Zero Reading And Noise On Some Ranges; Overload Reading On Some Ranges; High Or Low Reading On Some Ranges; 4-Wr Troubleshooting - HP 3456A Operating And Service Manual

Service
8-0-39. Zero RBading and Noise on some Ranges.
8-0-40. A Zero Reading normally shows that no cur­
rent is generated for the range with the zero reading.
This is normally caused by an open FET but can also be
caused by some shorted FETs. Noisy readings caused by
FETs normally shows up on the range which uses the
FET. To determine if a range(s) fails, measure a resistor
value which corresponds to the bad range (e.g. 100 ohm
resister for a bad l OO ohm Range). If a resistor value is
not equal to the full scale value of the range, the failure
symptoms may be different. The following are the faul­
ty ranges and causes.
I
a. Only the 100 and
ranges use I mA to measure resistance. The current is
derived by Q209. Make sure the FET is turned on and
not noisy, when either range is selected.
lOO,
b_ Only the I K, and 10 K ohm Ranges fail.
- 5.5 V Reference voltage determines the currents for
these ranges. Make sure Q201 is turned on and nOI
noisy, when any one of the ranges are selected. Also
make sure Q202 and Q205 are not shorted or leaky.
c_ Only the 10 K and 100 K ohm Ranges fail.
ranges use 100 �A and 50 pA currents to measure
resistance. The currents are derived by Q208 and the
- 5 . 5 V Reference for the 10 K ohm Range and the
- 9.25 V Reference for the 100 K ohm Range. Make
sure Q208 is turned on and not noisy, when either range
is selected.
I
d. Only the M ohm Range fails.
p.A current to measure resistance. The current is derived
by Q205. Make sure the FET is turned on and not noisy,
when the range is selected.
e. Only the 10 M, 100 M,
These ranges used 500 nA current to measure resistance.
The current is derived by Q203 . Make sure the FET is
turned on and not noisy, when any one of the ranges are
selected.
f. Only the 100 K through
The - 9.25 V Reference determines the currents for
these ranges. Make sure Q202 is turned on and not
noisy, when any one of the ranges are selected. Also
make sure Q201 is not shorted or leaky.
8·0·41. Overlold Heedingl on lome Rlnge
8-0-42. This failure can be caused by shorted or leaky
FETs. The bad component(s) can often be determined
by what range(s) fail. Make sure the overload condition
exists by measuring a resistor value which corresponds
to the bad range (e.g. 1 00 ohm resistor for a bad 1 00
ohm Range). If another value is used, the failure symp­
toms may be different. (Remember, an overload on the
1 00 M and 1000 M ohm Ranges can be a .. - 49.XXX
+ 6" reading.) The following are the bad ranges and
causes.
M , M,
•. I M, 10 100 and 1000
8-0-6
These
K ohm ranges fail.
The
These
This range uses 5
and 1 G ohm Ranges fail.
1000
M
ohm Ranges fail.
•.
M
ohm Ranges bad.
This failure can be caused by a shorted or leaky Q203 ,
Q205. Q208, and Q209. To determine the faulty FET,
check for the following symptoms.
I . I f the 1 00 through l OO K ohm Ranges are Low,
Q203 and/or Q205 is at fault.
2. I f the 1 00 and 1000 ohm Range is low and the 10 K
and 1 00 K ohm Range is good, Q208 is at fault.
3. If the 100 through lOO K ohm Ranges read pro­
gressively higher, Q209 is at fault.
b. 10 M. 100 M, and 1000
for a shorted or leaky Q204 andlor Q206.
c. 10 K ohm through 1000 M ohm Ranges bad.
for a shorted or leaky Q208 andlor Q209.
8·0·43. High Low Readingl on lome Rlngel.
or
8-0--44 . This type of failure can be caused by shorted or
leaky FETs. To make sure if a range fails, measure a
resistor value which corresponds to the failing range
(e.g. 100 ohm resistor for a defective 1 00 ohm Range).
I f a resistor value is not equal to the full scale value of
the range, the failure symptomes may be different. Once
it has been determined which range(s) fail and how they
fail, use Table 8-0-3 to determine the shorted or leaky
FET. Note that a FET can cause certain ranges to fail in
specific ways. However, because of the way the Ohms
Current Source works (some FETs are only partially
on), the table should only be used as a guide. The symp­
toms may change from instrument to instrument.
8-0-45. When using Table 8·0-3, note that a shorted or
leaky FET can cause a failure on more than one range.
Becuase of this, a defective FET can be determined by
what ranges fail and how they fail. One thing to keep in
mind. A range may read good but in actuality may be
inaccurate. This is true if the error of the range has been
adjusted out by a Calibration Potentiometer (Pot F to
J).
8·D·46. 4·WRO TROUBLESHOOTING.
8·0·41. General.
8-048. The following information is for 4-WRO
Failures that do not show up in the 2-WRO Function. I f
the failure i s also present i n 2·WRO, g o t o Paragraph
8-0-15 for troubleshooting. Also, troubleshoot any
2-WRO Failures first, before troubleshooting for any
4-WRO Failures.
8-0-49. The major difference between 4-WRO and
2-WRO is where the actual ohms measurement is made.
In the 2-WRO Function, the measurement is made at the
VOLTS Input Terminals. In the 4-- W RO Function, the
measurement is made at the RATIO REF (4WRO
SENSE) Terminals. Selecting the correct terminals is
done in the Input Switching Circuitry. Because of this, a
4-WRO Failure with no corresponding 2-WRO Fa ilure is
most likely caused by the Input Switching Circuitry .
Model 3456A
M ohm Ranges bad. Look
Look
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