To protect the picture tube and FBT,the
average beam current and peak beam current may not too high. The beam current limit information and High Voltage
(H.V) tracking information are derived from the H.V voltage winding of T401
1. The contrast and brightness reduction of RGB is proportional to
the voltage on the pin 48 of IC501.
2. As the beam current (slow),and H.V (fast) are changes, the voltage of point A is changed.
3. By the increase of beam current, the point A voltage is decreased and this information is fed to pin 48 of IC501.
> Contrast reduction begins when V49 is below than 3.1V.
> By the more increasing of the beam current, the V48 decreased and eventually when this voltage is less than 1.8V the brightness reduction is started.
[Check the value of R416, the resistor connected to +B for (open); if the ABL action is not detected. This is a common fault to this set.]
Deflection SECTION
3. By the increase of beam current, the point A voltage is decreased and this information is fed to pin 48 of IC501.
> Contrast reduction begins when V49 is below than 3.1V.
> By the more increasing of the beam current, the V48 decreased and eventually when this voltage is less than 1.8V the brightness reduction is started.
Deflection SECTION
1. The H drive output is open collector and active low, I.e. the
H-out TR should conduct during the low period of output.
2. Under normal operation condition the duty cycle of the output pulse is 50%.
3. To ensure a smooth start/stop behavior of the horizontal deflection and protect the H-out TR, there are 3 different condition of H-drive output.
2. Under normal operation condition the duty cycle of the output pulse is 50%.
3. To ensure a smooth start/stop behavior of the horizontal deflection and protect the H-out TR, there are 3 different condition of H-drive output.
During switch-on. The horizontal output starts with the double
frequency (31KHz) and with duty of 75 %(off),25%(on). After about 50mSec of switch-on. The frequency is changed to normal value (15.625KHz)
and duty cycle to 50%.
During switching-off. Also the frequency is switched to the double
value and R.G.B drive is set to Max. and after 100mSec R.G.B is set to Min. and
50mSec later the H drive is switch-off.
4. The H drive pulse from the MICOM (IC 501) is amplified by the horizontal drive circuit, to get sufficient base drive current for the high voltage switching TR (Q402).
4. The H drive pulse from the MICOM (IC 501) is amplified by the horizontal drive circuit, to get sufficient base drive current for the high voltage switching TR (Q402).
If the horizontal output transistor [2SC6093] found shorted (Collector to Emitter short); before replacing it with an other one, check the solder terminals of the C414; for any loose. Check the capacitance of this capacitor. Any failure (Capacitance value deteriation or Loose solder), to this capacitor will damage the horizontal output transistor.
You won't get any time to check any voltages. The damage to horizontal output transistor will be instantaneous, as you power on the circuit.
SCHEMATIC
