HP 3478A Technical Manual page 80

3478A
f. If the reading is good, do the following:
1. Measure for a stable 3V ac at pin 4 of U303.
2. If the reading is good, replace U303.
3. If the reading is wrong, lift the end of
capacitor C304 which is connected to pin 4 of U303.
Then measure for a stable 3V ac at the lifted end of
the capacitor.
4. If the voltage is good, replace U303.
5. If the voltage is still wrong, make sure the
voltage at pin 6 of U302 is a good stable 3V ac.
6. If the voltage is good, capacitor C304 or
C306 is defective.
7. If the voltage is unstable or wrong, connect
pin 6 to pin 2 of U302 (use a very short lead to
connect the pins, to prevent oscillations).
reading at pin 6 is now a stable .12V, the feedback
resistors of U302 are defective.
wrong, replace U302.
g. If the reading at JM303 is other than .12V ac or
unstable, replace U301. If the AC Volts Function is still
inoperative, U102 may be defective. Before replacing
U102, make sure the A/D Controller (U462) is good. Go
to paragraph 7-B-16 to check the A/D Controller. If the
controller is good, replace U102.
7-B-8. Inaccurate Reading on All Ranges
7-B-9. Since the 3478A's AC to DC Converter is
calibrated with an input voltage at a frequency of 1KHz,
inaccuracy can result from poor frequency response.
Since the high frequency is compensated by C302, make
sure the capacitor is good. If the capacitor is good, try
replacing U301 and if still inaccurate, try U102.
7-B-10. Overload, Inaccurate, Floating, or Noisy
Readings on Some Ranges
7-B-11. Since all ranging of the AC to DC Converter is
done in the Input Hybrid U102, the hybrid is the most
likely cause for ac failures on some ranges.
replacing U102, make sure it is not set to an incorrect
mode by the A/D Controller (U462). Go to paragraph 7-
B-16 to check the A/D Controller.
good, replace U102.
7-B-12. AC CURRENT TROUBLESHOOTING
7-B-13. Make sure the DC Current and AC Volts
Functions are operating correctly on all ranges, before
troubleshooting for ac current failures. Go to paragraph
7-B-3 to troubleshoot the AC Volts Failures and Service
Group A for the DC Current Failures. If the functions
If the
If the .12V is
Before
If the controller is
7-B-2
are good, the only component that can cause a failure is
the Input Hybrid U102. Before replacing U102, make
sure the A/D Controller is good. Go to paragraph 7-B-16
to check the A/D Controller. If the controller is good,
replace U102.
7-B-14. OVERLOAD PROTECTION CIRCUITRY
TROUBLESHOOTING
7-B-15. All of the ac functions overload protection
circuitry is in U102. Replace the hybrid, if defective.
7-B-16. CHECKING THE AID CONTROLLER (U462)
7-B-17. Do the following to check the A/D Controller.
Refer to Schematic 3.
a. Turn the 3478A off.
b. Move jumpers JM502, JM503, and JM403 to the
"D" position (JM403 pin 1 and 2), as shown in Figure
7-B-1.
c. Obtain a Signature Analyzer. Set and connect
as follows (shown in Figure 7-B-1):
Start: TP401 (JM403 pin 6) (
Stop: TP402 (JM403 pin 5) (
Clock: TP403 (JM403 pin 4) (
Hold: Out
Self-Test: Out
Gnd: Ground Pin (next to C203)
d. Turn the 3478A on and check for the following
signatures.
U462 pin 32: 7ACA
U462 pin 33: 20F0
U462 pin 34: 666H
e. If any signatures are wrong, go to Service Group
D (Flowchart D) for troubleshooting.
f. If the signatures are good, the Input Hybrid is
defective.
Figure 7-B-1. JM403 SA Connection
TM 11-6625-3071-14
)
)
)