9. If the voltages do not return to near their nominal range, the A6 Power Supply is
probably at fault.
10. If the
+5
V supply is low, suspect the feedback circuit. Measure the voltage of the
+5
V
reference (U305 pin 5) and the f7.5 V references (U306B pin 7 and U306D pin 14).
11. Check output of U306A pin 1. If the feedback circuit is working properly, the output of
U306A
should be near
+13
Vdc.
12. Check output of
U302;
its output should be high if the feedback circuit is working
properly.
High Voltage Supplies
1. Set the
luNE)
switch off, disconnect the power cord, and remove the power supply shield.
2. Connect a DVM’s negative lead to A6TP401 and positive lead to
A6TP405.
3. Set the (LINE) switch on.
4. If the voltage displayed on the DVM is approximately i-110 Vdc and the rear-panel CRT
+llO
VDC ON indicator is lit,
A6Al
HV Module is probably at fault.
Note
Ideally, the DVM should read the voltage written on the label of the
A6Al
HV Module. If necessary, perform the High Voltage Power Supply adjustment
in Chapter 2.
5. If the DVM does not read approximately
-l-110
Vdc, measure the voltage on
A6U401
pin
10. This is the HV-SHUT-DOWN signal and should be near
+5
Vdc.
6. If HV-SHUT-DOWN is low, suspect a bad connection along W8 between the A6 Power
Supply and the 17 CRT Driver.
7. If HV-SHUT-DOWN is correct, connect an oscilloscope to A6TP402. Connect the scope
probe’s negative lead to TP401. Set the oscilloscope to the following settings:
Sweep time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
ps/div
Vertical Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
V/div
8. A nearly-sinusoidal waveform, greater than 30 Vp-p, with an approximately i-18 Vdc
offset, should be observed.
9. If the waveform is a dc voltage near 0 Vdc with narrow, positive- and negative-going
pulses, the
A6Al
HV Module is faulty. If the waveform is a dc voltage near
+18
Vdc with
narrow, positive- and negative-going pulses, connect the probe to TP403.
10. If the waveform at TP403 is a sawtooth waveform with a 1.8 V amplitude, the
A6Al
HV
Module is faulty.
11. If the TP403 waveform has pulses similar to those on TP402, the A6 Power Supply is
probably faulty.
12-14 Display/Power Supply Section
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