Dell E177FPb - White Screen

[tom66]

I've just bought one of these for £11.04, faulty, with a white screen only. It's a 17" LCD monitor.

I fixed another monitor like this which had 7 bad caps in it, so I'll see if it's as simple as that.

I will post pictures when it arrives but before I get it does anyone have any ideas about what the problem may be? Is it as simple as bad capacitors?

[PlainBill]

Re: Dell E177FPb - White Screen

I've just bought one of these for £11.04, faulty, with a white screen only. It's a 17" LCD monitor.

I fixed another monitor like this which had 7 bad caps in it, so I'll see if it's as simple as that.

I will post pictures when it arrives but before I get it does anyone have any ideas about what the problem may be? Is it as simple as bad capacitors?

Probably, but it won't be electrolytic caps. A white screen is caused by a failed link between the signal card and the LCD panel itself. The most common cause is disconnected LVDS cable (between the signal card and the panel card (tcon)). The second is a blown fuse on the tcon, which may be caused by a shorted ceramic cap in the DC-DC converter on the tcon.

PlainBill

[tom66]

Re: Dell E177FPb - White Screen

Probably, but it won't be electrolytic caps. A white screen is caused by a failed link between the signal card and the LCD panel itself. The most common cause is disconnected LVDS cable (between the signal card and the panel card (tcon)). The second is a blown fuse on the tcon, which may be caused by a shorted ceramic cap in the DC-DC converter on the tcon.

PlainBill

Sorry, what I meant is I fixed a Xerox monitor, not this same model, which had similar symptoms. It would show the Xerox logo for about a tenth of a second and then fade to white (after about 5 seconds it was completely white.) I figure the supply was still able to power the inverter, but the main board was freaking out - opened it up to find 6 bulged caps plus one non-bulged one. Replaced them all with new Chemicon KZEs and Panasonic FCs which fixed the problem and it works great now. (Still does as my main monitor.)

If the screen is completely white then most likely it is a cable or t-con fault as you say.

[tom66]

Re: Dell E177FPb - White Screen

I just got the monitor today and the screen is completely white, no image at all. The monitor turns on and off using the button and the power indicator lights up.

It was difficult to tear down but it was nice to see some attention to detail paid to servicing. Although everything was taped down there were always accessible screws and sliding covers covers to unplug the CCFL cabling so the wires can be removed, before flipping it over. The unit is encased in a full metal shield, presumably for EMI, but probably not great for the caps as it's going to essentially be an oven inside the monitor.

But, you don't get everything - it's full of Lelon and Elite. There are two 1000u/10V Lelons, they look okay but their ESR is about 0.14 ohms which is much too high especially for two in parallel. I have some Panasonic FCs on hand, so I'll replace them. I don't expect it will fix the main problem, but it's good preventative maintenance. The Elites measure 0.02 ohm ESR, which for now is fine.

A rare sight, too - a regulator on the power supply board - not the main board. Looks like a DPAK. Might be tricky to replace if it is bad. It's a 3.3V regulator, 1117 type.

[tom66]

Re: Dell E177FPb - White Screen

I replaced the caps. 5V is 4.95V - 4.9V - varies depending on state of monitor. I tested the LDO on the power supply board, giving a nice clean 3.28V output, and the regulator on the main board gives 1.8V, so they seem okay.

I also learned it's very important to get the CCFL connectors right, specifically the polarity of the coloured wire and the white return wire. If you don't you get an awful hissing sound and a half-dim display; I guess the small thin wire isn't HV rated so is internally arcing somewhere.

Will have a look at T-con later, considering scoping the LVDS waveforms...

[momaka]

Re: Dell E177FPb - White Screen

Will have a look at T-con later, considering scoping the LVDS waveforms...

No need to scope the LVDS waveforms. Just check the fuse on the t-con (usually near the LVDS connector). Also check the primary step-up rail for short circuit or resistance less than 50 Ohms. The primary step-up rail is usually located on the cathode side of a diode that its anode side connected to a round inductor.
If you can't find it, post some pictures of the t-con. The area around the LVDS cable and the round inductor is of primary interest.

[selldoor]

Re: Dell E177FPb - White Screen

Hi Just wondered if you have seen this:http://www.youtube.com/watch?v=FfbmQRuqDD0
Its got to be worth a try?

