Floating Reading; Inaccurale Reading - HP 3456A Operating And Service Manual

Service
a. Turn the 34S6A Ofr.
b. Place A30J3S (on the Inguard logic Assembly) in
the "TEST" position (position 2 and 3).
NOTE
Make sure A30J35 (on the Inguard Logic i s
Assembly) placed back in the "RUN"
position ( position I and 2),
troubleshooting procedure is completed or
a f ter repairing the 3456A .
c. Turn the 34S6A On.
d. If the Overload condition disappears and zero is
displayed, replace A2OQlO2.
e. If the Overload condition remains, turn Off
A20QlO3 (by connecting its gate to A20TP lOS).
f. If the Overload condition remains. measure the
voltage on either side of A20RlO3 with a high im­
pedance Digital Voltmeter (like the 34S6A). If the
voltage switches between an approximately zero to some
negative value - 12 V). QI03 is shorted. Replace the
( >
FET.
g. If the Overload condition disappears. a FET on
the Upper Input Node is shorted. With a high im­
pedance Digital Voltmeter (like the 34S6A), measure the
voltage on the Upper Input Node (drains of A2OQI09.
Q l lO. QI12, Q I I S . and Q I 16). The vohage should be
larger than - 1 2 V for an overload condition. Do the
following:
I . Set the 3456A to the ACV Function.
2. If the voltage changes to a low value, Q I I O is
shorted. I f not. set the 34S6A to the 4-WRO Func­
tion.
3. If the voltage changes to a low value. Q I I S is
shorted. If nOt. Q I 1 2 is shorted.
4. If the reading is still an Overload, check for a
shorted Q1 12.
5 . A shorted QI09 and Q I 1 6 can also cause an
overload. However, both FETs have to be
shorted . If only one FET is shorted, the overload
should only be on some ranges.
8-e-l5. Floating Reading.
reading is displayed and changes very little or not at all
when an input (different from the reading) is applied to
the 34S6A. This may also show up as an overload condi­
tion on the lower ranges ( l OO mV and I V) and is nor­
mally caused by an open circuit in the main measuring
path. I f all ranges show the symptoms. check the
following :
a. Make sure the Gate Bias Amplifier is good. The
amplifier can be checked by applying + 5 V to its input
(pin 3 of A2OQ 120) and then checking its output
(A20TPI04). Since the gain of the amplifier is
output should be the same as the input ( + 5 V, within
S-C-4
when the
A noating reading is when a
the
X I ,
5 mY). If the output is incorrect, replace A2OQI20 or
UIOl.
b. Make sure A2OQI03 is not open. This can be
checked by turning the 3456A's Autozero off and shor­
ting the source to the drain of QlO3. I f the 3456A can
now make a normal DCV measurement, replace QlO3.
c. Try replacing Q102.
d. If the readings are still noating, check for an open
circuit path in the A20KlO2, KlO3, QI09, and Q1 I6 cir­
cuitry. An open can easily be checked by shorting across
the individual components. (An open path has to be in
both the 100 mV, I V, 10 V and 100 V, 1000 V circuit
paths to show up as a noating reading in all ranges.)
8-C-l6. Inaccurale Reading.
by leaky FETs in either the Upper or lower Input Node
or a defective gate Bias Amplifier. A FET which is leaky
from its gate (when turned off) to its drain (connected to
the Input Node), can cause a low reading. If that is the
case, connect the source (input) of the FET to the input
of the 34S6A and then turning the FET on (gate to
ground). If the reading is then correct, the FET is leaky.
To determine a leaky FET and a defective Gate Bias
Amplifier, do the following:
a. Press the 34S6A's RESET button.
b. Set the instrument to either the 1 00 mV (or I V)
Range, and the 6 Digit Mode (by storing "6" into the N
DIG DISP Register).
c. Apply a stable and accurate l OO mV (or I V) to the
VOLTS Input Terminals. If an accurate voltage is not
available, use one of which the exact value is known.
d. If the 3456A is inaccurate in the positive direction
(high reading), do the following:
I . Measure the voltage at A20TPI04. If the voltage is
higher (more than 500 mY) than the input voltage,
replace A20Q I20, UIOS.
2. If the voltage at TPl04 is close to the input voltage
(within lOO mV), short the High VOLTS Terminal
to the gate (pin 3) of Q120. If the reading on the
34S6A is now good, replace Q 1 20. I f still inac­
curate, replace Q103.
e. If the reading is inaccurate in the negative direc­
tion (Iow reading), remove the voltage from the 3456A
Input Terminals.
f. Short the VOLTS Input Terminals and note the
displayed reading.
g. If the 3456A displays a negative offset, do the
following:
I . Turn Autozero Off.
2. Short the gate of A20QI04 to the low VOLTS In­
put Terminal and nOle the displayed reading. If
the offset reading changes in the positive direction
200 counts) or to approximately zero «
( >
counts), replace QI04. Remove the short from
QI04.
Model 3456A
This failure can be caused
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