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LCD backlight problem

I have a dell 1504FP (LCD monitor). The backlight doesnt work for a period of time.

On close examination with a scope on the PSU board, BIT3105 (lamp controller IC) is getting confused. It has several pins to do with OLP (open lamp indicator)- basically the chip switches itself off if it thinks one of the lamps is open circuit. The lamps are definitely all working. I can recreate the problem by disconnecting any one lamp.

The OLP circuit waits for a cap to reach 2.5v before monitoring- if I drain that cap, the monitor works.

Could someone look at the BIT3105 data sheet and tell me how to disable the OLP circuitry- I dont care if it shortens my monitor life. Please help.

Thanks in advance.

Michael.

Reply to
Michael
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Perhaps a resistor across that capacitor would make it less sensitive? How are you draining it? Just shorting across it?

Reply to
James Sweet

The capacitor is on pin 19 of the chip. Its the soft start for the lamps. For normal function, I think the cap has to fully charge (goes to around 9v i think), but the OLP circuit doesnt cut in till 2.5v. I dont really want to tinker permanently with that cap.

Anyone know where I can get a BIT3105 (in the UK), I might change it out in case its faulty.

Michael.

Reply to
Michael

Sorted it! I soldered a 47k surface mount resistor over the surface mount capacitor (C25, on underside of PCD, near BIT3105- in case anyone has this problem in the future) and it works fine. OLP circuit disabled. I think I'll notice if any of the lamps die- dont need all that overengineered circuitry. Thanks for your help people.

Michael.

Reply to
Michael

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