Inaccurate Or Noisy Readings; Normal Mode Rejection; Range Failure 8 - HP 3456A Operating And Service Manual

Service
d. I f a Constant Zero is displayed, do the following:
I . Press the 3456A's RESET button.
2. Set the instrument to the IO V Range and the 6
Digit Mode (by storing 6 into the N DIGIT DISP
Register).
3. Make sure jumper A20J29 is in the GND position
(as shown in Figure 8-8-1), before doing Ihe next
step.
4. Turn Autozero Off.
5 . Remove jumper J29 from the GND position and
set it aside.
NOTE
Make sure the removed jumper is placed
back into the RU N position, when finished
with the procedure.
6. Apply an accurate and stable I O V dc to
A20TP40I . (Use the Low VOLTS Terminal as the
reference poinl or ground). If an accurate voltage
is not available, use one of which the exact value is
known.
7. If an overload or zero is displayed, the AID Con­
verter or Inguard Logic as at fault. Go
Paragraph 8-8-16 for further isolation.
8. If the reading is inaccurate (not within the 3456A's
IO V DCV specifications), the AID Converter is at
fau l t . Go to Paragraph
troubleshooting.
9. I f the IO V reading is good, the Input Switching or
Input Amplifier is at fault. Go to Service Group C
for troubleshooting.

Inaccurate or Noisy Readings.

8·B·10.
8-8-1 1 . Inaccurate or Noisy Readings can be caused by
the AID Converter, Input Amplifier, or Input Swit­
ching. These failures can also show up as a noisy
reading or an offset with the 3456A's Input Terminals
shorted. To determine the faulty circuitry, do the
following:
a. Press the 3456A's RESET button.
b. Set the instrument to the IO V range and the 6
Digit Mode (by storing 6 into the N DIGIT DISP
Register).
c. Turn Autozero Off.
d. Place jumper A20J29 in the GND test position, as
shown in Figure 8-B-1 (Schematic 4).
e. If the reading on the 3456A's display is other than
zero volts ( > ± 3 counts), the 3456A has an Inaccuracy
Failure caused by the AID Circuitry. Go to Paragraph
8-B-18 for troubleshooting.
f. If the reading is good, remove jumper 129 from the
GND position and set it aside.
8-B-2
Make sure the removed jumper is placed
back into the RU N position, when finished
with the procedure.
g. Apply an accurate and stable I O V dc
A20TP40 1 . (Use the Low VOLTS Terminal as the
reference point or ground). If an accurate voltage is not
available, use one of which the exact value is known.
h. If the reading is inaccurate or unstable (not within
the 3456A's IOV DCV specifications), the AID Con­
verter is at fault. Go to Paragraph 8-B-18 for
troubleshooting.
i. I f the I O V reading is good, the Input Switching or
Input Amplifier is at fault. Go
troubleshooting.
8·8·12.
8-8- 1 3 . Most Range Failures are caused by the
Inguard's
However, Range Failures can also be caused by the In­
guard Logic. This can be determined by checking for the
10
correct output of the Port Expander (A20U702, in the
Range and Function Control Logic). The correct out­
puts in the 3456A's functions and ranges are given in
various tables in Service Group C,
the Service Groups first, if a range failure is noted. Once
8 - 8 - 1 8 for
it has been determined that the Inguard Logic is at fault,
go to Paragraph 8-8-32 for troubleshooting.
8·8·14.
8-B- 15. This failure can be caused by the ALE Syn­
chronizer Circuitry or Main Clock in the Inguard Logic
(Schematic 8). If this failure is suspected, do the follow­
ing:
a. Make sure the 3456A has the 5.85 MHz crystal for
a 60 Hz power source
Hz power source (Schematic 8).
b. Make sure the ALE Synchronizer Circuitry is
good. Check the circuitry by making sure pins I and 2 of
A30U5 are toggling (the pins are changing from high to
low). If they are not toggling, replace U7 or U8. If they
are toggling, replace U5 or U6.
8-8·16.
CIRCUITRY.
8-8-17. An Overload, Constant Zero. or Test Failure
can be caused by the Inguard Logic or AID Converter.
Since the main purpose of the Inguard Logic is to con­
trol the AID Converter, the circuitry should be isolated
from each other. Do the following:
a. Turn the 3456A Off.
b. Set the FrontlRear Switch o f the instrument to the
FRONT position.
NOTE
10
Range Failure.
Range and Function

Normal Mode Rejection.

O J
the 4.875 MHz crystal for a 50
ISOLATING THE INGUARO LOGIC FROM THE AIO
Model 3456A
10
Service Group C for
Control Logic.
0,
and E. Refer to
�,