2. Service
Functional-Group-Diagnostic Trees (Diagnostic Section 2)
81
C-Field Diagnostic Tree
(Functional-Group Subsection 4)
This subsection describes the C-field diagnostic tree for the 5071A Primary
Frequency Standard. (Refer to the A2 connector pinout diagram ahead of
procedure 3.1.1.1.0 for pin number and location.)
2.4.1.0 C-Field Current Test
1 Remove all power from the 5071A.
2 Remove the jumper on the right-hand side of A2J2 labeled CFC
(C-Field Current jumper.).
3 Connect the Multimeter to the pins which were occupied by the jumper using
small clip-leads.
4 Set the DMM to read dc current.
5 Reapply instrument power and wait about 15 seconds. Use the
CONFIG/MODES menu to put the instrument in STANDBY.
6 Compare the current shown on the DMM with the nominal current displayed
on the instrument LCD using the INFO/C_field Menu. (The display units are
Milliamperes. The current value is set by the instrument to match the
parameters of the CBT.)
7 Remove instrument power and replace the CFC jumper.
QUESTION: Are the measured and indicated current magnitudes in
agreement to within 0.2 mA?
If Yes: The C-Field system is working correctly.
If No: Go to 2.4.2.0.
2.4.2.0 C-Field Current Source Test
1 Remove all power from the 5071A.
2. Disconnect CBT wiring harness from A2J2.
3. Replace the jumper on the right-hand side of A2J2 labeled CFC
(C-Field Current jumper.).
4. Connect the DMM between pins 4 and 14 of A2J2.
5. Set the DMM to measure dc current in the 10 to 100 mA range.
6. Reapply instrument power and wait about 15 seconds.
7. Measure the C-field current supplied.
8. Remove power and reconnect CBT wiring harness to A2J2.
QUESTION: Is the C-field current the same as measured in 2.4.1.0
above, within ±0.2 mA?
If Yes: Re-check results before replacing A17 CBT.
If No: Remove power, replace A7, and retest. Sustained failure indicates
that either A2 or traces on the A1 Motherboard are at fault.
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