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d. Check the vertical collector the same way you
checked the horizontal collector, except place
the incandescent light bulb a few inches from
the bottom right corner of the face of the CRT.
e. If no signal is observed, check J3 and J4 for
good connections.
f. If the signal is observed from one detector, but
not the other, replace the collector plate assem-
bly that has no signal output.
NOTE: Both vertical and horizontal circuits must be
operating before the touch control will respond to any
toyching.
21. If both collector plates fail the bulb test, the prob-
lem is most likely in the amplifier. Test the amplifier
as follows:
a. Remove the logic board from the monitor, re-
move the top shield from the amplifier, and re-
connect all logic board cable connectors.
b. Measure the amplifier transistor voltages. They
should be the values shown below. If the values
are not within 5% tolerance, replace the logic
board. The ratios of the voltages between the
base, collector, and emitter of each transistor
must be as shown.
Table 4-2
Logic Board Transistor Voltages
TRANSISTOR
VOLTS DC
DESIGNATION
BASE
EMITTER
COLLECTOR
Q201
6.0
5.4
6.6
Q203
3.5
2.9
8.2
Q204
8.2
7.6
12.0
Q205
3.5
2.9
8.2
Servicing
c. Set R204 (sensitivity) to mid-range and induce
a sine wave (10MHz, 10 mVpp) to pin 1 of the
J3 connector. Check 0201 and Q203 collector
voltages. The voltages should be 100 mVpp at
0201 and between 2 and 3 Vpp at 0203.
22. If the amplifier is satisfactory, check the collectors
as follows:
a. With the monitor turned off, disconnect the col-
lector plate cable from the logic board, and con-
nect a DC voltmeter between pins 1 and 3 of
the cable connector.
b. Position the monitor for the most room light to
reach the collector plate that is under test (up-
side down for the horizontal collector plate and
vertical collector plate side down for the vertical
collector plate).
c. Record the DC Voltmeter reading with room
light striking the the collector plate assembly
and then block off the complete front of the
monitor and record the meter reading again.
The meter reading should change by a factor
of 2 or more. With the room light blocked, the
meter reading should be 20 mV or less. Read-
ings with room light on the collector assembly
will vary depending upon the type and intensity
of the light. An example is, 150 mV with high
intensity fluorescent room lighting.
d. Repeat this test for the other collector.
23. After repair, remove the test IC, install an opera-
tional IC1 01, and set S101 switches for operation,
2 and 3 off (away from the board) and all other
switches on (toward the board).
24. Readjust the R204 sensitivity according to the
Touch Control Sensitivity Adjustment procedure.
Page 4-13
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