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Input Power Stages Troubleshooting (Figure 3-6)
There are eight identical input power stages connected in parallel. Figure 3-6 shows one of the eight. This stage consists
primarily of a power FET (in quad array Q1), a monitor amplifier (U14) and an error amplifier (U1). Schematic details are
shown on Figure 6-1, sheet 6.
To troubleshoot the input power circuits, place the Electronic Load in the S.A. mode by connecting the jumpers in test
headers TP201 and TP301 in the S.A. mode positions (see Figure 3-2). The waveform shown in Figure 3-6 at the output of
the error amplifier can only be generated in the S.A. mode. Check that this waveform appears at the output of the error
amplifier in each input power stage. Refer toFigure 6-1, sheet 6 to locate the output pin of each error amplifier. Checking
each stage may isolate the problem to a specific stage.
If the problem is isolated to a specific stage, turn the S.A. mode off (remove jumpers) and check the test points
through
that correspond with applicable circuit points in the defective stage. Use the measurement conditions specified in Table
3-3. Also, check the applicable fuses in the specific stage. As shown in Figure 3-6, fuses F1 and F9 are used by stage 1.
If all stages have a problem, check test points
and (see Table 3-3). Also, check voltage suppressor (VR9) and diode
(D14) which are connected across the + and - INPUT terminals. Make sure that SENSE switch S1 on the rear panel is set to
the LCL position if remote sensing is not being used.
Figure 3-6. Input Power Stages Troubleshooting
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