Ohms Protection Circuitry Troubleshooting - HP 3456A Operating And Service Manual

Service
r. orr
Turn A20K 101
A2OQ702 to ground. (Q702 is located in the Range and
Function Control Logic.)
g. Remove the red wire from A20J20 and the white
wire from J19.
h. Using a clip lead, connect the removed red wire to
the base of A2OQ31O (pin 3). (Q3 1O is in the Input
Amplifier Circuitry.)
i. Using another clip lead, connect the emitter of
A2OQ21O to the bas of Q31O.
j. If the ohms reading is good or improves
significantly (good within 20
Circuitry is at fault. Go to Paragraph 8-0-25 for
troubleshooting.
v
k. If the reading is still bad, acquire a 5 V dc Power
Supply and apply - 5 to the base of Q21O.
I. If the reading is good or improves significantly
(good within 20 cts), the Ohms Protection Circuitry is at
fault. Go to Paragraph 8-0-28 for troubleshooting.
m. If the reading is still bad, remove both clip leads
from the base of Q31O. Connect the removed leads to
either side of A20R 103.
n. Connect a high input impedance Digital Voltmeter
(like the 3456A) to either side of R 103 and ground.
Measure and note the voltage at R103.
o. 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
leaky Q310. If the reading remains the same, the Ohms
Current Source is at fault. Go to Paragraph 8-0-34 for
troubleshooting.
8·0·25. INPUT SWITCHING TROUBLESHOOTING.
8-0-26. Most Ohms Failures caused by the Input Swit­
ching Circuitry should also show up in the OCV Func­
tion. Make sure the
troubleshooting for any Ohms Failures. Go to Service
Group C, if the OCV Function is bad.
8-0-27. The Ohms Failures caused by the Input Swit­
ching is normally leakage to ground. This failure shows
up as a low reading on a range with the higher ranges
worse (lower reading). Only a few components in the In­
put Switching can cause leakage without affecting the
OCV Function. To isolate the components, do the
following:
a. Unsolder and lift one side of E l . (El is located at
the A40 Assembly on the bOHom of the chassis.)
b. If the defective ohms range is now good, replace
E l . If not, unsolder and lift one side of A20R I O I .
c. If the range is now good, replace A20K 102. I f not,
replace KIOI.
8-0-4
by shorting the base of
CU),
the Input Switching
function is good, before
8·0·28. OHMS PROTECTION
SHOOTING.
8-0-29. Use the following troubleshooting information
if it has previously been determined that the Ohms Pro­
tection Circuitry is at fault. (The procedUres to deter­
mine if this or other circuits are al fault are in
Paragraphs 8-0-18 and 8-0-20.)
8-0-30. the Ohms Protection Circuitry can cause Ohms
Failures due to leakage to ground, an open Protection
Circuitry, and noisy readings. Leakage to ground shows
up as a low reading on a range with the higher ranges
worse (lower reading). An open Protection Circuitry
shows up as a zero reading. Troubleshoot the Protection
Circuitry using the following information.
Only three components in the Ohms
a. Leakage.
Protection Circuitry can cause this failure. Do the
following:
1 . Unsolder and lift one side of A2OCR201 .
2. If the defective ohms range is now good, replace
CR201. If not, unsolder and lift E20 1 .
3 . I f the range is now good, replace E20 1 . I f not,
replace K 101.
b.
Open
Circuit or Noise.
component(s) in the Ohms Protection Circuitry can be
done by collectively shorting across the individual com­
ponents. Do the following:
I. Connect one end of a clip lead to the Ohms Input
at A20KIOI (red wire connected to KIOI).
2. Connect the other end of the clip lead to the other
side of KIOI. If the ohms failure was noise and the
reading is now good, replace KIOI. If the ohms
failure was a zero reading and is now good, KIOI
and/or its associated circuitry can be bad. Make
sure the relay is being turned on by Q702. If not,
check for the correct logic level. A bad U702
and/or the Inguard Logic can cause a wrong logic
level. Go to Service Group B if the Inguard Logic
is suspected.
3 . If the reading is still wrong, remove the clip lead
from the output of KIOI. (The output of the relay
is the lead opposite the one connected to the red
wire.)
4. Connect the removed lead to the cathode of
CR202. If the reading is good, CR202 or R222 is
open or noisy.
5 . If the reading is still wrong, check for an open or
noisy Q20 I and Q211 .
8·0·3 1 . INPUT AMPLIFIER TROUBLESHOOTING.
8-0-32. Use the following troubleshooting information
if it has previously been determined that the Input
Amplifier is at fault. (The procedures to determine if
this or other circuits are at fault are in Paragraph 8-0-18
and 8-0-20.)
Model 3456A
CIRCUITRY TROUBLE·
Isolating the defective