High Voltage Feedback Compensation Circuit - Acer 7299sl Service Manual

2.1.21 High voltage feedback compensation circuit

For the purpose of getting accurate CRT anode voltage 26kV in different modes and different
patterns, high voltage feedback compensation circuit is very important. IC302 functions as a integrator
which offer frequency compensation. R309, C307, C325 are frequency compensation elements which
make voltage regulation excellent. VR302 is offered to adjust correct anode voltage by factory.
R314, C308, D319 constitutes soft start circuit to assure high voltage up to setting value gradually
when power on, in order to protect high voltage transistor Q313.
The feedback compensation signal is taken from FBT inside bleed and capacitor and input to IC302
pin6.
The processed feedback compensation signal is fed to PWM comparator IC301 pin5 and compares
with sawtooth signal at pin6 of IC301 to output PWM pulse at pin7 of IC301. The PWM pulse is buffered
by Q302, Q323 and drives Q304 high voltage converter.
2.1.22 High voltage output circuit
In fig.8, we can find there is no H-DY and Cs capacitor, only FBT Lp and C317, C386, C384, C386
are exist. The negative pulse at emitter of Q313 is resonant by C317, C384, C385, C386 and FBT Lp
and the pulse width TR of negative pulse is
= π
t L
r
The emitter voltage Q313 is about -700Vpeak and subject to change with different patterns and different
modes.
Q312 and T302 constitutes high voltage drive circuit to offer optimum base current of Q313.
2.1.23 X-ray protective circuit
Q360(SCR) constitutes X-ray protection. If in abnormal condition, high voltage go rise to over
28.5kV, then the voltage at R342 go up to turn on ZD310 and Q360. So Q312 is off and Q313 is also
off.
2.1.24 ABL circuit
FBT
C301
0.1U
100V
(PE) SGND
Q301 constitutes ABL circuit and VR301 is offered for factory setting to adjust picture luminance
about 30Ft-L at full white pattern. When picture become bright, the voltage of Q301 base will decrease,
then the contrast voltage also decrease, so get automatic brightness limit function.
2.1.25 Dynamic Focus Circuit
Circuit Operation Theory
+
* ( C 317 || C 386 C
p
+12V
R381
D303 4.7K
1N4148
R304
47K
R301
C304 +
1K
R3991
1U
12K
100V
105C
(EL)
FIGURE.16 ABL circuit
No Copy/Reproduction Allowed
≈
384 || C 385 ) 3 us
C327
+
10U
R388
25V
5.1K
(EL)
R389
Q318
47K
A733
Q301
A733
D327
R305
1N4148
1.8K
C393 R306
330K
0.1U
50V
(D)
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