Input Switching Troubleshooting; Dcv Circuitry Isolation - HP 3456A Operating And Service Manual

Model 34S6A
8-C- 14. The faully circuitry can ofte" be determined by
what range(s) fail. look for the following symptoms.
a. If only one range or certain ranges fail, the cause is
most likely open or shorted FETs in the Input Switching
or Input Amplifier Circuitry. For failues in the Input
Switching Circuitry, go to Paragraph 8-C-17 for
Iroubleshooting. For failures in the Input Amplifier, go
10
Paragraph 8-C-3S for troubleshooting. The following
is a listing of the ranges and most likely causes.
Only Ranges Fail
1 00 mV
100 mY, I V, 10 V
V, 100 V
I
1000
IO V, I000 V
100 V, V
Only Ranges Good
1 00 mV
I V, 1 00 V
IO V, I000 V
b. I f the lower ranges fail or are less accurate than the
higher ranges, the failure can be caused by leaky or par­
I
tially shorted FETs. For example, the 10 V Range may
be good with the V and 100 mV Ranges reading getting
progressively worse. To determine the faulty circuitry,
further isolation is necessary. Go to the next paragraph
to isolate the circuitry.
8·C·1S. DCV Circuitry Isolation.
8-C-16. The following is a procedure to isolate the faul­
ty DCV Circuitry causing range failures due to leakage.
a. Press the 34S6A's RESET button.
b. Set the instrument to the Digit Mode (by storing
"6" inlO the N DIG D1SP Register).
c. Turn Autozero Off.
d. Apply a stable and accurate + 100 mV dc to the
VOLTS Input Terminals (an accurate and stable
+
can be used instead of 100 mY). I f an accurate voltage
is not available, use one of which the exact value is
known.
e. With a high impedance and accurate Digital
Voltmeter (like the 34S6A), measure the voltage at
either side of A20RI03.
f. If the voltage is a stable and accurate
I V), the Input Amplifier is at fault . Go to Paragraph
8-C-3S for troubleshooting.
g. If the voltage is still wrong, unplug the cable from
A20J l9.
h. If the voltage changes to another value or the same
value as the input voltage, the Input Amplifier is at
fault. Go to Paragraph 8-C-3S for troubleshooting.
i. I f the voltage is still wrong. the Input Switching is
at fault. Go to Paragraph 8-C-J7 fOr tr9ubleshooting.
Cause
Input Amplifier
Input Switching
Input Amplifier
Input Amplifier
Input Switching
Cause
Input Amplifier
Input Amplifier
Input Amplifier
I V
+
100 mV (or
+
B·C·17. INPUT SWITCHING TROUBLESHOOTING.
8·C·18. General.
8-C-19. Before troubleshooting the Input Switching
Circuitry, make sure the failure is not in the Input
Amplifier. Refer to Paragraph 8-C-8 and 8-C-IS to
determine the faulty circuitry.
8-C-20. Failures caused by the Input Switching can be a
low, High, Zero, or Overload (Ol) Reading. The
failures can be caused by Open FETs and Relays,
Shorted FETs and Relays, and/or Leaky FETs and
Relays. Failures can also be caused by the Gate Bias
Amplifier and Analog Filter. The following paragraphs
have the DCV Failures and possible causes.
NOTE
The Charge Dump and Charge Correction
Adjustments may have to be readjusted,
any components in the Input Switching Cir­
cuitry are replaced. Go to Service Group
for the Adjustment Procedure.
8·C·2 1 . All Ranges Inoperative.
8-C-22. This type of failure can be a Zero, Overload
(Ol), Inaccurate. (High or Low), or Noisy Readings.
Another possible failure is a noating reading (no change
when an input is connected or disconnected). To help
find the faulty component, first determine what the
failure is (Overload, Zero, etc.). This is done by apply­
ing a voltage with its amplitude the same as the full scale
value of a failing range ( I V for an inoperative I V
Range, etc.) and noting the failure. The following are
the failures and troubleshooting information.
Since this failure is on all
8-C-23. Zero Reading.
ranges, the cause is most likely in the lower Input Node
(drain side of A2OQ I03). Do the following:
a. Press the 34S6A's RESET button.
b. Set the instrument to the 10 V Range and turn Off
A2OQI03 (by connecting its gate to A20TPIOS).
c. With a high impedance Digital Voltmeter (like the
34S6A), measure the voltage on either side of A20R 103.
d. Set the 34S6A to the 4-WRO Function. If the
voltage read on the Test Voltmeter appears to be chang­
ing from approximately zero to some negative value,
A2OQ IOS is shorted or very leaky. If the voltage remains
steady, Q102 is shorted or very leaky.
8-C-24. Overload Reading.
noted when a high level voltage
the Upper (source of Q103) or lower (drain of Q103)
Input Node. This is normally caused when the gate of a
turned off FET shorts to the Input Node. To
troubleshoot Overload Failures on all ranges, do the
following:
Service
if
F
This failure is normally
1 2 V) is on either
( > ±
B-C-3