Dcv Troubleshooting; All Ranges Inoperative (Overload, Zero Reading, Accuracy Or Noise); Some Ranges Inoperative - HP 3456A Operating And Service Manual

Model 3456A
8-C- 1 . GENERAl.
8-C-2. This Service Group has the DCV. Ratio, and
Analog Filter troubleshooting information for the
3456A. The Service Group is symptoms oriented (what
fails) with different levels of troubleshooting. The first
level uses procedures to determine the general area in
the 3456A which causes the failure. The other levels
have troubleshooting information for the areas.
8-C-3. PRE-TROU8LESHOOTlNG INFORMATION.
8-C-4. Various Switching FETs and Relays are used in
the DCV Function. The switches are turned On and Off
by Comparators and Transistors. The Comparators and
FET
Transistors are controlled by the Inguard logic. I f a
or Relay is suspected defective, make sure the logic
is correct before replacing the FET or Relay. A FET is
normally Off with its gate at
gate bias level). Go to Service Group B if the Inguard
Logic is inoperative.
(
8-C-5. Some troubleshooting procedures in this Service
Group requires that the 3456A's Autozero must be
either On or Off, to determine the failure. Make sure
the instrument
troubleshooting.
8·C·6. SWITCH CLOSURE COMMAND TABLE.
8-C-7. Table 8-C-1 has the commands used to control
the state of the Inguard Switches. Use the table to deter­
mine which FET or Relay is On or Off in each function
and range.
B·C·8. DCV TROUBLESHOOTING.
8-C-9. The following information can be used to
troubleshoot the 3456A for any Inoperative DCV
Ranges, DCV Accuracy, and DCV Noise Failures.
8·C·l0. All Ranges Inoperative (Overload, Zero Relding,
Accuracy, or Noise).
8-C- I I . This failure can show up as a constant
Overload (Ol), Zero Reading, Noise, or Inaccuarcy on
all ranges. The Input Switching, Input Amplifier, and
AID can cause the failure. To determine the faulty cir­
cuitry, do the following:
a. Press the 3456A's RESET button.
b. Set the instrument to the 10 V Range and the 6
Digit Mode (by storing "6" into the N DIG DlSP
Register).
SERVICE GROUP C
- 18 V (and turns On at the
i s in the given state,
c. Position the jumper on A20J29 as shown below.
usoo
d. Turn Autozero Off.
e. I f the reading on the 3456A's display is other than
zero volts ( ± 3 counts), the AID Converter is in­
operative. Go to Service Group B for troubleshooting.
f. Remove the jumper from 129.
g. Apply a stable and accurate + 10 V dc to the
VOLTS Input Terminals. If an accurate voltage is not
available, use one of which the exact value is known.
h. Using a clip lead, connect the High Input (High
VOLTS Terminal) to A20TP401 .
i . If the reading is not a stable and accurate + 1 0 V,
make sure the Reference Voltages are good. Check for a
when
stable - 12 V at A20TP501 and TP502, and a stable
+ 1 2 V at TP503. If the vohages are good, the AID
Converter is defective. Go to Service Group
troubleshooting.
Reference is defective. Go to Service Group F for
troubleshooting.
j. With a high impedance and accurate Digital
Voltmeter (like the 3456A), measure for + 10 V at either
side of A20R103. If the voltage is good, the Input
Amplifier is at fault. Go to Paragraph 8-C-35 for
troubleshooting.
k. If the voltage is wrong or noisy, unplug the cable
from A20Jl9.
I. If the voltage is now good, the Input Amplifier is at
fault. Go to Paragraph 8-C-35 for troubleshooting.
m. I f the voltage is still wrong, the Input Switching is
at fault. Go to Paragraph 8-C-l7 for troubleshooting.
S·C·12. Some Ranges Inoperative.
8-C·B. This failure can show up as a Constant
Overload (Ol), Zero Reading, Noise, or Inaccuracy on
some ranges. This is often caused by certain defective
FETs and Switches in the Input Switching and Input
Amplifier Circuitry.
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0 0 0
J29
I f the voltages are wrong,
Rev B
Service
for
B
the
8-C-l