HP 3478A Technical Manual page 83

3478A 3478A
7-C-14. Floating Reading on 7-C-14. Floating Reading on All or Some
7-C-15. 7-C-15. A Floating A Floating Reading is Reading is normally
open circuit between the INPUT Terminals and the open circuit between the INPUT Terminals and the
DC/Ohms Input DC/Ohms In put Amplifier. Amplifier.
Function is operating correctly, before suspecting an Function is operating correctly, before suspecting an
ohms failure. ohms failure. If the failure If th e failure shows u
Function only Function only, U102 may , U102 may be defective.
U102, make sure U102, make sure the A/D Co the A/D Controller (U462) is
to paragraph 7-C-24 to check U462. to paragraph 7-C-24 to check U462.
7-C-16. Overload, Noise, or 7-C-16. Overload, Noise, or Constant Zero
on Some Ranges on Some Ranges
7-C-17. Failures on some range 7-C-17. Failures on some ranges can only
the Input the Input Hybrid Hybrid U102. U102. This is because
used to configure the current source for the different used to configure the current source for the different
ranges. ranges. If at least If at least one range is one range is good, the Ohms Curre
Source is operating. Source is operating. Before replacing Before replacing U102, make
it receives the correct information from the A/D Controller it receives the correct information from the A/D Controller
(U462). (U462). Go to paragr Go to paragraph 7-C-24 t aph 7-C-24 to check U462.
7-C-18. 7-C-18. OHMS OHMS PROTECTION PROTECTION CIRCUITRY TROUBLE-
SHOOTING SHOOTING
7-C-19. The Ohms Protection Circuitry 7-C-19. The Ohms Protection Circuitry is used to protect
the Ohms Current Source from excessive positive or the Ohms Current Source from excessive positive or
negative inpu negative input voltages. t voltages. To make sure t
operational, check the following: operational, check the following:
a. a. To check the circuitry To check the circuitry operation for
voltages, do the following: voltages, do the following:
1. 1. Set the Set the 3478A to 3478A to the 2-Wire Ohms Function
and the 3K ohm Range. and the 3K ohm Range.
2. 2. Connect a Connect a high impedance high impedance Digital Voltmete
(like the 3456A) across R205. (like the 3456A) across R205.
3. 3. Acquire Acquire a va a variable
the supply for a 0V output and connect it to the the supply for a 0V output and connect it to the
3478A's INPUT Terminals. 3478A's INPUT Terminals.
4. 4. Measure Measure for appr for approximately
R205. R205. If the v If the voltage is w oltage is wrong, the Cu
may be defective (go to paragraph 7-C-20 for may be defective (go to paragraph 7-C-20 for
troubleshooting). troubleshooting).
5. 5. While monitoring While monitoring the vo
adjust the power supply until it outpu adjust the power supp
output voltage of approximately +6V, the voltage output voltage of approximately +6V, the voltage
across R205 should go to 0V and remain at that across R205 should go to 0V and remain at that
level. level.
6. 6. If the voltage If the voltage does not
CR201. CR201.
7. 7. If the voltage If the voltage does go
Protection Circuitry does protect for positive input Protection Circuitry does protect for positive input
voltages. voltages.
All or Some Ranges Ranges
normally caused by caused by an
Make sure Make sure the the DC Volt DC Volts s
shows up in p in the the Ohms
be defective. Before replacing Before replacing
ntroller (U462) is good. good. Go
Constant Zero Readings Readings
s can only be caused by be caused by
This is because the hy the hybrid is
good, the Ohms Current nt
U102, make sure
o check U462.
CIRCUITRY TROUBLE-
is used to protect
To make sure the circuitry he circuitry is is
operation for positive input positive input
the 2-Wire Ohms Function
Digital Voltmeter r
riable 0-10V 0-10V power power supply. supply. Set
oximately .47V dc .47V dc across
rong, the Current Source rrent Source
the voltage acr ltage across R205, oss R205,
ly until it outputs +10V. ts +10V. At an
does not go to go to 0V, replace 0V, replace
does go to 0V, to 0V, the Ohms the Ohms
b. b.
input voltages, do the following: input voltages, do the following:
an
1. 1. Leave t
bring the bring the variable powe
voltage across R205 should again be .47V. voltage across R205 should again be .47V.
