HP Elitebook Screen blank - backlights but no image

[dicky96]

Hi guys
I'm trying to fix this laptop screen

Basically the owner is a computer shop and they brought it to me with two screens. They said the repair shop they use could not fix it, so I thought i would try to see if I am better than their current repairer!

OK so I have the original screen which works but has lines across the bottom quarter of the screen

I have the replacement screen, which they fitted and it worked. They then put the laptop on one side for a couple of weeks until they found a buyer. When they showed it to the buyer, the screen would not work - backlights OK but no image. An external VGA monitor works fine

So I thought I would try to fix the screen with no image. I checked the voltages on all the test points on the TCON PCB. They are the same, or very close, on both screens

I then booted into windows with the internal screen attached, using the external monitor as a second display. in both cases Windows knows the screen is there.

So basically I can't see any difference between the the one with lines on the display and the one with no display.

I identified the main chips (Backlight PSU, Gamma, Image?, and a couple of eeproms - see pics. I could only find datasheets for the EEPOMS.

I was thinking maybe the screen with no image would also not detect - but that is not the case. What do the EEproms do? Are they like a BIOS chip on a GPU for example? I'm now thinking of swapping the chip that I think handles the graphics (The parade Wise View chip) and maybe the EEproms to see if I can get the blank screen to work

In fairness I don't have as much knowledge as I would like -

Apart from the obvious voltage, PWM and BL_ON signals on the LVDS, there are also signals called DPRX_AUXN, DPRX_AUXP, DPRXOP and DPRXON. Any idea what these are? Is there a standard pin-out for LVDS or tutorial?

What do the EEPROMS do? Is the data unique to each model of screen (like a motherboard BIOS) or unique to each individual screen (like a HDD eprom)?

Any suggestions of anything else I could check, or should I just start swapping parts over?

Here are two videos (unlisted so you need these links to view) if you would to see what I have tried so far:

part 1 - https://youtu.be/dk3crSH8uo0
part 2 - https://youtu.be/pm-8aTGbKj0

[diif]

Re: HP Elitebook Screen blank - backlights but no image

Looks like a Dell laptop ? The eeprom holds the Extended Display Identification Data (EDID), I'm guessing this wrong on the non display screen.

[dicky96]

Re: HP Elitebook Screen blank - backlights but no image

Hi diif
Yeah that would make sense.

it is a HP Elite laptop

Given that it did work after the new screen was fitted, then put in storage for a few weeks and did not work when it came back out, what would cause the eeprom to fail?

There are two EEPROMS, do I need to swap both or just one?-

[piernov]

Re: HP Elitebook Screen blank - backlights but no image

Apart from the obvious voltage, PWM and BL_ON signals on the LVDS, there are also signals called DPRX_AUXN, DPRX_AUXP, DPRXOP and DPRXON. Any idea what these are? Is there a standard pin-out for LVDS or tutorial?

These are eDP not LVDS. AUX is for display identification (EDID is communicated over that bus), 0 is the first (and unique on this panel since it's low resolution) eDP data lane.
HPD is also part of eDP and it's the panel presence signal.
Can be confirmed by looking at the schematics of the laptop.

[dicky96]

Re: HP Elitebook Screen blank - backlights but no image

Hi Piernov
I didn't think to look for the laptop schematic as the laptop itself works

So the DPRX0N and DPRX0P (I used zeros instead of O this time) are Negative and Positive (differential signals) and the same for AUX P and N-? - these are like PCIe lanes, kinda?

The display identification works (as tested in my YT video) just no video. I will try swapping the Eeproms from the other panel first. It seems more likely that an Eeprom may be have got corrupted rather than the video output chip to fail.

I'll also go google for eDP see if I can learn something new.

Cheers.

[piernov]

Re: HP Elitebook Screen blank - backlights but no image

These are differential pairs yes.
Also you need to clarify what "no video" means. No image and/or no backlight and what color is it showing.

[dicky96]

Re: HP Elitebook Screen blank - backlights but no image

No image and backlight working. I did link to the videos which show the fault clearly. You can see that that it is a dark grey colour. There are a few feint horizontal lines when first powered on but they fade away to an even grey

This video
part 1 - https://youtu.be/dk3crSH8uo0
3mins : 0 secs

[SMDFlea]

Re: HP Elitebook Screen blank - backlights but no image

Whats the laptop model number and do you know the old screens part number .

[dicky96]

Re: HP Elitebook Screen blank - backlights but no image

Hi SMDFlea
I posted photos of both screens part numbers in the OP - you can see them both on the second photo - They are identical. Is that what you wanted or something else, in which case I will post the info

I didn't post the laptop pmoel number, I'll do it tomorrow morning

[dicky96]

Re: HP Elitebook Screen blank - backlights but no image

Also - today I looked at the signals DXPR0N and DXPR0P - there is too much noise on my scope to see the differential signals properly. But I looked at the signals coming out of the video chip to the panel and in both cases they are are sending data to the screen though I have no idea if the data signals are good or bad. However they look kinda similar

I've now removed both the Eeproms and the 'video' chip from the working panel that has lines in the bottom quarter. Tomorrow I will solder them to the panel with the blank screen and see if it makes any difference.

