Service manual

Replaces page 47 105
Input Power Stages Troubleshooting (Figure 3-6)
There are four, eight or sixteen identical input power stages (depending on the module) connected in parallel Figure 3-6
shows one, which consists primarily of a power FET (in quad array Q2), a monitor amplifier (U6) and an error amplifier
(U5). Schematic details are shown in the corresponding module Power Board schematics.
To troubleshoot the input power circuits, place the Electronic Load in the SA mode by connecting the jumpers in test
headers TP201 and TP301 in the SA 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 SA mode. Check that this waveform appears at the output of the error
amplifier in each input power stage. Refer to the Power Board schematic 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 SA 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 F5 and F13 are used by stage
Q2D.
If all stages have a problem, check test points
and (see Table 3-3). Also, check voltage suppressor (VR28) and
diode (D56) 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