4-Wr Noise Failures; Dc Ohms Failure - HP 3456A Operating And Service Manual

Service
the high input of the OVM to the other side R105.
Measure for any voltage across the resistor.
e. If no or very little (a few microvolts) voltage is
measured across R105, replace Q105.
f. If a positive voltage larger than 50 microvolts is
measured across R105, replace QI07.
g. If a negative voltage is measured across R105, do
the following:
I . Turn Autozero Off.
2. If the voltage across R 105 remains Ihe same,
replace Q 1 06.
3. If the voltage across Rl05 appears jumpy or noisy
with Autozero On and stable with Autozero Off,
replace Q105.
4. I f none of the above was noted and the 3456A still
has an ohms accuracy failure, Q105 may be defec­
tive.
8-0-8
S·D·55. 4·WRn Noise Failures.
8-0-56. Ohms Noise Failures in the 4-WRQ Function,
but not the 2-WRQ Function, is most likely caused by a
leaky Autozero FET (A20QI05) in the Input Switching
Circuitry. This failure should also show up as a low
ohms reading. Replace Q105.
H·D·57. D.e. DHMS FAILURE.
8-0-58. Before troubleshooting for any O.c. Ohms
Failures, make sure the 3456A is good in the OCV and
Ohms Functions. MOSI O.C. Ohms Failures should
show up in either of the other funclions, because the
O.c. Ohms Function is very similar
The only difference is that an ohms measurement is
taken with the Ohms Current Source alternately turned
On and Off. This is accomplished by turning the in­
dividual ohms current determining FETs (A20Q201 -
Q206, Q208, and Q209) On and Off. These FETs and
the other circuitry (Input, Current Source) are also used
in normal ohms. Therefore, an O.c. Ohms Failure is
most likely caused by a FET that is too slow in changing
states or the Range and Function Control Logic is not
operating correctly.
Model 3456A
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,
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10
normal ohms.