Dell E171FPb bulged cap & open fuse on backlight inverter

[blargg]

I was given a Dell E171FPb with a bulged 25V 220uF capacitor and blown fuse on the backlight inverter board. I read +25V and +3.2V going into it. Not sure whether +25V is too high or they skimped on capacitor voltage rating. Details below:

Dell E171FPb, 17" 1280x1024 LCD monitor, May 2003 manufacture

Powers on, green LED, black screen, not even a flicker of light when plugging in. Computer notices monitor when plugged in. Flashlight to screen shows that LCD is working and showing computer's output, just backlight isn't lit.

On opening, I see bulged C8 cap on backlight inverter board (Ambit T05I030.01). Fuse F1 also tests open. I connected known-working CCFL bulb to each socket and no flashes when powering up. Voltages to backlight inverter board are +3.23V and +25.03V (with no load). Bulged capacitor is 25V 220uF connected after fuse across +25V line. The two heat-sinked diodes on power board (upper-left and upper-center) test fine (one on left is for the +25V output). No bulged capacitors on power board. Outputs to video board look good: +3.23V and +5.01V.

So power board seems fine, outputs very close to 25V, so it is probably supposed to be this and not say 12V. Filter capacitor for this 25V has a 35V rating, again suggesting it's not supposed to be only 12V. But this line on the backlight inverter goes to the 220uF cap that's only 25V rating, which suggests very bad design or that the inverter runs on less and this 25V is what killed it and blew its fuse.

I'm thinking of just replacing the two 220uF caps with ones of higher voltage, replacing the fuse, and seeing whether it works. Maybe it was just the blown cap that blew the fuse? Cap doesn't read as shorted though. Also not sure how I'm going to find such a small fuse, and what its current rating was (just reads FS on the top).

Thanks for any ideas of what to check or try.

[retiredcaps]

Re: Dell E171FPb bulged cap & open fuse on backlight inverter

But this line on the backlight inverter goes to the 220uF cap that's only 25V rating, which suggests very bad design or that the inverter runs on less and this 25V is what killed it and blew its fuse.

Other threads for the same model say 25V is correct. If you are going to replace the caps, maybe use 220uF 35V for a bit of extra margin.

[shovenose]

Re: Dell E171FPb bulged cap & open fuse on backlight inverter

Dell monitors are BenQ made? EEK

[blargg]

Re: Dell E171FPb bulged cap & open fuse on backlight inverter

I traced further where the +25V went to on the inverter. It's filtered by the two caps, and then goes to pins 1-3 of Q10, the 8-pin chip noted on the picture. I finally figured out that it's a P-channel MOSFET, where pins 1-3 are the source, 4 the gate, and 5-8 the drain. I see it's also connected to diodes D9 and D8 to the right, and to the big inductor, so it's a buck voltage regulator. The MOSFET doesn't seem shorted (pin 1 to pin 8 for example). The internal reverse-diode across the source and drain checks out as well. The two diodes to the right look good, also not shorting to ground.

EDIT: Well, I removed the two 220uF caps and put a 470uF 35V cap in their place (they were in parallel), and then held a 10-ohm 1/2-watt resistor across the fuse and powered on and the backlights now light. I then put my DMM across the open fuse it in amps mode and measured about 1.43A when the backlights were lit. At 25V, this gives 35W. Seems a bit high for a display of this size. After running for several seconds, I felt components on the inverter and nothing is hot. So, maybe I just put a larger 2A fuse in (plenty of room to solder it in) and call it a day?

[Toasty]

Re: Dell E171FPb bulged cap & open fuse on backlight inverter

FS fuse is 4A

[blargg]

Re: Dell E171FPb bulged cap & open fuse on backlight inverter

Seems to be working so far (didn't bother putting the correct 4A fuse, partly so this 2A might protect it better in case of further problems).

The next problem I encountered was very unresponsive front-panel microswitches. Every single one barely worked, especially the power button. Finally pulled the PCB out and sprayed contact cleaner into them. After working it in, they work acceptably.

One other thing, at lowest brightness there is very noticeable variation in illumination, horizontally across the screen (so the brighter and darker bands are like columns). I tried unplugging one CCFL at a time (putting in an external one to carry the load) to see whether it was one particular bulb. I believe this panel's bulbs are in pairs, along the top and bottom edges. So this variation is either due to aging of the tubes (mercury migration), or possibly the light guide behind the LCD. I was really relieved that the problem with this unit wasn't bad CCFLs, and worry that they are on the way out, given how old this is. I made my standard mod to this unit where the backlight is kept on whenever the unit is on, rather than turning the light off every time the computer changes resolution etc. which is idiotic (it still turns off when the monitor goes to sleep of course).

[blargg]

Re: Dell E171FPb bulged cap & open fuse on backlight inverter

Almost a year later, this Dell is still working great, and I use it most of the time. I encountered a similar problem today with the front panel microswitches sticking on a Samsung SyncMaster 172T. I didn't use any contact cleaner spray this time, just steel wool, and it works great now.