Fluke 5220A 4-39. Resistors R11 and R12

5220A

Instruction Manual

4-16

4-39. Resistors R11 and R12

Resistors R11 and R12 are associated with the Input Offset Adjustment procedure and the

setting of the ZERO ADJ potentiometer R6. Use the following procedure to select the

proper replacements for R11 and R12:

1. Disconnect the 5220A from line power.

2. Remove the top and large inner covers from the unit.

3. Remove the two fuses (F1 and F2) from the A3 Cap Bus PCB Assembly.

4. Mount the A5 Preamplifier PCB Assembly on the extender board and install jumper

wires on the PCB at the following points:

a. TP1 to TP5

b. TP3 to TP4

c. Short across R11

d. Short across R12

5. Disconnect the red and black Current Shunt sense lead from the motherboard

(connections are located near the A5 Preamplifier connector). On A8 the Analog

Control, short TP9 to TP13 to prevent a false overcurrent trip.

6. Set the digital voltmeter to the 100 mV dc range and connect its input to TP4(+) and

TP5(-) on the A5 Preamplifier PCB Assembly.

7. Adjust R6 on the A5 Preamplifier to the full ccw (counterclockwise) position.

8. Connect the 5220A to line power and set the POWER switch to ON.

9. Select the operate mode using the STDBY/OPR switch.

10. Note the DVM reading. It should read with in the rang e of +/-1.300 mV dc.

11. Adjust R6 on the A5 Preamplifier PCB to the full cw (clockwise) position and note

the reading. The DVM reading should increase in a positive direction.

12. Calculate the difference between the DVM readings taken in steps 10 and 11. The

difference should be 350 µV +/- 70 µV dc.

13. Determine the average reading of steps 10 and 11 and adjust R6 for that reading on

the DVM. Then perform step a, b, or c as required:

a. If the DVM reading is within the range of +/-0.087 mV dc, replace both R11 and

R12 with shorting wires (0 Ω).

b. If the DVM reading is negative, refer to Table 4-4, and replace R12 with the

appropriate resistor. Replace R11 with a shorting wire.

c. If the DVM reading is positive, refer to Table 4-4 and replace R11 with the

appropriate resistor. Replace R12 with a shorting wire.

14. Set the POWER switch to OFF and disconnect the 5220A from line power.

15. Install the two fuses (F1 and F2) on the A3 Cap Bus PCB Assembly.

16. Remove the jumpers from TP1-TP5 and TP3-TP4.

17. Disconnect the DVM from TP4 and TP5.

18. Remove the A5 Preamplifier PCB from the extender board.

19. Reconnect the red and black curren t shunt sens e leads to the mothe rboa rd.

20. Install the A5 Preamplifier PCB Assembly in its slot.

21. Perform the Calibration Adjustment procedures.