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Old 01-11-2012, 11:52 PM   #21
stevenps
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Default Re: Dell 2405FPW won't power on

I managed to find another one of these monitors that had a cracked screen and thought I'd fixed the problem with this monitor. I swapped both the power supply and display board, and the monitor powered right up. I thought I'd take some measurements just in case, and I saw the same failure happen again. The backlight browned out, and I saw a bit of smoke coming from the general area of Q1.

I did get some measurements though - The drain of Q1 measured 5.07v, and the drain of Q8 measured 5.12v (same as the input to the USB board). Pins 5-8 (drains, I think) of Q10 measured a few hundred mV I think (I didn't get the exact number down), and pins 5-8 of Q16 measured somewhere between 3.25 and 3.3v (I don't recall exactly, but it was one of those two). The power supply voltages were also close to 7v and 20v, so I think I now have two good power supplies and two damaged display boards. I'll try to get back to tracing where the outputs of U1 and U2 go, though it's not easy as they switch between sides of the board.
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Old 01-31-2012, 07:32 PM   #22
stevenps
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Default Re: Dell 2405FPW won't power on

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Originally Posted by stevenps View Post
I managed to find another one of these monitors that had a cracked screen and thought I'd fixed the problem with this monitor. I swapped both the power supply and display board, and the monitor powered right up. I thought I'd take some measurements just in case, and I saw the same failure happen again. The backlight browned out, and I saw a bit of smoke coming from the general area of Q1.

I did get some measurements though - The drain of Q1 measured 5.07v, and the drain of Q8 measured 5.12v (same as the input to the USB board). Pins 5-8 (drains, I think) of Q10 measured a few hundred mV I think (I didn't get the exact number down), and pins 5-8 of Q16 measured somewhere between 3.25 and 3.3v (I don't recall exactly, but it was one of those two). The power supply voltages were also close to 7v and 20v, so I think I now have two good power supplies and two damaged display boards. I'll try to get back to tracing where the outputs of U1 and U2 go, though it's not easy as they switch between sides of the board.
I finally got around to actually testing the second display board, and it seems to mostly work! I put it back in the monitor with the cracked screen (along with all original parts from that monitor - it seems fine aside from having a broken screen) and it turned right on. It blew the fuse on the 20v output from the power supply, likely because Q1 is shorted source-drain. I should have replacement fuses and a new transistor tomorrow. Soldering Q1 without excessively heating the part may be tricky, as I ended up having to use maximum heat on my soldering station to get the old one off. I'll check a few neighboring parts too, hopefully nothing else shorted.
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Old 02-02-2012, 12:40 AM   #23
stevenps
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Default Re: Dell 2405FPW won't power on

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Originally Posted by stevenps View Post
I finally got around to actually testing the second display board, and it seems to mostly work! I put it back in the monitor with the cracked screen (along with all original parts from that monitor - it seems fine aside from having a broken screen) and it turned right on. It blew the fuse on the 20v output from the power supply, likely because Q1 is shorted source-drain. I should have replacement fuses and a new transistor tomorrow. Soldering Q1 without excessively heating the part may be tricky, as I ended up having to use maximum heat on my soldering station to get the old one off. I'll check a few neighboring parts too, hopefully nothing else shorted.
I didn't find any other obvious shorts, but apparently I wasn't looking in the right places as the board still blows the 20v fuse with just the power supply and display board attached (no USB board, LCD, or inverter). Q1 uses 20v as its input so I don't know if my replacement part is working properly, but it isn't shorted again so I'm pretty sure it's okay. Any thoughts on what might be causing the fuse to blow? Also, is it likely a problem that I'm using a 5A fast-blow fuse instead of a slow-blow fuse? The original was slow-blow, but they're a bit expensive at over $2 each (DigiKey).
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Old 02-02-2012, 04:00 PM   #24
Jeroen
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Default Re: Dell 2405FPW won't power on

Hi Guys, im working on a same monitor at this moment, i got 2 monitors, who died after 5 years continuous work, day and night. One is already working again, the other one has more problems. But the biggest problem with ageing monitors, is that the backlight CCFL tubes are wearing out, and starting to draw more current, that would declare why your power board from the monitor with cracked screen smoked.

In my monitors i replaced the big 450v 120uf cap, becouse it had leaked, and one monitor is working again. (you can see that the backlight is dimmer at the edges of the screen left and right, that's becouse the tubes are black at the ends)

It is possible to replace those as well, but it is tricky work taking the panel apart, but i managed to do this with a 42" LCD tv as well

Tomorow i'm getting parts for the second monitor, hope to fix this one as well

Hope you can use this info, i keep you informed
And thanks for all the information you provided here!
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Old 12-01-2015, 03:36 PM   #25
Oblak
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Default Re: Dell 2405FPW won't power on

I have 2405FPW and I have replaced booth controllers, bridge rectifier, mosfets and capcacitor on the power board. But it still doesn't turn on. When I was measuring voltage on output to the logic board, I noticed, that it was on almost all outputs a sine wave with frequency of 50Hz and RMS ob about 22V. What else should I check/replace?
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