Mitsubishi Electric VS-45605 Technical Training Manual page 66

During the VBI, all three
CRTs are cutoff by the blank-
ing pulse. The AKB pulse
drives the CRT to conduction.
At the point of conduction the
Current Detector, Q6G1, con-
ducts. The resulting pulse
from Q6G1 is applied to the
base of Q6G3.
All three of the Current De-
tectors, Red, Green and Blue,
connect to the base of Q6G3.
During the AKB period there
are three pulses in sequence
applied to Q6G3. Each pulse
denotes the turn on point of
its' respective CRT.
The AKB pulses from Q6G3
are fed back to pin 27 of the
VCJ. The pulses are com-
pared to the CRTs previous
Cutoff point, and the circuitry automatically read-
justs the cutoff point if a change has occurred.
If a problem occurs in the AKB, or Color Output
circuitry it is difficult to determine which circuit is at
fault. Typical symptoms can include:
• A bright white raster that can exceed beam
current limits, causing the set to shut down.
• No raster
• Delayed raster when first turned on.
Note: Similar symptoms can be caused by improp-
erly adjusted Screen Controls. Accurately setting
the CRT Cutoff Adjustments is critical for proper
operation and performance. Refer to the Service
Manual to perform the procedure.
Disabling the AKB loop helps simplify troubleshoot-
ing.
Opening the AKB Loop
Figure 7-9 illustrates how to disable the AKB cir-
cuitry.
1) Remove power from the TV
2) Disconnect pin 27 of the VCJ
3) Short pins 19, 21 and 25 of the VCJ together
and connect a Variable DC Supply to these
pins.
4) Set the DC Supply for 4.2 Volts.
5) Reapply power to the TV
The DC supply inversely effects brightness, increas-
ing the supply reduces brightness, and decreasing the
supply increases brightness. With the set in this con-
dition, the Output Amplifiers can be checked. Also
the AKB pulses can be checked, both from the VCJ,
and from the Current Detectors.
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