Ohms
2. 2. Reverse the
apply -10V dc to the 3478A's INPUT Terminals. apply -10V dc to the 3478A's INPUT Terminals.
Go
3. 3. The volta
.47V. .47V.
4. 4. If the
Ohms Protection Circuitry is operating correctly. Ohms Protection Circuitry is operating correctly.
brid is
5. 5. If the v
the Ohms Protection Circuitry is inoperative. the Ohms Protection Circuitry is inoperative.
Measure for approximately -.6V at the source and Measure for approximately -.6V at the source and
sure
drain of Q205. If the voltage is high or z drain of Q205.
Q205. Q205. If the
Q201 through Q204. Q201 through Q204.
7-C-20. 7-C-20.
SHOOTING SHOOTING
7-C-21. 7-C-21. The Ohms Current
Reference, Buffer and Range Resistors, and a Gate Bias Reference, Buffer and Range Resistors, and a Gate Bias
Amplifier. Amplifier.
make sure the + 10V make sure the
troubleshoot the Ohms Current Source, do the following troubleshoot the Ohms Current Source, do the following
procedure. procedure.
a. a. Set the 34 Set the 3478A to the
the 3K ohm Range. the 3K ohm Range.
b. b. Using a
the 3456A), measure the voltage at pin 2 and pin 3 of the 3456A), measure the voltage at pin 2 and pin 3 of
U203 (U102 U203 (U102 pins 17
the voltage on both pins is approximately +8V. the voltage on both pins is approximately +8V.
Set
c. c. If the v If the voltages are no
replace U203. replace U203.
across
d. d. If the voltages on both p If the voltages on both pins are wrong (other than
+8V), but are approximately the same, do +8V), but are approximately the same, do the following:
1. 1. Measure for
U201. U201.
At an
2. 2.
replace U102. replace U1
the A/D the A/D Controller (U462
7-C-24 to check U462. 7-C-24 to check U462.
3. 3. If the v
+12V, measure for approximately +8V at pin 3 of +12V, measure for approximately +8V at pin 3 of
U201. U201.
4. 4.
replace U1 replace U102.
7-C-3 7-C-3
To check the circuit To check the circuitry ry operation
Leave the same set he same set up as in
variable power supply r supply down to 0V
Reverse the power supply power supply output leads
The voltage across ge across R205 should
If the voltage remains voltage remains the same (.47
If the voltage changes oltage changes value and/or
If the voltage is high or zero, replace
If the voltage is go voltage is good, check for a od, check for a defective
OHMS CURRENT OHMS CURRENT SOURCE
The Ohms Current Source consists o Source consists of a Voltage
Before Before troubleshooting troubleshooting the
+ 10V reference is g reference is good at
78A to the 2-Wire Ohms Function and 2-Wire Ohms Function and
Using a high impedance high impedance Digital Volt
pins 17 and 18, and 18, respectively respectively). ). Make sure
oltages are not approximately t approximately the same,
ins are wrong (other than
Measure for approximately approximately + 12V at pin
If the If the voltage at voltage at pin 6 of pin 6 of U201 is + 1
02. Before rep Before replacing U102,
Controller (U462) is good. ) is good. Go to paragraph
If the voltage at oltage at pin 6 of pin 6 of U201 is other t
If the voltage at If the voltage at pin 3 pin 3 of U201
02. Before rep Before replacing U102,
TM 11-6625-3071-14 TM 11-6625-3071-14
operation for nega for negative tive
up as in step a, step a, except except
down to 0V. . The The
output leads and and
R205 should remain at remain at
the same (.47v), the v), the
value and/or polarity, polarity,
ero, replace
defective
SOURCE TROUBLE- TROUBLE-
f a Voltage
the current current source, source,
ood at JM201. JM201. To To
Digital Voltmeter (like meter (like
Make sure
the same,
the following:
+ 12V at pin 6 of 6 of
U201 is + 12V, 2V,
lacing U102, make sure make sure
Go to paragraph
U201 is other than han
of U201 is wrong, is wrong,
lacing U102, make sure make sure