Sony KV-32XBR400 Training Manual page 29

Component Video leaving the B board - Color Bar input
c h 1
1
c h 2
c h 3
2
C H 1 ! 5 0 0 m V ~ 3
C H 2 ! 2 .0 0 V =
C H 3 ! 2 .0 0 V = C H P M T B 1 0 .0 u s lin e
Channel Name Location
1 Mid Y CN3203/pin B8
2 Mid H CN3203/pin B14
3 Mid V CN3203/pin B15
Time base = 10usec/div
RGB Drive / AKB Circuit
The Y/C CRT Drive IC201 has several functions:
•
Amplifies the RGB signal and applies it to the CRT cathodes
•
Mixes the main signal with the RGB On-Screen Display (OSD)
•
Automatic Cathode Balance (AKB) or IK (cathode current)
The AKB circuit monitors the CRT cathode currents and adjusts the RBG
drive levels to compensate for CRT aging. By adjusting RGB drive levels
to simulate the same cathode currents, white balance can be maintained.
To accomplish this task, at power ON three IK drive pulses (about 3Vp-p)
from IC201 are sent to each CRT cathode (video is muted). The cathode
currents from all three cathodes are returned to IC201 on the single IK
line. The three pulses are used to adjust the RGB drive pulses (and RGB
gain) to produce equal amplitude IK return pulse levels. When the AKB
loop closes, the AKB drive pulse is reduced (1.8Vp-p - ch 2). Finally, the
video signal is unmuted to display a picture.
To see the full operation in the next scope shot, the red drive wire has
been opened at CN9001/pin 1. The CN9001/pin 1 connector is shorted to
ground to simulate a defect red cathode. Notice the red IK drive pulse
c h 1 p
Comments
0.7Vp-p
3.8Vp-p
3.8Vp-p
25
(vertical blanking area of ch 1) is still at 3Vp-p (power On level). The
normal green signal (ch 2) shows the IK signal is reduced to 1.8Vp-p
because the IK loop is complete. The last waveform (ch 3) does not show
the missing red IK signal because of sampling errors in the digital scope
used.
IK drive pulses
3 3 9 , U &
c h 1
c h 2
T
c h 3
1
2
C H 1 ! 2 .0 0 V = Vertical blanking
C H 2 ! 2 .0 0 V =
C H 3 ! 1 .0 0 V = C H P M T B 5 0 0 u s - 1 .0 8 d v c h 1 -
3
IK drive signal in the vertical interval - Color Bar input
Channel Name
1 R Drive
2 G Drive
3 B Drive
Time base = 0.5msec/div
Technical Note: If one or two cathodes falls below AKB adjustment range,
the video will NOT blank as in other AKB circuits. However, if a cathode
draws too much current, (Ik pulse gets large) the picture will blank, and
the standby light will blink five times and repeat.
In normal operation, if you increase the screen voltage, the IK return pulses
(ch 3) will increase in amplitude because more cathode current is drawn.
Because of the AKB closed loop, IC201's output IK drive pulses (ch 2) will
decrease to lower the cathode current.
S
Location (C board) Comments
4Vp-p (open
CN9001/pin 1
circuited)
CN9001/pin 3 3Vp-p
CN9001/pin 8 1.4Vp-p