HP 3478A Technical Manual page 76

3478A
also in Service Group D). If the procedure has been
performed already, ignore the procedure and go to
paragraph 7-D-21 for troubleshooting. If it has not been
performed, do the following:
a. Set the 3478A to the DC Volts Function and the
3V Range.
b. Turn Autozero off by pressing the blue Shift
button and then the INT/TRIG (AUTOZERO) button.
c. Apply a stable + 3V dc to the INPUT Terminals.
d. With a Digital Voltmeter (like the 3456A)
measure for +10V at JM101.
e. Make sure the reading on the test voltmeter is a
stable +10V. If the reading on the test voltmeter is a
stable +10V, the A/D Converter is at fault. Go to Service
Group D for troubleshooting.
f. If the reading on the test voltmeter is wrong
(overload, constant zero, floating, or noisy), unsolder and
lift the end of jumper JM101 which is connected to the
A/D Converter (toward the rear of the 3478A).
g. If the reading on the test voltmeter is now good,
the A/D Converter is at fault. Go to Service Group D for
troubleshooting.
h. If the reading is still wrong, the Input Circuitry
(Input Switching and DC/Ohms Input Amplifier) is at fault.
Go to paragraph 7-A-21 for troubleshooting.
i. Replace jumper JM101.
7-A-9. Constant Zero Readings (with no input applied)
on All Ranges
7-A-10. Since the 3478A's 30V and 300V Ranges will
normally have a constant zero reading with no input
applied, the other ranges will also be at zero if relay K102
is shorted. Make sure the relay is good and is not being
turned on by U102 (zero volts across the coil of K102). If
the relay is turned on in the 30mV through 3V Ranges,
U102 may be defective. Before replacing U102, make
sure the A/D Controller (U462) is good. Go to paragraph
7-A-33 for the SA procedure to check U462, before
replacing the hybrid.
7-A-11. Overload, Constant Zero, Floating, or Noisy
Readings on Some Ranges
7-A-12. The Input Hybrid (U102) in conjunction with the
Input Relays (KI01 through K104) is used to select
different paths to connect the input signals with the
DC/Ohms Input Amplifier. The hybrid is also used to
select the amplifier's different gain configurations.
Because of this, a relay or U102 can make certain
ranges fail.
7-A-13. Overload. An overload condition exists if the
DC/Ohms Input Amplifier's feedback circuitry is open.
Since the feedback circuitry is in U102, the hybrid is most
likely defective. Before replacing U102, make sure the
A/D Controller (U462) is good. Go to paragraph 7-A-33
for the SA procedure to check U462, before replacing the
hybrid.
7-A-14. Constant Zero Reading.
cause is the Input Hybrid (U102).
U102, make sure the A/D Controller (U462) is good. Go
to paragraph 7-A-33 for the SA procedure to check
U462, before replacing the hybrid.
7-A-15. Floating Readings.
normally caused when an input path to the DC/Ohms
Input Amplifier is open. Check for the following:
a. If the 30mV, 300mV, and 3V Ranges are
defective, do the following:
1. Short across the contacts of relay K101.
2. If the ranges are now good, make sure KI01
is energized (5V dc across the coil).
energized, replace the relay. If not, U102 may be
defective. Go to paragraph 7-A-33 before replacing
U102.
3. If, after replacing K101, the ranges still fail,
U102 may be defective. Go to paragraph 7-A-33
before replacing U102.
b. If the 30V and 300V Ranges are defective, do
the following:
1. Short across the contacts of relay K102.
2. If the ranges are now good, make sure K102
is energized (5V dc across the coil).
energized, replace the relay. If not, U102 may be
defective. Go to paragraph 7-A-33 before replacing
U102.
3. If, after replacing K102, the ranges still fail,
U102 may be defective. Go to paragraph 7-A-33
before replacing U102.
7-A-16. Noise. Noise can be caused by a FET switch
internal to the Input Hybrid (U102) and UI01. Replace
U101 and then U102 if noise is noted on some ranges.
7-A-17. Inaccurate Readings on All Ranges
7-A-18. Inaccurate readings normally show up as other
than positive full scale readings. This is because the
3478A is calibrated using zero and positive full scale (or
1/3 scale) inputs. Inaccurate readings can be caused by
the Input Circuitry (Input Hybrid or DC/Ohms Input
Amplifier). Try replacing U101 and then U102.
7-A-2
TM 11-6625-3071-14
The most likely
Before replacing
A floating reading is
If K101 is
If K102 is