lcd inverter schematics
Hello w_m0zart!
Thank you very very much for this schematic.
Symptom: Backlight flashes short after start and then shut down.
I've invested several hours:
power board
- checked out power supply capacitors
- fuse 25V / 2.5A
inverterboard
- replaced C11 and C12 = 180nF / 250V
- check fuse F1
When the monitor was warm > 25° - it works
Next day - dark screen.
replaced the whole inverter board --> the same
Then I found your schematic:
CN1 / 7: 250 Hz square 8Vss = duty cycle for brightness
The protection circuit causes shut down the backlight.
Each lamp connector CN2 – CN5 has a feedback with a rectifier. The DC voltage is measured above C13, C22, C24 and C25 and should be about 5V.
The comparator U1C and U1D compare this voltage against 1.55V (Pin 8,10 – C16).
The feedback of CN5.2 ( C25) had only 1V in cold condition. So the backlight shut down.
Next try only with power OFF / ON.
I change the connector CN5 and CN4 and also CN2 an CN3 – oh, oh – this fault moves to C24.
Okay, the backlight has a little problem. This is okay, because it has plenty of hours on duty every working day the last five years.
Change R28 from 680Ohm to 1kOhm, and increase C16 from 1µF to 220µF
Now the backlight turns on for 10sec. ignoring the protection circuit.
Backlight warm up and turns to normal operation.
Attention: This modification is no general repairing method for the dark backlight.
You’ve carefully checkout the behaviour with an oscilloscope before your change the components. Check the power consumption on the 12V CN1 / 1 (1.2A @brightness=50%; 2.2A @ brightness= 100%).
In case of disabling the protection circuit even for 10 seconds you might destroy the power circuit or the high voltage inverter.
:idea: This case: more or less weakness of the backlight bulb.
Good Luck!
Werner