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Automatic Brightness Limiting (ABL)
The purpose of this circuit is to prevent sudden bright scenes from short-
ening the life of the picture tube. It does this automatically by monitoring
the picture tube's current and then using this voltage to limit the bright-
ness of the picture.
The flyback transformer T504 (secondary) supplies the picture tube's
high voltage. The ground end of the secondary at T504/pin 11 is current
limited by a 100k ohm resistor, R535. As the picture gets brighter, the
high voltage current increases, causing a voltage drop across R535. This
ABL voltage decreases with increasing brightness.
A B L V o lt a g e s – M o d e l K V 2 0 M 4 0
C o n d it io n
T 5 0 4 / p in 1 1 V o lt a g e
B la c k s c r e e n
6 . 2 V d c
( g e n e r a t o r )
C o l o r b a r s
2 . 9 6 V d c
W h it e s c r e e n
2 . 1 V d c
91
Voltage divider R533, R534 and R532 bias this ABL line with a positive
voltage that is applied to Jungle IC301/pin 3. A lower ABL voltage de-
creases the level and gain of the RGB waveform. This is how ABL sec-
tion uses the Jungle IC to keep the brightness within a reasonable oper-
ating range.
OCP Protection
One part of the protection circuit is shown here in block form. The over
current Protection (OCP) stage shuts OFF the TV during a fault. Q571 is
used to monitor the current flowing through the FBT and horizontal out-
put transistor. If the current becomes excessive, Q571 conducts, ground-
ing out the DC voltage from the Jungle IC301/pin 18. This ground causes
IC301/pin 34 to send data to Micro IC001/pin 37. The data instructs
IC001 to shut OFF the TV, allowing the horizontal output transistor to
cool down.
I C 3 0 1 / p in 3 V o lt a g e
6 . 2 V d c
3 V d c
2 . 1 V d c
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