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An isolation transformer provides ac voltage that is not referenced to earth ground, thereby reducing the
possibility of accidentally touching two points having high ac potential between them. Failure to use an
isolation transformer as shown in Figure 3-5 will cause the ac mains voltage to be connected directly to
many components and circuits within the power supply, including the FET heatsinks, as well as to the
terminals of the external dc power supply. Failure to use an isolation transformer is a definite personal-
injury hazard.
The troubleshooting setup of Figure 3-5 connects high ac voltage to A1F1, A1S2, the fan, and
other components and circuits along the left edge of the A1 main board.
As a convenience in implementing the troubleshooting setup, modify a spare mains cord set as shown in Figure 3-6. This
facilitates connecting the unit's power receptacle to the external supply and connecting the bias transformer to the ac mains.
With the mains cord unplugged proceed as follows:
a.
Remove the top cover and the inside cover per Page 31. Set switch S4 (front-left corner of the A1 main board) in the
TEST position.
Failure to set switch S4 in the TEST position will result in damage to the power supply, damage to the
external dc supply, and is an electrical shock hazard to you.
a.
Install control board test connector onto the A2J3 card-edge fingers.
b.
Connect a 50Ω 10-W load resistor to the unit's output terminals.
c.
With the LINE switch off, connect an external dc power supply to the outside prongs of the unit's power receptacle.
Ignore polarity as the unit's input rectifying diodes steer the dc power to the correct nodes.
d.
Complete the setup of Figure 3-5 by attaching an ac mains cord to test points TP1 (L, black wire) and TP2 (N, white
wire) and connect the green ground wire to the unit's case ground terminal or a suitably grounded cabinet screw. TP1
and TP2 are accessible through the cutout on the left side of the unit and are at the left edge of the A1 main board.
Troubleshooting No-Output Failures
No-output failures often include failure of the A4Q4 PFETs and their fuses A4F1 and A4F2. When either the off-pulses or
the power-limit comparator fails, the PFETs can fail from excessive power dissipation. The strategy for localizing no-output
failures is to check the voltages and waveforms at the control board test connector to predict if that circuit failure would
cause the FETs to fail. This makes it possible to develop your troubleshooting approach without an extensive equipment
setup. Proceed as follows:
a.
With the mains cord disconnected remove the A4 FET board per Page 32. Connect the mains cord and switch on
power.
b.
Using Table 3-9 check the bias voltages, the PWM-OFF and PWM-ON Control signals and other signals of interest at
the A2 control board test fingers, A2J3.
c.
Check for the presence of program voltages, VP and IP, at the rear panel.
d.
Check for presence of the 320Vdc rail voltage between the cathodes of diodes A1CR1 and A1CR2 and the anode of the
diodes A1CR3 and A1CR4. If there is no rail voltage, check diodes A1CR1 through A1CR4.
Diodes AlCR1 through A1CR4 connect to the ac mains voltage. Use a voltmeter with both input
terminals floating to measure the rail voltage.
a.
Select the functional circuit for troubleshooting based on your measurements and Table 3-11, which provides direction
based on the status of the PWM OFF and PWM ON signals.
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Table of Contents
Troubleshooting