Skip first 3 minutes - just shows its white screen then dissassembly,

[tom66]

Re: Dell E177FPb - White Screen

Interesting... it does have an AU Optronics LCD panel like in the video... Plus that PSU and tiny logic board looks familiar. Thanks for the tip.

[tom66]

Re: Dell E177FPb - White Screen

Cleaning the connector didn't help. I found the T-con was only getting 1.25V or so. The fuse is okay, so that puts suspicions back on the main board. What would be the worst case if I just wired it straight to the 5V rail?

I see a few SMT transistors on the main board and one of them looks dodgy. It's tiny, so I'll probably just short it to 5V. I don't see a problem with that - if the backlights are off, the display won't be visible. Maybe energy saving?

I'm of course assuming the T-con uses 5V although I haven't seen any use 3.3V yet.

[tom66]

Re: Dell E177FPb - White Screen

I traced the circuit, and found the T-con does work off 5V. So, I wired it straight in... and... nothing...

It blew the fuse on the T-con. Monitor still powers up fine, but with same white screen.

So, what is going on here?? Something must be bad on the T-con board.

[momaka]

Re: Dell E177FPb - White Screen

I traced the circuit, and found the T-con does work off 5V. So, I wired it straight in... and... nothing...
It blew the fuse on the T-con. Monitor still powers up fine, but with same white screen.
So, what is going on here?? Something must be bad on the T-con board.

Yes.
There are 3 step-up rails on the t-con that provide various voltages to the TFT: a main step-up rail (which boosts the 5v supply up to 9 to 15V typically) and 2 secondary step-up rails (one of them further boots the main step-up to 15-25V, and the other makes a negative voltage typically in the range of -5 to -10V).

The main step-up provides a fairly large amount of current - typically 100 to 300mA. When it fails, the fuse on the t-con usually blows.

The secondary step-up rails are low current. When one of them fails, the fuse may not necessarily blow (in fact, out of the 2 monitors I've seen with this failure, in both cases the fuse did not blow).

In all of the failures I've seen on these 3 rails, the problem was always caused by a shorted ceramic capacitor. That doesn't not mean to say another component can't be bad, but a shorted ceramic capacitor is very probable. Fairly easy fix (once you find the shorted capacitor). I normally grab my replacement ceramic capacitors from dead motherboards and video cards. Just match the physical size and it's all good.

I see a few SMT transistors on the main board and one of them looks dodgy. It's tiny, so I'll probably just short it to 5V. I don't see a problem with that - if the backlights are off, the display won't be visible. Maybe energy saving?

Yes, energy savings indeed. The t-con can draw up to 5W, depending on the design.
I actually had to short one of those transistors like you mentioned because it was completely burned and I didn't know what to replace it with. Of course I fixed the issue on my t-con before shorting that transistor, so I didn't end up with a blown fuse .

Post some pictures of your t-con so we can start troubleshooting it. The area of most interest is around the LVDS connector and the round inductor.

[tom66]

Re: Dell E177FPb - White Screen

Tested the caps... most measured in the 100kohm range - but there was one that measured 115 ohms... which seems low but might be due to other components on the T-con.

No short seems to be present from 5V out (after fuse) to GND, but I don't have another fuse to test yet.

[tom66]

Re: Dell E177FPb - White Screen

Detailed pics of T-con...

[retiredcaps]

Re: Dell E177FPb - White Screen

Detailed pics of T-con...

Momaka is the expert this area and the only contribution I have is to caution you to be really carefully with those tab bonds.

I finally took apart a lcd screen to the ccfl tubes, found the bad one, put in the replacement from a donor screen, etc, etc only to find that I had somehow ripped the tab bond off the board. Now I get a black vertical stripe.

It was a 19 inch wide screen too. I hadn't tried glue or anything yet to repair it.

[momaka]

Re: Dell E177FPb - White Screen

There we go. The first picture (this one) has pretty much all of the test points you need, so I'll use that as reference in this post.
...
The main step-up voltage rail is connected to test point AVDD1 (just a little above the center of the picture). Using your multimeter on the lowest resistance scale (typically 200 Ohms for manual ones), check the resistance between test point AVDD1 and ground. Post what resistance you get.
When the monitor is working normally, this rail will be typically between 9v and 15v (depending on design).

