HP 3478A Technical Manual page 77

3478A 3478A
7-A-19. 7-A-19. PROTECTION PROTECTION
SHOOTING SHOOTING
7-A-20. 7-A-20. This circuit This circuitry ry consists
diodes in U102. diodes in U102. If all If all ranges fail, ranges fail, try
the ranges still fail or some ranges fail, the most likely the ranges still fail or some ranges fail, the most likely
cause is U102. cause is U102.
7-A-21. INPUT 7-A-21. INPUT CIRCUITRY CIRCUITRY TROUBLESHOOTING
7-A-22. 7-A-22. The Input The Input Circuitry
Switching Circuitry and the DC/Ohms Input Amplifier. Switching Circuitry and the DC/Ohms Input Amplifier.
Before troubleshooting the circuitry, make sure the Before troubleshooting the circuitry, make sure the
failure is failure is not ca not caused by used by the A/
paragraph 7-A-5 to determine the faulty circuitry, if it has paragraph 7-A-5 to determine the faulty circuitry, if it has
not been done already. not been done already.
7-A-23. Overload Readings 7-A-23. Overload Readings on All
7-A-24. 7-A-24. An overload An overload can be caused can be caused when the out
the DC/Ohms the DC/Ohms Input Input Amplifier is t Amplifier is too high.
caused by an excessive input to the amplifier, open caused by an excessive input to the amplifier, open
feedback, or feedback, or a defective a defective amplifier.
a. a. Set the 3 Set the 3478A to the DC 478A to the DC Volts Function and t
3V Range. 3V Range.
b. b. Make sure Autoz Make sure Autozero is turned o
7-A-8 step b). 7-A-8 step b).
c. c. Short the Short the 3478A's INPUT 3478A's INPUT Terminals.
d. d. With a high impeda With a high impedance Digital Volt
3456A) measure for zero volts (±1mV) at U101 pin 3 3456A) measure for zero volts (±1mV) at U101 pin 3
(U102 pin 10). (U102 pin 10).
e. e. If the reading on If the reading on the test voltmet
zero volts (i.e., above I mV), the zero volts (i.e., above I mV), the Input Hybrid is defective.
Replace U102. Replace U102.
f. f. If the If the reading is goo reading is good, connect pin
(U102 pin 3 and 9) of U101 to (U102 pin 3 and 9) of U101 to each other.
voltmeter, measure for zero volts (± 3mV) at pin 6 of voltmeter, measure for zero volts (± 3mV) at pin 6 of
U101. U101.
g. g. If the If the reading is go reading is good (less tha
amplifier feedback amplifier feedback circuitry is def circuitry is defective.
h. h. If the reading If the reading is wrong (grea is wrong (greater than 3mV),
amplifier is amplifier is defective. defective. Replace U101. Replace U101.
7-A-25. Constant Zero 7-A-25. Constant Zero Readings on
7-A-26. 7-A-26. A const A constant z ant zero re
when the input or output of the DC/Ohms Input Amplifier when the input or output of the DC/Ohms Input Amplifier
is shorted is shorted to ground. to ground. Make sure p
U102 and pin U102 and pin 6 of U101 are not sh 6 of U101 are not shorting to ground
shorts are noted, do the following: shorts are noted, do the following:
a. a. Set the 3 Set the 3478A to the DC 478A to the DC Volts Function and t
3V Range. 3V Range.
CIRCUITRY CIRCUITRY TROUBLE- TROUBLE-
consists of of E101 E101 and v and various
try replacing E1 replacing E101.
TROUBLESHOOTING
Circuitry consists consists of of the
the A/D Convert D Converter. er.
on All Ranges Ranges
when the output of
oo high. This can be This can be
amplifier. Do the fo Do the following: llowing:
Volts Function and the
ero is turned off (see paragraph ff (see paragraph
Terminals.
nce Digital Voltmeter (like the meter (like the
the test voltmeter is other than er is other than
Input Hybrid is defective.
d, connect pin 6 and pin 6 and pin 2 2
each other. Using the test Using the test
od (less than ± n ± 3mV), the 3mV), the
ective. Replace U102. Replace U102.
ter than 3mV), the
Readings on All Ranges All Ranges
ero reading is ading is normally normally caused
Make sure pin 1 and in 1 and pin 13 of
orting to ground. . If no
Volts Function and the
b. b. Make sure Autoz
7-A-8 step b). 7-A-8 step b).