Rich

[dicky96]

Re: HP Elitebook Screen blank - backlights but no image

OK, the model number as requested HP Elitebook 8440p

I swapped the large QFN (Parade DP619) and both the Eeproms from the screen that had a picture but lines on it.

The result of this is that it made no difference. The backlight still comes on, there are some initial feint horizontal lines across the screen that fade away to an even grey after several seconds. Using an external VGA I can see that the laptop still detects both screens in display Properties.

I'm at a loss now to understand why it does not show a picture. I feel like I am missing something obvious - but what? Can anyone suggest what else I can test, just for the sake of sanity?

Rich

[piernov]

Re: HP Elitebook Screen blank - backlights but no image

If all the usual LCD driving voltages are good (VGH/VGL/VCOM/…) I'd guess it's a defective row/column driver or FFC.

[dicky96]

Re: HP Elitebook Screen blank - backlights but no image

Hi Piernov
I am home now but I just looked at the photos I uploaded. There is a test point called VCOM so that will be the same on both panels as I measured all test points and compared them.

I can't see any test points for VGH/VGL,though I need to look a the actual boards to double check. So quite possibly I did not measure those.

I found this link that taught me a bit about these voltages and how they are generated.
https://pcbartists.com/design/power-...matic-tft-lcd/

I saw some voltages around -6.5V on several test points on both panels so that sounds like it could be VGL

Do I also need AVDD ,DVDD, VIH, VIL? Or is that for TFT panels only, which mine could be I guess.

I'm pretty sure all the Gamma voltages are there - they measured the same on both panels, and the test points are clearly marked. I just noticed there is a coil L1 near the RT9965 chip which I assumed generates Gamma, and I didn't check voltage there.

I failed to locate a datasheet for RT9965, but is it likely that this IC generates gamma and also generates VGH, VGL? There is another chip marked OP1 near here that I should look at, plus a few transistors. The other four 6 pin devices nearby are marked D11, D12, D32, D33 which I am assuming are diode arrays, and this kinda makes sense if OP1 is in a circuit similar to the example on in the above link ,rather than RT9965.

Although quite a pointless repair, I am learning quite a lot about LCD panels here, so it's a useful if somewhat frustrating experience so far

Thanks for your assistance.

Rich
PS what's a FFC? Google says it's Fulham Football Club

[mon2]

Re: HP Elitebook Screen blank - backlights but no image

FFC :

https://en.wikipedia.org/wiki/Flexible_flat_cable

[vinceroger69]

Re: HP Elitebook Screen blank - backlights but no image

FFC i think means Flat Flexible Cable

[dicky96]

Re: HP Elitebook Screen blank - backlights but no image

Thanks Mon2, Vince. Google did not tell me that for some reason, and I thought Google knew everything.

OK, there are test points at all the inputs from the FFC on the panel PCB, and they are reading the same voltage on both panels. Plus I also examined the socket on both and they look fine. Obviously the cable is OK as one of the panels works ( albeit with lines on the screen)

[SMDFlea]

Re: HP Elitebook Screen blank - backlights but no image

Whats the part number on IC5

[dicky96]

Re: HP Elitebook Screen blank - backlights but no image

Here is the chip IC5, and also the other one OP1

I could not find a datasheet

[SMDFlea]

Re: HP Elitebook Screen blank - backlights but no image

iML7975 and iML7822 ,theres no datasheets.From what ive read the 7975 is there to reduce screen flicker.

https://cdn.badcaps-static.com/pdfs/...bba237f832.pdf
https://monitor.net.ru/forum/threads/483355/

[dicky96]

Re: HP Elitebook Screen blank - backlights but no image

I was given an bin file for the EDID eprom ,and compared that with the contents of the 402WP Eproms from both my screens - both the eprom contents are identical to the file

I decided to have another look on both screens to see if I could find the LCD supply voltages

Vcom has a test point and is 3.326V
VGL has a test point and is -6.94V
VGH, DVDD, AVDD, VIH, VIL I can't find.

Is the information on this link correct for LCD screens in general? As it gives min, max and typical voltages for all of these.

https://pcbartists.com/design/power-...matic-tft-lcd/

This says that VGH would be ~ 16V and AVDD would be ~ 10.4V but I can't see any voltages on either panel in that range.

Basically all the voltages I see are either less than 8V apart from 22.37V on test point VON, and a 22.18V and 19.4V in the area of the circuit that appears to be for Back light voltage generation.

So that does not make sense either - I have looked for 16V and 10.4V on all the capacitors in the RT9965 / iML7975/iML7822 area of the board and can't find anything close to those values - on either panel.

It's so annoying when I can't fix something because I can't figure out what's wrong with it! I don't care now if I can fix it or not, I just want to understand exactly why it does not work