Acv Troubleshooting; Inaccuracies At All Frequencies; Inaccuracies At High Frequencies - HP 3456A Operating And Service Manual

Model 3456A
8. If any of the transistors are not turned on, replace
the defective transistor.
9. I f the transistors are turned on, try replacing Q 17.
ID.Turn the 3456A Off and remove the clip lead bet­
ween TP8 and QI6A.
h. If the signal at TPI6 is good, check for the signal
shown in Figure 8-E-IB at TPI7.
i. If the signal is not present, different, or unstable,
do the following:
I . Make sure QI6B is turned on (approximately .6 V
across the emiller-base junction).
2. If the voltage is good. replace Q 17. I f not, replace
U I 3 .
3 . Replace U 1 3 .
j . I f the signal at TPI7 is good, do the following:
I . Leave the gate of Ql4 connected to ground. Using
another clip lead, carefully connect TPID to pin 2
of U I 2 (or short across C28).
2. Using a high impedance Digital Voltmeter (like the
3456A), measure the voltage at TPID.
3. If the voltage is not zero (>
replace U12.
4. If the voltage is good, replace Q14.
r
8·E·38. ACV TROU8LESHOOTlNG.
8·E·39. Generll.
8-E-40. Use the following procedures to troubleshoot
the 3456A if the intrument only fails in the ACV Func­
tion. Also use the procedures if the instrument fails with
ac inputs (in both
Functions).
8-E-41 . If the ACV Function is inoperative and the
ACV + DCV Function is good, A40KI may be open.
This can be checked by shorting across the relay'S con­
tacts. If the function is then good, make sure KI is turn­
ed on by QI9 and UIO. If Ql9 and UIO are good,
replace K I .
8-E-42. AC Failures i n both the ACV and ACV
Functions with no corresponding DC Failures in the
+
ACV DCV Functions,
Readings. This can show up as wrong readings at all fre­
lOO
quencies, lower «
( > kHz) frequencies. The following are the failures
and troubleshooting information.
8·E-43. Inlccurecitl At All Frequ.ncitl.
8-E-44. Check and do the following:
a. If the failure is in all ranges, do the following:
1 .
Press the 3456A's RESET button.
10
mY) or unstable,
the ACV and ACV
are normally Inaccurate
400 Hz) frequencies, or higher
2. Set the 3456A to the ACV
1 0 V Range.
3. Apply a
Input Terminals.
4. Make sure the signals at A4OTP3 and pin 6 of U3
are a clean half-wave sine wave.
5. If the signals are good (not oscillating), the RMS
Converter is most likely at fault. Go to Paragraph
8-E-31 for troubleshooting.
6. If the signals are oscillating, measure the signal at
TP9.
7. If the signal is a clean sine wave, make sure CI6,
CI7, and CI8 are good. The capacitors can be
checked by connecting another capacitor with the
same value across the individual capacitors. If the
capacitors are good, replace U4.
S. If the signal is oscillating, make sure C25 is good.
If good, U7 or U6 may be defective.
b. If the failure shows up on some ranges, the Input
Attenuator is at fault. Go to Paragraph 8-£-IS for
troubleshooting.
8·E·45. Inaccuracie. At High.r frequencies.
8-E-46. Check and do the following:
a. If the failure is in all Ranges, check the following:
I . Make sure capacitors A4OC16, CI7, and CIS are
good.
2 . Check for a frequency sensitive CR5 and CR7.
3. Make sure QI7 and QI6 are not frequency sen­
sitive.
b. If the failure is in only the I V ,
+
DCV
Ranges, check and do the following:
I . Make sure capacitors A4OC22, and C25 are good.
The capacitors can be checked, if open, by con­
necting another capacitor with the same value
across the individual capacitor.
2. Repad C26, as given in Servie Group F (ACV Ad­
justment). If another value C26 improves the ac­
curacy, replace U7. (Note: C26 may need to be
repadded, if U7 is replaced_)
+
DCV
3. If the capacitors and U7 appear to be good, try
replacing Q2 or U6.
c. If the failure is in either the I V, I D V, or 100 V
Range, the failure can be caused by a defective A4OCIO,
Cl I, or C12. The faulty capacitor can be determined by
which range fails. The following lists the ranges and
faulty component.
Only Range Fails
IO V
100 V
d. If the failure is o n only the 1000 V Range, check
and do the following:
+ DCV Function and
10
V, 1000 Hz sine wave to the VOLTS
ID
Cause
I V
A40C l l
A4OC1 O
A4OC1 2
Service
V. and 100 V
8-E-5