HP 3478A Technical Manual page 82

3478a 3478a
7-C-8. Inaccurate Readings 7-C-8. Inaccurate Readings on All or Some
7-C-9. This failure 7-C-9. This failure is normally is normally caused when
current changes value current cha nges value due to
following procedure. following procedure.
a. a. Set the 3 Set the 3478A to the 478A to the 2-Wire Ohms Function and
the 3K ohm Range. the 3K ohm Range.
b. b. Connect Connect a 3K a 3K ohm Resistor
INPUT Terminals. INPUT Terminals.
c. c. Using a Using a high impedan high impedance Digital
the 3456A), measure the 3456A ), measure the volt the voltage across
the ohms current on the 3K ohm Range is 1mA, the the ohms current on the 3K ohm Range is 1mA, the
voltage should be approximately .47V dc. voltage should be approximately .47V dc.
d. d. If the voltage If the voltage is radically wrong, t is radically wrong, the ohms current
is incorrect. is incorrect. Use the over Use the overload troubleshootin
(in paragraph 7-C-6) to determine the faulty circuitry. (in paragraph 7-C-6) to determine the faulty
e. e. If the If the voltage is voltage is good, re
Resistor from th Resistor from the e INPUT Terminals. INPUT Terminals.
INPUT Terminals. INPUT Terminals.
f. f. If the If the voltage across voltage across R205 changes,
PMOS FET in U1 PMOS FET in U102 may be 02 may be defective.
g. g. If the If the voltage remains voltage remains the same,
most likely in the Ohms Protection most likely in the Ohms Protection Circuitry.
Q201 through Q204 are good. Q201 through Q204 are good.
7-C-10. 7-C-10. Constant Zero Constant Zero Reading on
7-C-11. A Constan 7-C-11. A Constant Zero t Zero Reading is
when the Ohms Current Source supplies no ohms when the Ohms Current Source supplies no ohms
current (current at current (curren t at zero value). zero value). Since no current
through the resistor to be measured, no voltage drop through the resistor to be measured, no voltage drop
across the resistor is developed and the 3478A across the resistor is developed and the 3478A
measures zero measures zero volts volts (zero
condition can be caused by an open circuit between the condition can be caused by an open circuit between the
Ohms Current Source and the INPUT Terminals, or by a Ohms Current Source and the INPUT Terminals, or by a
defective Current Source. defective Current Source. Do the following
a. a. Set the 3 Set the 3478A to the 478A to the 2-Wire Ohms Function and
the 3K ohm Range. the 3K ohm Range.
b. b. Connect Connect a 3K a 3K ohm Resistor
INPUT Terminals. INPUT Terminals.
c. c. Using a clip lead, Using a clip lead, connect pin 15
3478A's HI INPUT Terminal. 3478A's HI INPUT Terminal.
d. d. If the 3478A still sho If the 3478A still shows a constant z
the Ohms the Ohms Current Current Source Source is is inoperative.
paragraph 7-C-20 for troubleshooting. paragraph 7-C-20 for troubleshooting.
e. e. If the Ohms Fun If the Ohms Function is now operatin
do the following: do the following:
on All or Some Ranges Ranges
caused when the ohms the ohms
due to a load cha a load change. nge. Do the
2-Wire Ohms Function and
ohm Resistor to the to the 3478A's
ce Digital Voltmeter Voltmeter (like
age across R205. R205. Since
he ohms current
load troubleshooting procedure g procedure
circuitry.
good, remove the move the 3K ohm
Then shor Then short t
R205 changes, the Output the Output
defective. Replace U102. Replace U102.
the same, the failure is the failure is
Circuitry. Make sure Make sure
Reading on All Ranges All Ranges
Reading is normally cau normally caused
Since no current goes
(zero reading). reading). A A no-current no-current
Do the following procedure. procedure.
2-Wire Ohms Function and
ohm Resistor to the to the 3478A's
connect pin 15 of U102 to t of U102 to the
ws a constant zero reading, ero reading,
inoperative.
ction is now operating correctly, g correctly,
and connect it to the collector of Q204 (junction of and connect it to the collector of Q204 (junction of
Q204, R203, and cathod Q204, R203,
other end of the clip lead connected to the input other end of the clip lead connected to the input
Do the
terminal for the checks that follow. terminal for the checks that follow.
for an open R205 or an open in Q201 through for an open R205 or an open in Q201 through
Q204. Q204.
