Isolating The Ohms Circuitry - HP 3456A Operating And Service Manual

Model 3456A
also show up as an inoperative OCV Function. Go to
Service Group C for troubleshooting.
h. If the reading is Low (accuracy), the failure can be
in the Ohms Current Source, Input Circuitry, Input
Amplifier, or the Ohms Protection Circuitry. To isolate
the defective area, do the following:
Turn the 3456A's Autozero Off.
I .
2. Turn A20KIOI Off by shoTting the base of
A2OQ702 to ground. (Q702 is located in the
Range and Function Control Logic.)
3. Remove the red wire from A20J20 and the white
wire from J19.
4. Using a clip lead, connect the removed red wire
to the base of A2OQ31O (pin 3). (Q31O is in the
Input Amplifier Circuitry.)
5. Using another clip lead, connect the emitter of
A2OQ2 1O to the base of
6. If the reading is good or improves significantly
(good within 20 C1S), the Input Switching Cir­
cuitry is at fault. Go to Paragraph 8-0-25 for
troubleshooting.
7 . If the reading is still wrong, acquire a 5 V dc
Power Supply and apply - 5 V to the base of
Q210.
CIS),
8. If the reading is good or improves significantly
(good within 20
cuitry is at fault. Go to Paragraph 8-0-28 for
troubleshooting.
9. I f the reading is still wrong, remove both clip
leads from the base of Q3 10. Connect the remov­
ed leads to either side of A20R 103.
10. Connect a high
Voltmeter (like the 3456A) to either side of R
and ground. Measure and note the voltage at
R to3. The voltage should be very close to
if the recommended lOO
used.
1 1 . Reconnect the white wire to J 19. If the reading of
the test voltmeter changes to a lower value (less
negative), the Input Amplifier is at fault. Most
likely a leakly Q3to. I f the reading remains the
same, the Ohms Current Source is at fault. 00 to
Paragraph 8-0·34 for troubleshooting.
8·0·20. Some Rlnges Inoperltive.
8-0-2 1 . This failure can show up as a constant
Overload (OL), Zero Reading, or Inaccuracy on some
ranges. A failure can be caused by the 3456A Input
Switching, Input Amplifier, Ohms Protection Circuitry,
or the Ohms Current Source. The following symptoms
can be used to help isolate the faulty area.
a. If a group of ranges fail, the cause is most likely in
the Ohms Current Source or Input Amplifier. For ex­
ample, the lOO, I and to
K,
zero ohms reading). In this case, the faulty ranges all de-
Q3IO.
the Ohms Protection Cir­
to3
input impedance Digital
- 5 V,
ohm test resistor is
K
ohm Ranges fail (Iow or
K
pend on the - 5.5 V Reference Voltage (in the Ohms
Current source) for current. The most likely cause is an
open A2OQ201. The following is a listing of the ranges,
and the faulty circuitry. For a defective Input
Amplifier, go to Paragraph 8-0-3 1 for troubleshooting.
For a defective Ohms Current Source, go to Paragraph
8·0·34 for troubleshooting.
Only Ranges Fall
100 Input Amplifier
Current Source
100,IK
100,IK, IOK Current Source
Input Amplifier
IK,IOK
Current Source
IOK,looK
Current Source
lOOK
Input Amplifier, Current Source
lOOK through 10
Current Source
through 1 G
IOM
Only Ranges Good
Input Amplifier
100
Input Amplifier
100,IOK
lOOK through 1 0 Input Amplifier
b. A low reading on a range which gets progressively
worse on higher ranges is normally caused by leakage to
ground. Leakage can be caused by the 3456A Input
Switching Circuitry, Input Amplifier, Ohms Protection
Circuitry, and/or the Ohms Current Source. If this type
of failure is noted, go to Paragraph 8-0-23 to determine
the faulty circuitry.
8-0-22. Since a faulty circuitry can often be isolated by
the failing ranges, one other thing should be kept in
mind. Sometimes a range may appear good, but is in ac­
tuality bad. For example, the
Ranges show low readings. However, the 1 00
Range may also be low. This may not show if the error
causing the low reading has been adjusted out by
Calibration Pot H . It is sometimes possible to adjust an
inaccurate range to tolerance.
8·0·23. Ilollting the Ohlll l Circuitry.
8-0-24. To isolate the faulty circuitry causing Ohms
Range Failures due to leakage, do the following:
a. Press the RESET button on the 3456A.
b. Acquire an accurate resistor with the same value as
the defective ohms range (e.g. 100
bad 100 ohm Range). It is best to use a resistor value
K
that corresponds to the worst range. I f an accurate
resistor is not available, use one of which the exact value
is known.
c. Connect the resistor at the VOLTS (2WRO,
4WRO) Input Terminals.
d. Set the instrument to the 2-WRO Function, the
defective Range, and the 6 Digit Mode (by storing "6"
into the N DIG DlSP Register).
e. Turn the 3456A's Autozero Off.
Service
Cause
Cause
M through 0 ohm
I
1
ohm
K
ohm resistor for a
K
8·0·3