arious
c. c. Using an external po
01. If If
resistor in serie resistor in series, apply
one end of the resistor to pin 3 of U101 and the other one end of the resistor to pin 3 of U101 and the other
end to the power supply.) end to the power supply.)
d. d. Using a
the 3456A), make sure the voltage at the resistor end the 3456A), make sure the voltage at the resistor end
the Input Input
connected to pin 3 of U101 is +3V. connected to pin 3 of U101 is +3V.
e. e. If the voltage is loa
Go to Go to
defective. defective. Replace
f. f. If the
voltmeter) for approximately +10V at U101 pin 6. 6. voltmeter) for approximately +10V at U101 pin
g. g. If the volta
put of
fault. fault. Replace
h. h. If the
may be may be defective.
A/D Controller (U462) is good. A/D Controller
he
to check the A/D to check
replace U102. replace U102.
7-A-27. Floating Reading 7-A-27.
7-A-28. A floating 7-A-28.
open exists between the DC/Ohms Input Amplifier and open exists between the DC/Ohms Input Amplifier and
the the
troubleshooting the Input Circuitry, make sure the wire troubleshooting the Input Circuitry, make sure the wire
connected from the Front/Rear Switch to J108 is not connected from the Front/Rear Switch to J108 is not
open. If the wire is good open.
a. a. Set the 34
3V Range. 3V Range.
b. b. Make sure Autoz
7-A-8 step b). 7-A-8 step b).
c. c. Using an external
3478A's INPUT Terminals. 3478A's INPUT Terminals.
d. d.
U101 pin 3 (U102 pin 10). U101 pin 3 (U102 pin 10).
the
e. e. If the disp
floating reading, replace U101. floating reading, replace U101.
f. f. If the display
caused
Input Hybrid (U102) Input Hy
U102, make sur U102, make sure the A/D
pin 13 of
to paragraph 7-A-33 to check the to paragraph
If no
controller is good, replace U102. controller is good, replace U102.
7-A-29. 7-A-29. Noisy Readings
he
7-A-30. 7-A-30. Noisy readings can be c
7-A-3 7-A-3
Make sure Autozero is turned ero is turned off (see paragraph
Using an external power supply wit wer supply with a 100K ohm
s, apply +3V to pin +3V to pin 3 of U101.
Using a high impedance high impedance Digital Volt
If the voltage is loaded down, the I ded down, the Input Hybrid is
Replace U102. U102.
If the voltage voltage is +3V, is +3V, measure (using
If the voltage is incorrect, t ge is incorrect, the Input Amplifier is at
Replace U101. U101.
If the voltage is good voltage is good, the Inp
defective. Before replacing U10 Before replacing U102, make sure the
(U462) is good. Go to paragraph
the A/D Controller. Controller. If the
Floating Reading on All on All Ranges
A floating reading is nor reading is normally caused wh
instrument's instrument's INPUT INPUT
If the wire is good, then do the f , then do the following:
Set the 3478A to the DC 78A to the DC Volts Function and t
Make sure Autozero is turned ero is turned off (see paragraph
Using an external power supply power supply, apply +3V t
Connect Connect the the 3478A's 3478A's HI HI INPUT Terminal
If the displayed read layed reading on the
If the displayed reading on the 347 ed reading on the 3478A is + 3V, the
brid (U102) may be may be at fault.
e the A/D Controller (U462) Controller (U462) is good.
7-A-33 to check the A/D Controller.
Noisy Readings on All on All Ranges
Noisy readings can be caused by the Input
TM 11-6625-3071-14 TM 11-6625-3071-14
off (see paragraph
h a 100K ohm
3 of U101. (Connect (Connect
Digital Voltmeter (like meter (like
nput Hybrid is
measure (using the test the test
he Input Amplifier is at
, the Input Hybrid ut Hybrid (U102) (U102)
2, make sure the
Go to paragraph 7-A-33 7-A-33
If the controller is g controller is good, ood,
Ranges
mally caused when an en an
Terminals. Terminals. Before Before
ollowing:
Volts Function and the
off (see paragraph
, apply +3V to the o the
INPUT Terminal to to
ing on the 3478A is still 3478A is still a a
8A is + 3V, the
at fault. Before replacing Before replacing
is good. Go
A/D Controller. If the If the
Ranges
aused by the Input
he
Go