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Touch Control Fault
Isolation Procedure
Use the following procedure if the touch control screen
on the monitor fails and the computer does not beep.
1. Set R204 sensitivity according to the Touch Con-
trol Sensitivity Adjustment procedure before start-
ing fault isolation. If the touch control circuits are
not functioning well enough to allow sensitivity set-
ting, proceed to step 2.
2. Connect the monitor and the computer as shown
in Figure 1-2.
3. Boot the PLATO Diagnostic disk into the computer
and display the PLATO TOUCH grid. The CB-
5063-28 Diagnostic package also can be used to
display the PLATO TOUCH grid (early copies of
the diagnostic package may not have this feature).
4. Touch each square in a diagonal line starting at
the upper-left corner to the bottom-right corner.
If there is no response when you touch a square,
disconnect the power and signal cables from the
monitor and remove the back and top covers.
5. Reconnect the cables as shown in Figure 1-2 and
display the PLATO TOUCH grid on the monitor.
6. Locate the red LED (CR109) on the logic board
(see Figure 4-23).
7. Observe CR109 while you touch a square. If
CR109 flashes each time you touch a square, valid
touches are being detected, indicating any prob-
lem may be in the data cable to the computer or
in the computer (software). Check the cable by
replacing it with a good cable.
Servicing
8. If CR1 09 does not flash, check the following power
supplies on the logic board. Replace the Power
Supply Assembly if the indicated voltages are not
present.
+5 VDC
TP4
+ 12 VDC
TP5
-12VDC
TP6
(Common connection of
R116 and R117)
(Cathode of CR107)
(Anode of CR106)
9. If the power supplies are satisfactory, check for
proper horizontal sync pulse waveforms as fol-
lows:
a. Connect an oscilloscope to display waveforms
X and D according to Figure 4-24.
b. Replace IC106 if the waveform is not correct
at pin 39 of IC1 01.
10. If CR109 still fails to flash, check for a 6 MHz sine
wave at pins 2 and 3 of IC101. Replace crystal
Y101 if the sine wave is not present.
11. If the horizontal sync pulse waveforms and IC101
are satisfactory, check for an oscilloscope indica-
tion of pulses at TP1 on the logic board. See Fig-
ure 4-24, waveform A, and proceed as follows:
a. Turn off (up) switch 6 of S101 on the logic
board.
b. Replace IC101 on the logic board with Test IC
Assembly A-14136.
c. Turn the monitor off, then back on.
d. CR109 (Red LED) should be flashing.
Page 4-11
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