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27. Troubleshooting the AKB Circuits
If you could actually “see” the IK pulses, troubleshooting would be a snap. If the service mode is entered on a
direct-view television, the VPOS can be adjusted to bring the picture centering down and the three IK pulses can
be seen as faint lines. If a problem occurred, one or two of the lines would be missing, very faint, or very bright.
All three lines could also be either extremely bright or missing completely. It depends on what is causing the AKB
problem. A faint line indicates low cathode emission for that color. A bright line means the cathode is healthy but
the return pulse is not making it back to the processor. If it is successfully returning, then the AKB circuit is
malfunctioning. If all three lines are bright, the IK return line has a problem or the AKB circuit is malfunctioning.
No pulses at all would be caused by a failure of whatever is common to the three cathodes, such as loss of G2,
no high voltage or no RGB drive from the video processor.
Projection televisions are unique in that the raster displayed in each CRT is not over-scanned. Over-scanning is
accomplished at the edges of the screen behind the bezel. The IK pulses are clearly visible when looking down
into the CRT lenses. This can be an important troubleshooting technique. If one of the CRTs has a missing or
dim line, chances are that particular CRT or its video line source is defective.
Unfortunately, viewing of the IK lines is not always practical. In a direct-view set, the service mode would need
to be entered “blindly” since there is no video to use as a guide. Many models start in the VPOS by default, but
others will begin in HPOS and that would require looking in the service manual and pressing the “1” key the
appropriate number of times to reach the vertical centering mode. Looking into a projection CRT is not always
practical either. If the CRT is weak, the IK line may appear bright even though it is just below the acceptable level
of the AKB circuit.
The visual descriptions mentioned above are meant to provide a better understanding of how the cathodes can
display the IK pulse being sent and what the reaction of the AKB circuit is, based on different failures. A more
objective method involves a simple set of procedures that can be universally applied to projection and direct-view
sets.
No Video - AKB Problem Verified: Check the G2 voltage line for loss due to failure or short to G1. If OK, read the
DC voltage on each cathode. NOTE: It is very important to verify that G2 cutoff is set properly for this check. The
AKB circuit will correct for improper screen voltage by increasing or decreasing video drive. If G2 is set too low,
the AKB circuit can blank the video. Adjust each CRT to retrace lines and back off until they disappear. On a
properly functioning unit, with no video input and the G2 cutoff set properly, the cathode voltage will read
approximately 185VDC ± 5V. Three possible scenarios will occur with an AKB failure:
1. One of the cathodes reads considerably lower than 185V: This reading will be below 165V. If a bright
raster cannot be obtained by turning up the screen control (projection television), the CRT is the problem.
On a direct-view CRT, the weak cathode color will be missing from the raster when the G2 is turned up.
A weak or dead cathode will cause the AKB circuit to send a large IK pulse and cause the driver to pull the
cathode down to try and get a sufficient return pulse. If a bright raster is obtainable, the IK pulse is not
returning to the IK line (highly unlikely), or the AKB circuit cannot recognize the pulse. This is rare, but
has been encountered. NOTE: The above symptom could also occur on two of the cathodes indicating
low emission. It is unusual but can happen.
2. All three cathodes read low: In a projection television, this indicates a high probability of a failure in the IK
return line. Since it is unlikely that all three tubes have failed, troubleshoot the IK line after looking into the
CRTs for IK lines. On a direct-view set, this can signal a gassy tube. Turning up the G2 will assist in
determining if the problem is emissions related or on the IK return line. Measuring the DC voltage on the
IK return line is a good way to detect a potential problem here. The voltage is generally between 2 and 3
volts. This line is subject to damage when a CRT arcs and this voltage will read 0V or 5V if this happens
depending on what damage the arc has done.
3. All three cathodes read 185VDC or more: The video processor is not outputting RGB drive, there is a
break in the connection from the processor to the driver circuits or all three CRT drivers have opened.
Intermittent AKB Blanking: This symptom is indicated by multiple AKB failures appearing in the failure history of
the self-diagnostics screen (if the chassis has this feature). Otherwise, you will have to rely on your own observation
or the customer’s description. This can be caused by several possibilities: A weak CRT, intermittent shorts in the
