3478A TM 11-6625-3071-14
the A/D Controller (U462) is good. Go to paragraph
7-C-24 to check U462.
5. If the voltage is good, connect pin 6 to pin 3
of U201 (U102 pin 6 to pin 7).
6. Measure for approximately + 8V at pin 6 of
U201.
7. If the voltage at pin 6 of U201 is +8V, replace
U102. Before replacing U102, make sure the A/D
Controller (U462) is good. Go to paragraph 7-C-24
to check U462.
8. If the voltage is other than +8V, replace
U201.
e. If the voltages on both pins of U203 are
approximately +8V, do the following:
1. Measure for approximately +12V at pin 6 of
U202.
2. If the voltage is wrong, connect pin 6 to pin 3
of U202.
3. If the voltage is now good, replace U102.
Before replacing U102, make sure the A/D
Controller (U462) is good. Go to paragraph 7-C-24
to check U462.
4. If the voltage is still wrong, replace U202.
f. If the Ohms Current Source is still inoperative,
replace U203 and then U102.
7-C-22. 4-WIRE OHMS TROUBLESHOOTING
7-C-23. The only difference between the two ohms
functions is that the 4-Wire Ohms Function uses the
Ohms SENSE Terminals and a different input path in
U102. Make sure the Front/Rear Switch is good and that
the wires from the terminals are connected correctly.
Also, make sure the lead resistance of the test used in
the 4-Wire Ohms Function is not excessive (<1/30 of full
scale reading in the LO INPUT lead and <1/3 of full scale
in the HI INPUT lead). If everything appears to be good,
U102 may be defective. Before
replacing U102, make sure the A/D Controller (U462) is
good. Go to paragraph 7-C-24 to check U462.
7-C-24. CHECKING THE A/D CONTROLLER (U462)
7-C-25. 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-
C-1.
c. Obtain a Signature Analyzer. Set and connect
as follows (shown in Figure 7-C-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)
Figure 7-C-1. JM403 SA Connection
d. Turn the 3478A on and check the following
signatures:
U462 pin 32: 7ACA
U462 pin 33: 20FO
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.
7-C-4
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