Sudden loss of backlight - Page 1
Forum section: electronics / general

author: blueskull
author_rank: Unknown
timestamp: March 05, 2020, 07:37:39 am
content: It's again my cursed Surface Pro 7.Just a few minutes ago it experienced some backlight flashing, then half of its backlight LEDs stopped working.After a quick backup and a reboot it was temporarily fixed, but why is that happening? Is MSFT using software controlled two sets of BL PWM, or is it just one LED boost converter glitching out?The real question for computer repairing gurus is, how long can I expect the tablet to last?I'm asking because this is the last computer I have as my X1 Extreme is on its way to Lenovo service, so if this one dies, I will be out of computers.If I have a good confidence that it will die soon, I'd better order a new one today to not to interrupt my workflow. But if it doesn't die, I just wasted $2000 for a third computer that I have no use of.Any expert recommendations?

author: Nominal Animal
author_rank: Super Contributor
timestamp: March 05, 2020, 09:51:37 pm
content: Not expert, more like bumblefuck rec'd: Have you checked your mains voltage, to see if it has severe spikes that could affect your devices? Or odd harmonics or something, that allows spikes to get through switchmode supplies, and damage the boost converters?

author: Nominal Animal
author_rank: Super Contributor
timestamp: March 06, 2020, 03:58:51 am
content: Quote from: blueskull on March 06, 2020, 03:53:39 amI was thinking more along the lines of a neighbor in the same mains section having something (with an induction motor, perhaps; something broken, but functioning well enough for the neighbor to ignore) that sometimes backfeeds spikes to your mains network, that are too short for the SMPS to suppress, and degrades caps and boost supplies.

author: Whales
author_rank: Super Contributor
timestamp: March 06, 2020, 04:30:02 am
content: There are multiple power supplies between mains and that screen backlight; if the device hasn't crashed then it's highly unlikely that massive mains spikes are the cause. The backlight is not the most sensitive part of this device.More likely a physical fault , eg half of the backlight LEDs (one string) going off due to bad solder joint or failed driver. Unlikely to be software.

author: magic
author_rank: Super Contributor
timestamp: March 06, 2020, 06:39:08 am
content: Pirate schematics of laptops can sometimes be found on the net.No idea how this circuit works and why a reboot would fix it.

author: Nominal Animal
author_rank: Super Contributor
timestamp: March 06, 2020, 06:53:23 am
content: Quote from: Whales on March 06, 2020, 04:30:02 amAbsolutely true. I was only thinking that the others are typically buck converters, and the backlight is the first boost converter in the chain, and maybe some kind of occasional spike or harmonic might perhaps explain the other issues blueskull has had with computers.Thinking about it further, I agree, and don't think it can be a mains issue... but maybe it could be a local electrostatic potential issue, with the problems caused by sporadic ESD?It might be worth investigating, for example putting together a megaohm resistor to ground for grounding oneself, and some kind of ESD or peak voltage detector, to notice if one accumulates a static charge.

author: magic
author_rank: Super Contributor
timestamp: March 06, 2020, 07:17:21 am
content: Quote from: blueskull on March 06, 2020, 06:42:53 amSee how it works in the older one. Chances are it may offer some hints. If both models use LED backlight they could be similar.

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 06, 2020, 03:21:08 pm
content: Looks like:- you really have bad luck with computers lately,- the Surface Pro 7 is a big pile of shit all in all.(I would doubt it was a driver/OS-side bug here. I doubt there is any way to control backlight other than globally from a driver's POV. It may be an hardware issue as you said, or just a firmware issue in the panel driver itself.)

author: KrudyZ
author_rank: Frequent Contributor
timestamp: March 10, 2020, 02:04:56 am
content: When it's in the broken state try adjusting the brightness.This will require the sw to rewrite the LED driver chip.If the remaining LEDs change their brightness, but the missing ones don't come alive, then you can be fairly sure that it is a hw issue, like a broken cable, etc...