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27. Troubleshooting the AKB Circuits
Chapter 27 - Troubleshooting the AKB Circuits
Overview
The AKB (Automatic Cathode Bias) circuit, often referred to as IK, has been incorporated into Sony televisions
for many years. In the beginning it created problems for technicians because they were either unaware of it or
did not understand how it worked. The subject was extensively covered in training courses, but the circuit
continued to be a difficult one to identify and diagnose. This was because the video signal would be cut off, or
“blanked”, as the term came to be known.
Why the video is cut off when a problem occurs in this circuit is anyone’s guess. There is no practical reason to
do it. In fact, the RA-3 projection chassis did not blank the video when a failure occurred. It did remain blanked
at turn on and would display video in approximately six to ten seconds once the CRT cathodes were sufficiently
warm. It would appear that the designers of this circuit intended for the video to remain off until full cathode
warm-up was achieved so the video would appear in a correctly white-balanced state.
The concept of AKB is rather simple. Once proper grayscale balance has been aligned, the circuit will monitor
the current draw of each cathode based on the bias assigned after the adjustments. This will help to maintain
proper balance throughout the life of the set as the cathodes age. The current in the cathodes fluctuates
dramatically based on video levels. In order for this circuit to function properly, a sample of cathode current must
be performed when there is no video and the vertical blanking interval provides the needed area to do this.
The video processing circuits will generate 63us pulses (one horizontal line duration) for each of the RGB drive
signals. The pulses are located near the end of the vertical blanking interval and are slightly above pedestal level
in amplitude and will vary in amplitude based on the drive settings performed in the service mode. This makes
them visible but they will be in the over-scan region of the picture. Each of the primary color drives will have its
own pulse. The location of these pulses will vary, depending on video processor design. They can be moved
within the blanking interval on some televisions. They are usually located on lines 19, 20, and 21 so this will be
used to explain the circuit further. The pulse for the R signal will be on line 19, the green on line 20, and the blue
on line 21. The video processor now has a timing reference in order to view each of these pulses as they are
returned from their respective cathode. When a cathode draws more current, the pulse that it received will be
returned at a higher level. The AKB circuit will respond by reducing video drive. The opposite is true if the
cathode current decreases.
Each of the pulses returned by the cathodes are combined into a single return line and sent to the video processing
circuit for the AKB section to analyze. Figure 27-1 provides a clear illustration of how this works. It is a diagram
of the CG board found in the RA-3 chassis. Since discrete components are used, a clear explanation of how the
cathode returns current information can be described. Most television designs, particularly the newer sets, use
an IC for driving the cathode(s) and all of the components for current detection are located inside of it.
Q733 provides the necessary pull-down of the cathode bias to control electron emission. The collector is connected
to ground by R743. The more current drawn, the greater the voltage drop across R743. This creates a voltage
level reference of the current. The voltage drop consists of an AC component since Q733 is driven by a video
signal. This video signal “image” is sent to Q734-B where the signals from the red and blue CRT are merged with
it. The signal at this point resembles a composite signal but it is actually a combination of the RGB video,
therefore, it is technically not a conventional signal since composite video does not contain the G component.
Included in this signal are the three IK pulses from each cathode. Since the video process circuits provided the
location timing for each pulse, it will simply extract and analyze each one for cathode current level.
When the unit is first turned on, the cathodes are cold and no current is being drawn. The IK pulses at each of
the RGB drive outputs of the video processor will be high in amplitude since no return pulses are being received.
As the cathodes warm up, the output pulses will begin to drop in amplitude as each cathode returns the expected
pulse level. This event is an important tool to use for troubleshooting intermittent AKB problems and will be
covered later in this chapter.
