETs are good and operate correctly, remove the
short from pin 1 0 of U408 (or pin 6 or P30) and
continue with the next step.
7. Using a clip lead, carefully short across A2OC403
(Integrator Capacitor).
8. Make sure jumper A20J29 is still in the OND
position.
9. Using an external power supply, carefully apply
+ 5 V dc to pin 6 (gate) of Q406.
10. Using a high impedance Digital Voltmeter (like
( ±
the 3456A). measure for approximately + 5 V
l OO mY) at TP402.
1 1 . I f the voltage is other than 5 V, the Integrator
may be at fault. Replace U405 or Q406.
12. I f the voltage is good, measure for approximate­
ly - .7 V at TP403.
13. I f the voltage is zero or another value, make sure
diodes CR402 and CR403 are not shorted.
Replace, if defective. If the diodes appear good,
replace U406.
14. I f the votlage is good, measure for approximate­
ly zero volts at TP405.
1 5 . I f the voltage is not zero, replace U409.
16. I f the voltage is good, remove the + 5 V from
Q406 and apply - 5 V to pin 6 of Q406.
17. Measure for approximately + .7 V at TP403.
18. I f the voltage is wrong, replace CR403.
19. If the voltage is good, measure for + 5 V at
TP40S .
20. If the voltage is wrong, replace U409.
8-8·24. Inaccuracy or Noise.
8-8-25. Inaccuracy is most likely caused by the AID
Analog Circuitry, including resistor network A20U500
or Inverter U402. If the 3456A is very inaccurate
( > 501171), use the Overload Troubleshooting Procedure
in Paragraph 8-B-20.
8-B-26. Noise is most likely caused by the AID Analog
Circuitry. However, an intermittent in the Digital Cir­
cuitry can also cause noise. This can be checked using
Signature Routines. To troubleshoot for noise and inac­
curacy, do the following:
a. Determine the type of failure.
b. If the 3456A is noisy or jumpy, do the following:
Service
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