31
d. With the LINE switch off, connect an external dc 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.
e.
Complete the setup of Figure 3-1 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 A4Q3 and 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 PFETs to fail. This makes it possible to develop your troubleshooting approach without an
extensive equipment setup. Proceed as follows:
a.
With the mains cord unplugged remove the A4 power mesh board as described on page 27. Plug in the mains cord
and switch on power.
b.
Using Table 3-1 check the bias voltages, the PWM-OFF, PWM-ON 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 A1CR3 and A1CR4. If there is no rail
voltage, check diodes A1CR1 through A1CR4.
Diodes A1CR1 through A1CR4 connect to the ac mains voltage. Use a voltmeter with both input terminals
floating to measure the rail voltage.
e. Select the functional circuit for troubleshooting based on your measurements and Table 3-2, which provides direction
based on the status of the PWM OFF and PWM ON signals .
Front-Panel Troubleshooting
The A3 front-panel board can be troubleshot by first doing the following setup.
a.
Remove the top cover of the unit.
b.
Remove the 4 side screws holding the front-panel assembly to the power supply chassis and pull the entire assembly
forward.
c.
Disconnect the W1 ribbon cable from connector A1J1 on the A1 main board and remove the ground wire screw
holding the green/yellow ground wire. Unplug the four wires to the LINE switch noting the configuration.
d.
Detach the A3 board from the front-panel assembly by removing the five mounting screws.
e.
Reconnect the W1 jumper to connector A1J1 and place the A3 board vertically against the supply with a piece of
insulating material between. The test connector can then be attached to the A3 board. The rest of the front-panel
assembly can stand vertically so that the pots and the switches can be accessed while troubleshooting.
f.
Attach the external line cord and place switch A1S4 in the TEST position.
The ac mains voltage connects directly to the LINE switch and to components and traces at the front of
the A1 main board. Be extremely careful to avoid touching the ac mains voltage.
Start troubleshooting by performing the tests given in Table 3-3. This table provides the measurements for the test points on
the test connector as well as the source components for that measurement. Switch A1S4 should be in the TEST position for
all measurements except where noted. Table 3-4 gives front-panel symptoms as well as the circuits or components that
may cause the supply to exhibit those symptoms. Both Tables 3-3 and 3-4 should be used to check out and troubleshoot the
front-panel.
