HP compaq NC6400, LA-2952P, white screen

[msilvap]

good morning

I've had this laptop sitting at home for many years with this defect, and now I have some free time and I'd like to fix it.

9 years ago I tried to sell it to a woman, she kept it at her house for a few days and then returned it to me broken (hinge). I bought the hinge and a new flat, thinking the problem was in the flat, and it wasn't...

The screen connector part is in the photo below, I measured the voltages with a simple multimeter and the measurements were as follows:


1: 1,3V
2: 1,2V
4: 1,3V
5: 1,3V
7: 1,1V
8: 1,4V
10: 1,1V
11: 1,4V
13: 0,1V
14: 0,1V
16: 0,1V
17: 0,1V
19: 0,1V
20: 0,1V
22: 0,1V
23: 0,1V

25-27: 3,3V
28-29: 5V
32-36: 3,3V
38-40: 19V



What could this be? I found it strange that some voltages were so low, 0.1V, but I don't have an oscilloscope to take a look at these signals... Does anyone know what could have happened?

[mon2]

I tried to sell it to a woman

Here is your problem.

The low voltage measurements are due to the pins that you have measured are very high speed LVDS lines = low voltage differential signals. In order to reach the high speed, the logic swing is kept low by design. Respectively, you must have a good oscilloscope to properly measure each of these high speed signals. Suggest to test with a known good display.

Other ideas are to disconnect the LCD (so that we can confirm the motherboard who is the driver for this display is mating upto this connector ok) -> meter in DIODE mode. Red meter probe to ground (yes ground). Black meter probe to the pin to test. Be sure there is no power to the board. Test each pin. If you see 'OL' this means there is a disconnect to that connector pin from the logic board side. Check in the schematic but suspecting that there will be EMI filters (coils) to clean up the video signals for these LVDS lines. These will then trace back to the GPU or CPU for the video signals.

[msilvap]

Here is your problem.

The low voltage measurements are due to the pins that you have measured are very high speed LVDS lines = low voltage differential signals. In order to reach the high speed, the logic swing is kept low by design. Respectively, you must have a good oscilloscope to properly measure each of these high speed signals. Suggest to test with a known good display.

Other ideas are to disconnect the LCD (so that we can confirm the motherboard who is the driver for this display is mating upto this connector ok) -> meter in DIODE mode. Red meter probe to ground (yes ground). Black meter probe to the pin to test. Be sure there is no power to the board. Test each pin. If you see 'OL' this means there is a disconnect to that connector pin from the logic board side. Check in the schematic but suspecting that there will be EMI filters (coils) to clean up the video signals for these LVDS lines. These will then trace back to the GPU or CPU for the video signals.

Thanks for replying!

I tested the continuity of the flat cable using the multimeter on the diode scale and it is OK. One thing I noticed is that the notebook does not display video on the external monitor either, but on the other hand the keyboard responds to the num lock and caps lock commands, indicating that the system has booted...

Any ideas?

[mon2]

Can you link the schematics? Not available in my database. Post it inside of the dedicated schematics forum.

Often, there will be a MUX (multiplexer) to steer the graphics data flow to either the internal or external screen. Check if there is one onboard and check the datasheet to determine the select pin and also which side this video data is directed. To me, a white screen could be missing out on even one line of the LVDS lines that are drawing the screen pixels. White screen confirms your backlight is ok.

[msilvap]

Can you link the schematics? Not available in my database. Post it inside of the dedicated schematics forum.

Often, there will be a MUX (multiplexer) to steer the graphics data flow to either the internal or external screen. Check if there is one onboard and check the datasheet to determine the select pin and also which side this video data is directed. To me, a white screen could be missing out on even one line of the LVDS lines that are drawing the screen pixels. White screen confirms your backlight is ok.

https://www.badcaps.net/forum/docume...ptop-schematic


I was thinking, this notebook probably suffered a fall, could something have shorted out and damaged part of the north bridge?