Range Failures And Defective Fets; Constant Overload, Or High Positive Or Negative Readings; 4-Wr Accuracy Failures - HP 3456A Operating And Service Manual

Model 3456A
T.ble 8·0·3. R.nge Failures .nd Defective FEll.
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8-0-50. Failures in the 4-WRO Function can be caused
by Shorted, Open, or Leaky FETs in the Input Swit­
ching Circuitry. The FETs involved are, A2OQI04-
QI08, Q 1 I 5 , and Q 1 l 7 . The faulty FETs can often be
determined by the failure symptoms. The following
paragraphs have some possible failures, troubleshooting
information, and most likely causes.
NOTE
TS
Make sure Jhe VOL
RA
and Low Terminals are connected to the
TIO REF (4WRO S E N S E ) High and Low
T erminals, res peclively.
8·0·51. CO"ltlnt Onrlo.d, or High POl
ReedinRl·
8-0-52. This failure can be caused by Open or Shorted
FETs in the RATIO REF (4WRO SENSE) Input Swit­
ching Circuitry. This fa ilure can also be caused by the
Front/Rear Switch. Make sure the switch is making
good contact. Once is has been determined that the
swtich is good, do the following to determine the faulty
component.
a. Press the RESET button and then set the 3456A to
the 1 K ohm Range.
b. Connect a 1000 ohm resistor to the Input Ter­
minals. Note the displayed reading (Overload, High
Positive, etc.).
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( 2 WRO, 4 W R O ) High
tin or •••• tive
i
c. Remove the 1000 ohm resistor.
d. If the reading remains the same, continue with
Step e. If the reading goes to
more than 1000 counts (with the 3456A in the 5 digit
mode), do the following:
1 . With a high input impedance Digital Voltmeter
(like the 3456A),
A20R105, gate of Q107, and source and drain of
QI06.
2. If the reading on the Test DVM is zero volts,
replace A2OQ105.
3. If the reading on the Test DVM is a high negative
voltage (approximately
4. If the reading on the Test DVM is a high positive
voltage (approximately 1 3 V), replace Q107.
e. I f the reading on the 3456A under test remains ap­
proximately the same when the
removed, do the following:
I . With a high impedance Digital Voltmeter (like the
3456A), measure the junction of A20RI06, gate of
Q I 1 7 , and source and drain of QI08.
2. If the reading on the Test OVM is a high negative
voltage (approximately - 1 3 V), replace Q108.
3. If the measured reading is a high positive voltage
approximately 1 3 V), replace Q I 1 7 .
4 . If the measured reading is jumpy or very unstable,
1000
replace Q 1 15. An open Q I 15 can easily be checked
by reconnecting the
Terminals and shorting the source to the drain of
Q 1 I 5 . If the 3456A (under test) now displays the
resistor value, Q I 1 5 is open.
·
8·0·53. 4 WRO Accur.cy Failurtl.
8-0-54. These failures are normally caused by leaky
FETs in the 3456A's Input Switching Circuitry. To find
the leaky FET, do the following:
a. Set the 3456A to the 4WRO Function and short the
Input Terminals. Make sure the instrument is on the I
ohm Range with Autozero On.
b. Connect the low input of a high input impedance
Digital Voltmeter (like the 3456A) to the input side of
A20RI06 (orange wire connected at J23). Connect the
high input of the Test DVM to the other side or RI06.
Measure for any voltage across the resistor.
c. If no voltage or less than 5 microvolts is measured
across RI06, continue with Step d. If a voltage is
measured across RI06, check and do the following:
I . If the voltage across R 106 is negative. replace
Q108.
2. If the yotlage across RI06 is positive, replace
Q 1 I 7 .
d. Connect the low input of the Test DVM to the in­
put of A20R 105 (gray wire connected to 122). Connect
Service
"000 . 000 "
or changes
measure the junction of
- 1 3 V). replace QI06.
1000
ohm resistor is
ohm resistor to the Input
8-0-7
K