Dell 1704FPTt - Backlight goes out after a couple of secs

[bob12345]

Hi there, I have a Dell 1704FPTt LCD monitor which has developed a fault. When I turn it on, everything appears as normal, ie I can see the desktop if connected/or see the Dell screen logo, however the backlight goes out after about 2 secs - power light still on. The display is still visible if i shine a bright light at the panel. I have tested another identical panel and it does the same thing with the same PSU and electronics of the 1704. I've been reading on here that its a problem with one or more of the 4 inverter transistors - all caps on the board appear to be in good condition with no bulging etc. The transistor model in my LCD is AOU402 (x4) in a TO-251 package. I intend to replace these however over here in the UK so far my search for this part has been unsuccessful, I can only find some from the Ebay USA. So my question is can i subsitute this part (AOU402) with a C5707 (2SC5707) part? If not are there any alternatives to the AOU402 transistor? Thanks

[Dickey]

Re: Dell 1704FPTt - Backlight goes out after a couple of secs

Hi, I have a Dell 1704FPt with the same problem. The screen comes on for 2 seconds and then it goes to black. I opened it up but didn't find any bad caps. Being a stubborn cuss I replaced a bunch of likely suspects anyway. No luck, so I ordered a new inverter board and put it in. I still get the same symptoms when I turn it on. I know I'm putting more money into this monitor than its worth, but did I mention I'm stubborn? Anyway, I'm looking for ideas from the kind and smart folks on this forum.

[alexanna]

Re: Dell 1704FPTt - Backlight goes out after a couple of secs

Hi, I have a Dell 1704FPt with the same problem. The screen comes on for 2 seconds and then it goes to black. I opened it up but didn't find any bad caps. Being a stubborn cuss I replaced a bunch of likely suspects anyway. No luck, so I ordered a new inverter board and put it in. I still get the same symptoms when I turn it on. I know I'm putting more money into this monitor than its worth, but did I mention I'm stubborn? Anyway, I'm looking for ideas from the kind and smart folks on this forum.

The fastest and Easiest way would be to substitute one or two known good ccfls one at a time to your inverter board see if the 2 sec to black problem goes away.
Retiredcaps has written a good article on this,
Use the search feature and look for troubleshooting 2 sec to black.
Al.

[ecking767]

Re: Dell 1704FPTt - Backlight goes out after a couple of secs

So my question is can i substitute this part (AOU402) with a C5707 (2SC5707) part? If not are there any alternatives to the AOU402 transistor? Thanks

No you can't substitute the 2SC5707 ( NPN Transistor) for the AOU402 (N-Channel MOSFET).

[Dickey]

Re: Dell 1704FPTt - Backlight goes out after a couple of secs

Well I kind of thought that since I put in a brand new inverter board my troubles would be in another area.

[PlainBill]

Re: Dell 1704FPTt - Backlight goes out after a couple of secs

Well I kind of thought that since I put in a brand new inverter board my troubles would be in another area.

You may be correct. There are several causes for the 'two seconds to black' problem. One is aging CCFLs. Another is a problem with the wiring to the CCFLs such as bad solder joints, deteriorated insulation, etc. A third is the signal driving the CCFLs - typiccally a bad transformer or bad caps. Lastly, there can be problems with the circuitry sensing the status of the CCFLs.

ASSUMING the inverter you purchased is actually new, working perfectly, the problem is one of the first two.

PlainBill

[Dickey]

Re: Dell 1704FPTt - Backlight goes out after a couple of secs

The fastest and Easiest way would be to substitute one or two known good ccfls one at a time to your inverter board see if the 2 sec to black problem goes away.
Retiredcaps has written a good article on this,
Use the search feature and look for troubleshooting 2 sec to black.
Al.

You called it! I finally got around to opening the thing up and took the lamps out and plugged them into the inverter. The lower set of lights were arching across between the bases of the two lamps and the inverter shut down in self defense.

Now where is a good place to get a couple new ccfls?

[jetadm123]

Re: Dell 1704FPTt - Backlight goes out after a couple of secs

You called it! I finally got around to opening the thing up and took the lamps out and plugged them into the inverter. The lower set of lights were arching across between the bases of the two lamps and the inverter shut down in self defense.

Now where is a good place to get a couple new ccfls?

Several members here have had good experiences with ccflwarehouse.com

[Dickey]

Re: Dell 1704FPTt - Backlight goes out after a couple of secs

Great, I will check them out.

Thanks to everyone who responded!

[Dickey]

Re: Dell 1704FPTt - Backlight goes out after a couple of secs

Got the new ccfl's. Put them in and it fired up just fine. Now I have another problem. The picture resolution is poor and I can see shimmer around the edges of objects. I tried all the menu adjustments on the monitor and screen settings in XP. I also swapped it out with a computer with a good flat screen monitor, cords and all. No change. I'm considering putting the old inverter back in though I would think the trouble would be in the board that drives the screen. I appreciate the continuing help.

[retiredcaps]

Re: Dell 1704FPTt - Backlight goes out after a couple of secs

I'm considering putting the old inverter back in though I would think the trouble would be in the board that drives the screen.

Sure, try the old inverter.