HP 3465B - Page 52

Model 3465B
Section VII
7-20. Signal Conditioning Faults.
7-21. AC/DC Volts — Common Fault. If the fault is com
mon only to the ac and dc volt function, check the input
attenuator and associated switches.
7-22. AC Volts/AC Current Common Fault. The most ex
pedient way to check out the impedance and ac/dc con
verters is to trace the input signal through with an oscillo
scope. With I V ac applied to the input in the 2 V ac range,
the following are normal.
a. The waveform on U19, pin 3, and C49 should be the
same as the input or about 3 volts peak-to-peak.
b. At the ACTP test point, the amplitude should he
about twice the input or about 6 volts peak-to-peak with a
dc offset of -I- 1.6 volts.
c. The signal at the junction of RlOO and R118 should
be a half wave rectification of the signal seen in (B).
d. The voltage across C38 should be about 0.75 volts
dc.
e. If one-half full scale readings are alright but full scale
is in error, check C41 for leakage.
f. Out-of-spec readings could be caused by leakage in
the filter capacitors C36, C37 and C38. This condition can
be readily checked by applying an external 1 V dc at the
junction of RlOO and R118 and verifying 1 V dc across
C38.
g. If an inaccuracy is unique to the 500 V range, check
R75.
7-23. AC/DC Current Common Fault. Check AMPS fuse,
and check that contact is being made through the terminal
fuse spring to the PC board.
7-24. AC Current. If the fault is unique to ac current,
check R20 and R39.
7-25. Ohms. Short the junction of C29 and R78 (Q25A
gate) to ground. Rotating R69 (10 Mf2 adj.) through its
extremes should result in the output (pin 6) of U15 to span
the voltage range of about - 2.5 V dc to -i- 10 V dc. This
establishes that Q25 and U15 are functional. Reset R69 in
accordance with the prescribed adjustment procedure of
the manual.
a. Depress the T2 and 20 m buttons, and short the input
f2 and COM terminals. If the instrument stops sampling or
the front panel count is greater than ± 15 counts, check the
ohms calibration.
b. If the 200 n range zero is out of tolerance (± 3
counts) check C31 for leakage.
c. If all readings are progressively increasing out of spec
as the measured resistance is increased, check diodes (par
ticularly CR15, CR24, CR25 and CR26) and capacitors
C29 and C31 for leakage.
7-26. Analog-to-Digital Converter Faults.
7-27. The Input Amplifier was basically treated in the
general isolation section. If the problem is attributed to the
input amplifier, the fault can be further localized as fol
lows:
a. Short the 10 signal at the input of U2 to ground.
(The instrument is now locked in the run-up phase.) Moni
tor the voltage at the DCTP point. The dc level should be
essentially zero with the input shorted and should vary pro
portionally to 2 V as the instrument's input is varied to full
scale of any particular range. If it does not, the fault is for
ward of the DCTP point and then proceed to Step b,
otherwise, go to Paragraph 7-29.
b. Short the input gate of Q16 (coming from Q20) to
ground. With the instrument on the 2 V range, check the dc
voltage at the DCTP point. It should he less than 1 mV. If
it is not. short U1 pins 2 and 3 together. If the voltage has
not essentially zeroed, check UI. If the latter did zero the
DCTP voltage, check Q16 and/or adjust R50 to yield the
desired less than 1 mV offset.
7-28. The outputs of U2 should also be verified in the nor
mal running mode. The waveforms should appear as:
-7V-
0»ls
* 0«3» m
at U2 pin 4, and
OV-
-7V^—
at U2 pin 3. If the 10 control signal is present and either of
these are incorrect, check U2, Q19 and Q20.
7-29. Integrator/Slope Amplifier/Comparator/Auto-Zero.
7-30. Proceed as follows to ascertain the condition of the
integrator, slope amplifier, comparator, and auto-zero cir
cuits.
7-3