Acv And Acv+Dcv Troubleshooting - HP 3456A Operating And Service Manual

Service
R·E·I 1 . ACV AND ACV+oCV TROUBLESHOOTING.
8-£-12. Use the following troubleshooting information
if both the ACV (ac inputs) and ACV + DCV (ac and dc
inputs) Functions fail.
8·E·13. All R.ngas Inoperative (Dverload, laro Reading,
Accuracy, or Noise).
8-£-14. The Range Failure can show up as a constant
Overload (OL), Zero Reading, Noise, or Inaccuracy on
all ranges. This can be caused by the Input Attenuator,
Absolute Value Amp, or the RMS Converter. Except
for Inaccuracy, use the following procedure to deter­
mine the faulty circuitry. If the ranges are inaccurate in
either the higher ( > 100 kHz) or lower «
quencies, and appear in only the ACV Function, go to
Paragraph 8-£-38 for troubleshooting. If inaccuracy
and any of the previous symptoms are noted in both
functions with ac (ACV and ACV + DCV Function) or
dc (ACV + DCV Function only) inputs, do the follow·
ing:
a. Press the 3456A's RES£T button.
b. Set the instrument to the ACV
and I V Range.
c. Apply a stable and accurate
VOLTS Input Terminals. Use the 3456A's DCV Func­
tion to determine the accuracy.
d. Using a high impedance Digital Voltmeter (like the
3456A), measure for + 1.25 V ( ± 2 mY) at A40TP3.
e. If the voltage is wrong or unstable, do the follow·
ing:
I . Measure for + .5 V ( ± I mV) at TP9.
2. If the voltage is stable and within limits, the Ab­
solute Value Amp is at fault. Go to Paragraph
8-£-28 for troubleshooting.
3. If the voltage is wrong or noisy, the Input At­
tenuator is at fault. Go to Paragraph 8-£-18 for
troubleshooting.
f. If the + 1.25 V is stable and within limits, do the
following:
l .
Remove the - 1 V from the Input Terminals and
apply an accurate and stable +
minals. Use the 3456A's DCV Function to deter­
mine the accuracy.
2. Measure for + 1.25 V ( ± 2 mY) at pin 6 of U3.
3. If the vohage is stable and within limits, the RMS
Converter is at fault. Go to Paragraph 8-£-31 for
troubleshooting.
4. If the voltage is wrong or noisy, measure for
- .5 V ( ± I mY) at TP9.
5. If the voltage is stable and within limits, the Ab­
solute Value Amp is at fault. Go to Paragraph
8-E-28 for troubleshooting.
8-E-2
8·E·15. Some Ranges Inoperative.
8-E-16. A Range Failure can show up as a Constant
Overload (OL), Zero Reading, Noise, or Inaccuracy on
some ranges. Since only some ranges fail, the mosl like­
Iy cause is the Input Attenuator Circuitry.
8-£-1 7 . The Input Attenuator consists of two different
circuitry: High Voltage and Low Voltage. The circuitry
can be isolated by what ranges fail. Check and do the
following:
400 Hz) fre­
a. Only the I V, 10 V, and 100 V Ranges
Low Voltage Attenuator (A40U6, Q2,
associated circuitry) is at fault. Check and do the
following:
2. Set the instrument to the ACV + DCV Function.
3. Check the
+
DCV Function
-
I V dc to the
4. Using a high impedance Digital Voltmeter (like the
5 . If the voltage on all ranges are wrong, the Input
6. If the voltage is wrong on only some ranges, the
b. Only the 1000 V Range fails.
Attenuator is at fault. Check for the following:
I
V dc to the ter­
2. If a zero reading «
3 . I f the range is inaccurate, make sure the 2.5
4. If the 1000 V Range is noisy, carefully short the
6. If the voltage is wrong or noisy, the Input At­
tenuator is at fault. Go to Paragraph 8-£-18 for
troubleshooting.
I . Press the 3456A's R£S£T button.
I
V, 10 V, and 100 V Ranges by apply­
ing alo:curate and stable - 1 V,
V dc to the VOLTS Input Terminals, respectively.
Use the 3456A's DCV Function to make sure the
voltages are accurate.
3456A), measure for + .5 V ( ± 1 mY) at A40TP9
for all ranges.
Attenuator itself is at fault. Go to Paragraph
8-£-18 for troubleshooting.
Feedback FETs and/or Feedback Resistors are
defective. Go to Paragraph
troubleshooting.
I . If an overload condition is noted. check for
oscillation at A40TP2. If oscillation is present, C3
may be open. To check for an open capacitor,
connect another capacitor with the same value in
parallel with C3. If the oscillation stops, replace
C3. If no oscillation is present, make sure the 2.5
ohm resistor in V3 is good. If good, V I may be
K
defective.
20 counts in the 5 Digit Mode)
is noted, A4OQ8 may be open. Make sure the F£T
is not held open by its logic. Use Table 8-£-1 to
determine the correct level. If Q8 is good, try
replacing U I .
V3
ohm resistor in is good. If good, U I may be at
fault.
source to drain of Q8. If the range is now good,
replace Q8. If still noisy, replace
Model 3456A
V7,
fail. The
and
-
10 V, and - 1 00
8-£-18 for
The High Voltage
K
")
V I .