Fluke 8842a instruction manual Track/Hold Troubleshooting, Ohms Current Source Troubleshooting

8842A

Instruction Manual

6-63. Track/Hold Troubleshooting

If a problem is suspected in the Track/Hold (T/H) circuit, first check the power supply voltages of all active components. (See Table 6-22.)

Next, check the T/H output waveform at TP103 with an oscilloscope. Set the 8842A to the VDC function and 2V dc range, apply +1V dc across the HI and LO INPUT terminals, and trigger the scope from the falling edge of line not-TR (TP201). The waveform should look like that in Figure 6-13. The circuit may be checked as follows:

1.Short the HI and LO INPUT terminals, and select the 2V dc range.

2.Short U301-14 to ground (Reference Low). The 8842A should read within 10 counts of zero. (The actual value is not as important as its stability.)

3.Connect U307-6 to U303-18, and monitor the voltage at TP103 using another multimeter. The 8842A should read about the same as the external multimeter, but with opposite sign.

If the 8842A fails step 2 but not step 3, then U303 is bad. If the 8842A fails both steps 2 and 3, then the fault is in the A/D Converter or the T/H Amplifier. To tell which, lift the end of R318 closest to the front panel (connected to U307-6) and apply an input of less than +2V to TP103. If the A/D Converter is OK, the 8842A will display the applied voltage with the opposite polarity. (For example, if you apply +1V, it should display - 1V.) The readings may differ by a slight offset.

6-64. Ohms Current Source Troubleshooting

Malfunctions in the ohms functions can be caused by a fault in the Precision Voltage Reference, Ohms Current Source, or Ohms Protection. Malfunctions can also be caused by a fault in the DC Scaling circuit which loads the Ohms Current Source.

First check the power-supply levels (see schematic). Then check all digital logic input pins of the quad analog switches (U402 and U403). These should show digital signals with high =>+3V and low =<+0.5V.

To determine whether the Ohms Current Source is being loaded down by the DC Scaling circuit, select the VDC function and connect a 10 kΩ resistor between the collector of Q404 and ground (Reference Low). (Selecting the VDC function opens K401, and configures the Ohms Current Source in the 20 kΩ range.) If the voltage across the 10 kΩ resistor is 1V, then the Ohms Current Source is working (at least in the 20 kΩ range), and the problem is probably due to a defect in the DC Scaling circuit.

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