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What is broken? No display and Power / Fn buttons flickering

I'm asking because I've never encountered these symptoms with any computer ever before.

Symptoms:

  • flickering power and Fn buttons
  • laptop screen blank

I replaced the mainboard of my X270 with one bought from a Chinese store. I've done that before on other laptops of the same model and with different models. although no professional hardware technician, I'm familiar with that kind of work.

I installed the replacement and booted up. The symptoms are as described above; the laptop screen is staying blank and the power and Fn buttons are flickering. Not blinking, flickering. This does not at all look like a post error blinking, it's way too fast.

The flickering might have a pattern that repeats after a pause, but that's just a feeling and I first need to actually record the timings.

With an external screen attached, the system can be used fine, but the flickering of the power button persists.

It's a very weird error. Do you have any clue about what could be broken and how does it cause the symptoms. Or are they unrelated?

I'm baffled.

p.s.

I rechecked the flickering of the Power and Fn buttons; there is a pattern and it's five flashes in about 0.5s, every 1s.

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Sounds like a loose connection somewhere. Recheck all especially for screen.

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Did that, will do again.

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I checked all connections, they seem fine. I also checked with another panel and got a similar issue (flickering picture instead of no picture, but still no working picture). I checked the panel on another laptop and the panel seems fine.

To me it currently looks like there's something wrong with the motherboard itself.

Thanks for your help, much appreciated!

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Hi @esarbe

With the laptop turned on and no display, try shining a torch at an angle close to the screen and check for any images. They will be very faint so trying this in a darkened room may help to see them if they are there.

If they are then then there's a backlight problem with the screen.

Since it is a new motherboard and assuming that the display is OK first check if the backlight power fuse F2 on the motherboard is OK and hasn't blown for some reason. If the fuse is blown it is a 3A_32V_ERBRD3R00X fuse. The backlight power is fed to pins 13 and 14 of the LCD connector (video cable)

There is always backlight power supplied to the screen and it is turned on and off in the screen by a signal voltage sent on the Backlight_On lead which appears on pin 23 of the LCD connector. This signal is controlled by the BIOS/OS and the backlights can be turned off for instance if the lid sensor detects if the lid is closed or if the OS goes into sleep mode etc.

here's an image form the schematic that may help with the backlights if that's what it is.

Block Image

(click on image to enlarge)

Here's the schematic for the motherboard (if it is a Lenovo X270 NM-B061 R04 board) that may help you further.

I have no idea why the LEDs are flickering but if the laptop works OK other then the display of course, it may be a problem with the power supply that feeds the LEDs. Hopefully the schematic will help track it down.

Hopefully a start.

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Thanks a bunch for these hints and suggestions!

I tried the approach you described but could not detect an image, even in a very dark room.

Luckily I have another x270 at hand, so I decided to investigate further, opened the bottom case and attached the screen connector to the new motherboard.

With this panel, there *is* an image, but it's flickering in the same repetitive pattern (5 pulses in about 0.5 seconds then another 0.5s pause) as the power button.

I also tried to connect the panel of the broken laptop to the motherboard of the working one and I got a picture; the panel and cable seem fine.

So, there definitively seems to be something broken with the motherboard.

I will check the fuses and see if I can find anything.

Thanks again for your very thorough reply!

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@esarbe

Just curious if you can get into BIOS (press F1 on startup) with a display on the external monitor?

Really you should not be able to as the external display is only after Win loads

maybe a BIOS problem - try updating perhaps or a power refresh by removing /disconnecting the main battery and RTC battery (CMOS battery) for 5 minutes

por

I was able to connect an external display, but I didn't get to the BIOS, unfortunately.

I disconnected the power supply from the motherboard but was not able to get a picture through to the panel.

I'm going to return the motherboard.

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