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Overview
When troubleshooting a blanking problem, you must first understand the
video drive and IK blanking circuit and how they interrelate. Second, by
applying the troubleshooting techniques in this book you can determine if
blanking is caused by the video drive circuit or the IK.
Video Drive
The RGB video signals exit IC511 Video Processor at pins 35, 37 and 39.
The RGB signals pass through buffers and enter the CR board at CN7101.
The G and B signals are sent to their respective C boards. Each of these
signals are input to their respective Video Out IC, then amplified and out-
put to the cathode of their tube.
IK
The purpose of the AKB or IK circuit is to keep the picture's white balance
correct by controlling the bias or input level at the picture tubes cathode.
When serial data is present at the IC511 Video Processor, the IK circuit
begins operation. Three pulses are output during vertical blanking, one
at each of the RGB outputs. These pulses turn the tubes fully on. The
beam current is sensed at pin 5 of each Video Out and applied to the IK
buffer. This signal is sent to IC511/41 IK In, which is the reference return
for the loop. The RGB output voltage levels are determined by this input.
If the voltages of the pulses returned are not within a certain window, the
video signal will be blanked. Internally these three pulses are applied to
their own sample and hold circuit. They all contain an external capacitor,
which is connected to pins 36, 38 and 40 respectively. The voltage at
these pins corresponds to the efficiency of each cathode. The more cur-
rent drawn, the lower the voltage output at the sample and hold ca-
pacitors. This differs from previous models.
When a normally working set is first turned ON, the Timer LED blinks for
approximately 8 to 10 seconds. This is because IC511 Video Processor
is holding the data bus in a busy state. When the voltages at IC511/36,
38 and 40 are between 2.5 and 6 volts, the IC511 Video Processor will
IK/AKB
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signal the micro that it is no longer in a busy state and unblank the set.
When IC511 unblanks the video, it outputs RGB of approximately 2 Vp-p
from pins 35, 37 and 39. If there is a problem, the Timer LED will flash
five times in accordance with the Self-Diagnostic function. As an ex-
ample, if the red tube were drawing no current, IC7101/5 would fall to
about 2 volts. This would cause the voltage at IC511/36 to rise to around
6.5 volts. These voltages are indications that the red tube is not drawing
enough current.
Troubleshooting
The first thing that must be determined is if the problem is with the video
drive or the IK return. There are several ways to determine if the IK pulse
is driving the tube. The quickest way is to take a mirror and place it at
about a 45-degree angle above and behind the tube and look for a bright
horizontal line at the bottom of the tube (this would be at the top of the
mirror). Remember the IK pulse is done during vertical blanking, so al-
though it cannot be seen on the screen, it can be seen in the tube. If one
of the lines is missing or is significantly dimmer than the other two, that
color has the problem. Check the color's output from the Video Proces-
sor to decide if it or the video drive circuit is at fault. If the video drive
circuit is at fault, the IK pulse can be followed to signal trace the circuit.
If all three lines can be seen, then you can check the sample and hold
voltages at IC301/19, 21 and 23 to see if the IK pulses are being returned.
These voltages are normally around 4 Vdc, but any voltage between 2.5
and 6 volts should not cause blanking. If all three lines are seen and the
S/H voltages are correct, suspect the IC511 Video Processor. However,
when one of the S/H voltages is incorrect, there are a few possibilities:
The screen control could be mis-adjusted. Simply turn the screen adjust-
ment up or down. If this is the problem, the set should unblank before the
adjustment brings the tube to retrace. If the voltage at Q501/B is not
within the levels mentioned above, there could be a problem with that
picture tube or its biasing. The voltage at Q501/B depends on the volt-
ages output by each pin 5 on the video drive IC's.
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Table of Contents
Troubleshooting