Next, check diode D8 to see if it's shorted. D8 is located to the right of that round inductor, L1. Diode D8 is the rectifier for the main step-up voltage rail.

Finally, there's the two secondary step-up voltage rails. The positive one is connected to test point VGH1. The negative one is likely VEE1.
Again, using the first picture as a reference, test point VGH1 is close to the lower-right corner of the t-con and test point VEE1 is just slightly above the screw hole.
Just like you did for AVDD1, check the resistance between test point VGH1 and ground as well as test point VEE1 and ground. Post the results you get here.

Lastly, should things get more complicated, write down the part number of U11 (located to the right of the round inductor). U11 is the controller for the t-con's power supply. The data sheets for these controllers usually give you sample circuits and a lot of other useful info. Sometimes, the circuit on the t-con can be very similar to the sample circuit, so it's always worth having a look.

[tom66]

Re: Dell E177FPb - White Screen

Well, this monitor got put on the back burner, but I decided to have one last shot at it again.

I did the unthinkable, of course - I bridged the fuse with some solder. Heh, I thought the monitor may as well be scrap, anyway. So, plugged it in, and it seems like it didn't want to come on, but then the screen started flickering and it powered up but no power LED because 5V was shorted. I heard the occasional SMPS click from a shorted output.

Then smoke comes from one of the MLCCs. I guess that was the shorted one then... I must have missed it in my tests or maybe it was only short circuit with a higher voltage.

Removed it... Got a display (no signal!)

BUT... it's very blurred and pretty poor. And after a while, it goes away and goes white again. And the PCB got a bit... melted... from that MLCC. So I'll have to figure a way out of putting a cap on there.

[momaka]

Re: Dell E177FPb - White Screen

That shorted ceramic cap was mostly likely on the main step-up rail, AVDD1. Would have taken 5 seconds to check it with a multimeter and avoid all of that damage above... Oh well, too late now .
I hope you get fixed, though. Looks like you're almost there. And at least the monitor works somewhat now.

[tom66]

Re: Dell E177FPb - White Screen

That shorted ceramic cap was mostly likely on the main step-up rail, AVDD1. Would have taken 5 seconds to check it with a multimeter and avoid all of that damage above... Oh well, too late now .
I hope you get fixed, though. Looks like you're almost there. And at least the monitor works somewhat now.

It was between the ribbons, probably AVDD1. Did check for shorts from 5V to ground but now I think about it: for a boost converter there's a series diode, and my 0.3V test voltage from the multimeter wouldn't bias the diode so the short wouldn't be found.

[vela]

Re: Dell E177FPb - White Screen

I am a newbie at fixing monitors, but have been reading a number of threads with great interest. Very impressed with the knowledge and expertise of the poster and even more so with everyone's willingness to share. This particular thread was especially interesting as my Dell w2207 monitor is also showing the White Screen issue and I was hoping to follow this thread and momaka's expert diagnosis to conclusion, but unfortunately tom66 took a shortcut

I did measure the resistance from AVDD1 to ground on my t_con and it is just 1 ohm. The resistances for VGH1 (71.2Kohms) and VEE1 (6.08Kohms) are much higher. Does this shed light on where the problem might be? Any help will be greatly appreciated.

I am also testing the ceramic caps on the t_con for shorts, just by measuring the resistance using a DMM. Hope that is the right way to do it.

[momaka]

Re: Dell E177FPb - White Screen

I did measure the resistance from AVDD1 to ground on my t_con and it is just 1 ohm... Does this shed light on where the problem might be?

Is it a "1" on the left side of the screen or 1.0 Ohms?

The latter means that there's a short on AVDD1. If that's the case, then more than likely one of the ceramic caps connected between AVDD1 and ground is shorted. Finding out which is the hard part since there more than one and they are all connected in parallel (so if one is shorted, they all shorted).

If you have decent SMD soldering skills, then start removing the ceramic caps connected to AVDD1, one by one. (I would say, start with the ones next to L1 as those are the most likely suspects.) Make sure to check each ceramic cap once you remove it from the board - if the cap doesn't show open circuit (infinite resistance) out of the board, it's bad. Also make sure to check resistance between AVDD1 and ground after removing each cap. You should get about 100 Ohms or more once the short is gone.