Dell 173fp inverter feedback voltage too low

[robrock1]

Hello,

this monitor has a backlight problem, it goes off after 10 seconds sometimes earlier. Transistors are ok. What i realized is that if i remove the dim signal the light stays on but is flickering.
The feedback voltage drops from 1.42 volts to 0.73 volts when the light goes off. if i put 1.4 volts on the feedback pins the light will stay on nothing is burning or smelling or anything but i want to fix it the right way. i measured all the resistors but they look ok, i tried with different lamps but is also the same. I have the schematic but i can't really figure it out, because also the time varies when the backlight goes of its not always the same.

[selldoor]

Re: Dell 173fp inverter feedback voltage too low

Perhaps its just worn out ccfls?
Can you test them on another monitor or do you have spare bulbs to test
on this monitor.
Are there 2 lamps? if so try changing the top lamp for the bottom one
( just swap the connectors over) - The top ones tend to wear out faster
due to more heat at the top.
Have you changed any caps on the inverter circuit.
Perhaps good clear pictures would help us to help you.

[Rtech]

Re: Dell 173fp inverter feedback voltage too low

Is the Model number as you state OR is there a another letter eg FPf etc ?? FPf being one PCB and FPc having a separte Logic Board

[robrock1]

Re: Dell 173fp inverter feedback voltage too low

The model is fpc, this is the schematic to it. I already tried with other bulbs, even this panel on another monitor works fine.

[Rtech]

Re: Dell 173fp inverter feedback voltage too low

The 'feedback' you refer to ..is that the On/off you are talking about or,if not where are you measuring this ?

[robrock1]

Re: Dell 173fp inverter feedback voltage too low

It is pin 5 and pin 12 of the U201 the inverter ic.

[Davi.p]

Re: Dell 173fp inverter feedback voltage too low

I'm not so an expert but to me it seems that the problem comes from the main logic board that for some reason doesn't provide the correct voltage to the inverter, are you sure that after the lamps goes off you can see dark images on screen? If it is not so try to test the pin 16 REF that must be 2,5v , maybe is useful also to control D203 - D204.

Selldoor: recently i have serviced one Acer al1717 a 17" lcd where i have replaced 2 of the 4 lamps, 2 on top 2 on bottom, there was 1 with marked darkened end at top, 1 at bottom, now monitor is fully functional, so maybe it's important also the position on the series of lamps... (?)