47
Video Process D
The B board contains the Video Processor IC3005 and the Main Micro (not
shown). The Main Micro controls the video processor, TV power and inter-
faces with the user. The Video Processor:
1. Selects component video to be input
2. Converts component video into RGB voltage for the picture tube
3. Monitors picture tube cathode current to adjust RGB drive level (IK,
AKB, or white balance circuitry)
4. Adds on screen display (explained in another section)
The first two functions are part of the video processing. The third function
is the automatic cathode balance circuit. It has changed dramatically in
this set for ease of servicing. The fourth is for user information such as
menus.
Video Processing
Several inputs are available for selection by the Video Processor IC3005.
The selection is based upon serial data from the Main Micro IC3251/pins
28 and 31 into IC3005/pins 55-56. The inputs are outlined below:
IC3005 Input Selection
Input
Operating Mode
DTV from set-back box at pins
13-23.
Pins 13-17 contain identical
information as pins 19-23.
Main Micro has instructed the set-
back box to tune to a UHF channel
and output that channel as
component video (Y, R-Y, B-Y).
Single or Twin View picture
information from the V board
into pins 65-69
Either VHF TV, UHF TV, cable TV,
Video, or DVD has been chosen
from the remote control.
Rear panel HD (1080i)
component video
HD is selected from the TV/Video
remote control button.
OSD (see the OSD diagram)
When menu or display is pressed.
Once the input is chosen by IC3005, this video is internally matrixed into
individual RGB levels. They output IC3005/pins 35, 37 and 39 for the pic-
ture tube (cathodes).
Automatic Cathode Balancing (AKB) Circuitry
The purpose of the AKB circuit (also known as the IK circuit) is to adjust
the level of R, G or B drive signal to compensate for picture tube aging.
The AKB circuit starts in IC3005 at turn on by outputting a one-line pulse to
each cathode (from RGB pins 35, 37 and 39). These three pulses occur
on different lines and in the blanking area (above the picture) so they are
never seen. The three drive pulses are output as long as the video pro-
cessor IC3005 receives Vcc (supply voltage).
While the picture remains blanked, CRT drive current from these pulses
produces a corresponding voltage pulse at each video output IC. Although
these three AKB pulses are combined and buffered, their individual ampli-
tudes represent the potency of each cathode.
The three pulses are returned to IC3005/pin 41 to charge their respective
capacitors at pins 36, 38 and 40. The average charge is used for analysis
and drive signal adjustment. For example, a drive output will be boosted to
compensate for a reduced cathode output so the colors appear equal in
performance. This is why this AKB circuit is commonly called a white
balance circuit.
A picture tube cathode loss results in this circuit’s inability to balance.
However, once the cathode emission is out of operating range, the AKB
circuit will not blank the picture like previous AKB circuits. The standby
light will blink five times, pause and repeat, indicating a white balance ad-
justment failure. The standby light will blink with the TV on or off. The
circuit is reset when the TV is unplugged, but the light will blink again if the
problem persists.
The voltages measured at the three capacitors of this sample TV repre-
sent the efficacy of the cathodes. For example, when the red cathode is
disabled, its capacitor voltage rises (see bold).
IK Capacitor Voltages
IC3005/pin
Normal
R drive shorted CN9001/pin 1 = gnd
36. Red IK voltage
5.6V
6.8V (abnormal)
38. Green IK voltage
5.9V
4.8V
40. Blue IK voltage
5.6V
5.5V
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