3478A's
disconnect the clip lead from Q204 and connect it to to disconnect the clip lead from Q204 and connect it
(like
the anode of CR201. the anode of CR201.
Since
for an open CR201. for an open CR201.
check for an ope check for
energized (+5V across the relay coil), before energized (+5V across the relay coil), before
replacing the relay. replacing the
3K ohm
U102 may U102 may be de
t the he
make sure the A/D Con make sure the A/D Controller (U462) is good
paragraph 7-C-24 for the SA procedure to check paragraph 7-C-24 for the SA procedure to check
U462. U462.
7-C-12. Noise on 7-C-12.
7-C-13. Noise on 7-C-13.
noisy ohms noisy ohms current.
a. a. Set the 34
the 3K ohm Range. the 3K ohm Range.
sed
b. b.
INPUT Terminals. INPUT Terminals.
goes
c. c. Using a clip lea
3478A's HI INPUT Terminal. 3478A's HI INPUT Terminal.
d. d. If the Ohms Function
the Ohms Prote the Ohms Protection
Q204. Q204.
e. e.
following checks. following checks.
3478A's
(like the 3456A), make sure the voltage at U201 pin (like the 3456A), make sure the voltage at U201 pin
6 is a quiet +8V (<10 6 is a quiet +8V (<10
noisy. If the noisy.
he
for a quiet +12V at U202 pin 6 (<10 for a quiet +12V at U202 pin 6 (<10
Replace U202, if noisy. Replace U202,
Go t Go to o
replace U102. replace U102.
U203. If the U203.
U102. U102.
7-C-2 7-C-2
1. 1. Disconnect the Disconnect the clip lead from pin clip lead from pin 15 of U102
and cathode of CR2 e of CR201).
2. 2. If the If the Ohms Function is now Ohms Function is now operating, check
3. 3. If the Constan If the Constant Zero Reading is still display t Zero Reading is still displayed,
4. 4. If the If the Ohms Function is now Ohms Function is now operating, check
5. 5. If the Constan If the Constant Zero Reading is still display t Zero Reading is still displayed,
an open K103 n K103. .
relay. If the r If the relay is no
be defective. fective.
troller (U462) is good. . Go to
Noise on All Ranges All Ranges
Noise on all ranges all ranges is normally
current. Do the following Do the following procedure.
Set the 3478A to the 78A to the 2-Wire Ohms Function and 2-Wire Ohms Function and
Connect a 3K Connect a 3K ohm Resistor ohm Resistor to the
Using a clip lead, connect p d, connect pin 15 of U10
If the Ohms Function is quiet, noise is quiet, noise is caused by
ction Circuitry. Circuitry.
If the Ohms If th e Ohms Function is Function is still noisy
1. 1. Using a h Using a high impedance igh impedance Digital
V change). V chang
µ µ
If the voltage is still voltage is still noisy, try
2. 2. If the volta If the voltage at U201 ge at U201 pin 6 is good, measure
if noisy. If the vo If the voltage is still n
3. 3. If the v If the voltage at oltage at U202 pin 6 U202 pin 6 is good, re
If the ohms function ohms function is still
TM 11-6625-3071-14 TM 11-6625-3071-14
15 of U102
01). Leave the Leave the
operating, check
ed,
operating, check
ed,
Make sure K103 is Make sure K103 is
elay is not energized, t energized,
Before replacing U102, Before r eplacing U102,
Go to
is normally caused by caused by a a
procedure.
to the 3478A's 3478A's
in 15 of U102 to the 2 to the
is caused by
Check Q201 Check Q201 through through
still noisy, do , do the the
Digital Voltmeter Voltmeter
e). Replace U2 Replace U201, if 01, if
noisy, try U102. U102.
pin 6 is good, measure
V change). V change).
µ µ
ltage is still noisy, oisy,
is good, replace place
is still noisy, noisy, replace replace