Fluke 8842a instruction manual Test Point, Minimum Maximum

Maintenance	6
TROUBLESHOOTING
f6-18.wmf

Figure 6-18. Calculating the A/D Reading From TP102 Waveform

	Table 6-23. Power Supply Voltages
TEST POINT		LIMITS (in volts)

MINIMUM

MAXIMUM

+5V

+7.5V

+15V

+30V

-8.2

-15V

-30V

4.75

7.00

14.25

28.45

-8.61

-15.75

-31.55

5.25

7.87

15.75

31.55

-7.60

-14.25

-28.45

If a supply is too low, there are a number of possible causes. First check the input to the affected regulator. If it is not at least 1V above the maximum output given in Table 6-23, the cause may be a bad transformer winding (check the resistance), open or shorted rectifiers, a shorted filter capacitor, or a shorted regulator. The latter two failures will usually blow the line fuse.

All regulators incorporate current-limiting which allows them to shut down in the event of a load failure. Therefore if the power supply output is too low, the first step should be to determine if it is due to a high load caused by a failure elsewhere in the instrument.

Frequently the faulty component can be found by using a multimeter with at least 5 digits resolution to check the supply pins of all components powered from that supply. Connect one lead of the voltmeter to the appropriate test point for the power supply under test and use the other lead to probe the loads. Small voltage drops across the PCA traces can be detected in this way, and the fault isolated. If any component other than one of the regulators is too hot to touch, there is something wrong with it or with something connected to it.

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