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27. Grand Wega Lamp Control
Chapter 27 - Grand Wega Lamp Control
DANGER
: There is High Voltage at very High Current, dangerous enough to cause
Death On Contact
. Keep
away from the Power Block and lamp socket when the TV is turned ON (see Figure 27-3). Follow the repair
procedure closely and be aware that the
HV periodically turns ON automatically.
This Lamp Control document is divided into these four major sections:
1.
Lamp Operation
•
Gas Discharge Tube Concept
•
Lamp Starting Sequence
•
New HV Detector Circuit
2.
Lamp On Time Display
•
Lamp On Time Reset
3.
Power Block Location & Removal
4.
Troubleshooting:
•
Defect – Unreliable backlight starting
•
Defect – No backlight repair procedure
•
Defect – No backlight repair flow chart
1. Lamp Operation
Gas Discharge Tube Concept
This LCD TV screen display panel is similar to most computer laptop displays because they both have a colored
LCD panel and backlight. To produce an image on the screen, the LCD panel’s colored cells will either allow or
block light from the backlight. In this TV, the source of light is a long life gas discharge tube (lamp). The gas
discharge tube is similar to a fluorescent tube in operation and testing. The tube’s two terminals (that are used
for the light discharge) measure open circuit (infinity resistance).
The gas discharge tube requires a high voltage to start but once started (lamp current is detected), the applied
voltage is reduced across its terminals. Not only is the applied voltage reduced when current is drawn (Power
Block relay clicks), but the voltage across the tube / lamp drops because the tube’s resistance has dropped at
ignition like a fluorescent tube. The Power Block voltage reduction relay clicks one second after the lamp ignites.
The phosphor that coats the tube takes about one minute to glow at a uniform level, near maximum brightness.
Lamp Starting Sequence
The lamp is instructed to turn ON when the TV turns ON. The Micro (B board) signals the Power Block to
generate 25kV necessary to start the Lamp. Refer to Figure 27-1 when following this lamp ON sequence:
1. When Main Micro IC9004 receives a power ON command (0Vdc at A board’s CN1901/pin 4), the Main Micro
informs Lamp Control IC9502 via serial data.
2. Lamp/Fan IC9502 checks to see if the fan is rotating before turning ON the lamp.
3. The Lamp Control IC9502 in turn outputs a HIGH (5Vdc) which passes through the A board (CN1908/pin 1)
to turn on the Lamp Driver Board.
4. The Power Block Board generates an initial 25kV at low current to start the gas discharge tube. The lamp
ignites.
5. Once the tube starts, the increased current is detected and the Power Block:
•
Increases the lamp current (internal to the Power Block) so the lamp can go to maximum brightness
•
Grounds the PRT line (protect line at CN1908/pin 3) so the Main Micro knows the lamp is working.
6. The Main Micro (M board) can now unmute the audio and video because the fan and lamp are both working.
7. The lamp reaches almost full screen brightness in one minute.
