Service manual

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.

Transient Generator Troubleshooting (Figure 3-7)

The transient generator (U349-U355) allows the input power stages to switch between two load levels. It produces the

HIGH control signal which is sent to the DAC circuits to switch the transient DAC output.

Troubleshooting the transient circuit consists of performing the general troubleshooting procedures if the transient circuit

will not perform any functions, or performing the frequency or toggle/pulse mode troubleshooting if there is a problem in

those areas.

General Troubleshooting

First, check the data bus and internal clock. Use signature analysis to check the SD 0-7 data lines at U354 and U355 (see

Table 3-10). Check for the presence of the 1MHz clock signal at U349-1, U350-1, U351-13, and U352-4, 12 (see Figure 3-

7).

Next, perform the front panel actions indicated in the Checkout table using a scope and logic probe to monitor the results.

Make sure that the unit is at the factory default setting of 1000Hz , 50% duty cycle.

Transient Generator Frequency

If the transient generator will not change frequency, press [TRAN ON] on the front panel and program the transient

frequencies according to the FSEL table. Check FSEL inputs at U351-1,2,3 with a logic probe. Check the 1

µs pulse

intervals at U350-14, and U351-14 with a scope.