Input Amplifier Troubleshooting - HP 3456A Operating And Service Manual

Service
c. I f the 100 V and 1000 V Ranges fail, the failure is
an open in the high vollage input circuitry (A20KI02,
U200, and QI09). To determine if KI02 or QI09 is
open, short across each one (source to drain of QI09).
To determine an open U2oo, measure for the correct
value resistor between pins I and 2. and I and 25 on
U200.
S-C-33. Inaccurate Reading.
caused by defective FETs in either the low or high
voltage input circuitry, depending on what ranges fail.
Do the following;
a. Press the 3456A's RESET bunon.
b. Determine what ranges fail.
c. I f the 100 mV, I V, and 1 0 V Ranges are inac­
curate, set the 3456A to the 1 0 V Range and apply an ac­
curate and stable 10 V dc to the VOLTS Input Ter­
minals. I f an accurate 10 V is not available, use one of
which the exact value is known. Then do the following:
I . I f the displayed reading is high, A2OQ I 1 9 may be
leaky.
2. I f the reading is low, turn on the Filter (by press­
ing the FILTER button). Replace Q l l 4 if the
reading is now good.
3. I f the reading is still low, remove the 10 V from
the input and short the Input Terminals.
4. Connect the gate of QI09 to the Low VOLTS In­
put Terminal.
5 . Using a high impedance Digital Voltmeter (like the
3456A), measure the voltage at the junction of the
source of Q1 16, gate of Q I 1 9 and source and
drain of Q 1 1 8 .
6 . I f the voltage is approximately zero «
replace QI09. If not, replace Q l lS.
d. I f the lOO V and 1000 V Ranges are inaccurate, set
the 3456A to the 100 V Range and apply an accurate and
stable + 100 V dc to the VOLTS Input Terminals. I f an
accurate l OO V is not available, use one of which the ex­
act value is known. Then do the following:
I . If the displayed reading is high, make sure
A20U2oo is good. This can be done by measuring
pin I of U200 with a high impedance Digital
Voltmeter (like the 3456A). With an accurate
100 V input the voltage should be an accurate I V.
2. If the reading is low, turn on the Filter (by press­
ing the FILTER button).
3. If the reading is now good, replace QIOI.
4. If the reading is still wrong, replace Q1 16.
This failure is normally caused
8-C-34. Noisy Reading.
by defective FETs in either the low or high voltage input
circuitry, depending on what ranges fail. Do the follow­
ing:
a. Press the 3456A's RESET button.
8-C-6
This failure is normally
±
�
10 V).
b. Determine what ranges fail.
c. If the 100 mV, I V, and 10 V Ranges are noisy, set
the 3456A to the 10 V Range and apply a stable 10 V dc
to the VOLTS Input Terminals. Then do the following:
I . Short the high input (High VOLTS Terminal) to
the source of A20Q I 1 6 Uunction of QI 16, Q1 19,
QI IS, and RI20).
2. If the reading is now stable, relay KI03 or resistors
R102, R109, RI 10, and R120 may be noisy. The
noisy component can be determined by shorting
across the resistors. Replace the component that
makes the reading good, when shorted. If the
resistors are good, replace K 103.
3. If the reading is still noisy, measure the voltage at
the source of A20Q I 1 6 Uunction of Q1 16. QI 19,
Q 1 1 8, and R120) using a high impedance Digital
Voltmeter (like the 3456A).
4. If the reading on the Test Voltmeter is noisy,
replace Q I I S and Q1 19.
5 . If the reading on the Test Voltmeter is good, try
replacing QI 16.
d. I f the 100 V and 1000 V Ranges are noisy, set the
3456A to the l OO V Range and apply a stable 100 V dc to
the VOLTS Input Terminals. Then do the following:
I . Using a high impedance Digital Voltmeter (like the
3456A), measure the voltage at the source of
A2OQI09 (pin 1 of U2(0).
2. I f the voltage is stable, replace QI09.
3. If the voltage is noisy, replace KI02 or U200.
B-C-35. INPUT AMPLIFIER TROUBLESHOOTING.
S·C·36. General.
8-C-37. Before troubleshooting the Input Amplifier,
make sure the failure is not in the Input Switching Cir­
cuitry. Refer to Paragraph S-C-8 and S-C-15 to deter­
mine the faulty circuitry.
8-C-3S. Failures caused by the Input Amplifier can be
Inaccurate, Zero, Overload
The failures can be caused by the Gate Bias Amplifier,
Feedback FETs and Resistors, and the Input Amplifier
itself. The following paragraphs have the failures and
troubleshooting information.
NOTE
be
The O f f set A d justment of the Input
A mplifier has to
nent in the amplifier has been replaced.
S·C·39. All Ranges Inoperative.
8-C-40. Use the following procedure to find and
troubleshoot the faulty area.
a. Press the 3456A's RESET button.
b. Set the instrument to the Ohms Function and
measure a 100 K ohm or higher value resistor. If the
Model 3456A
(Ol),
and Noisy Readings.
read justed. if a compo­
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