Dell Latitude 7420 2 in 1 (P136G) - no boot

[Mirenia]

Hello everyone,

A while ago my Dell Latitude 7420 did a bios update. Since then it no longer boots. The keyboard backlight is on, the display backlight too, but there is no Dell logo.

What I have tried so far:
All the troubleshooting guides on the Dell website (external monitor connected, booting without battery, NVRAM reset, tried flashing BIOS via USB stick, all key combinations while booting).
The only thing the laptop responds to is the BIST (integrated display test). Then the display lights up in all colors one after the other.

I have already disassembled the laptop into all its individual parts and reassembled it. Just in case a plug wasn't plugged in properly somewhere.

My fear is that the BIOS is bricked.

I came across posts in the forum where the BIOS chip is desoldered and then plugged into a programmer for reflashing.
Would that also make sense in my case or do you have any other ideas?
(I have quite a bit of soldering experience and am also not completely unfamiliar with electronics)

If it makes sense:
What exactly do I need for this? The bios chip is a winbond 25R256JVEN.
I have found the following programmer:
https://www.amazon.de/Hailege-EZP201...%2C182&sr=8-14
Would this be suitable or do I need a completely different programmer for the chip?

Unfortunately, I have not yet found any detailed instructions in the depths of the forum.

Thank you very much

[Vesko356]

What exactly do I need for this? The bios chip is a winbond 25R256JVEN.
Unfortunately, I have not yet found any detailed instructions in the depths of the forum.

It's a sticky thread in BIOS Requests sub-section -> BIOS guides, methods, resources and tools.
Winbond 25R256JVEN is a WSON8 package SPI, you'll need an adapter to read/program the chip, and a hot air station to unsolder/solder, or WSON8 pogo adapter (not very reliable method), if you want to do it without desoldering the chip.

[Mirenia]

Oh, I completely overlooked that! Thanks for the link.

I don't have a hot air station but a good soldering station. Hopefully I can manage to desolder the chip without damage.

By the way, the Latitude also performs the NVRAM reset. After pressing the power button for just under 30 seconds, it flashes three times (but no post, no boot)

Do you think it's really a bricked BIOS or could the error have other causes?
It will still be a few days before the programmer arrives. If you have any tips, I could continue troubleshooting in the meantime

[mcplslg123]

You cant de solder a wson chip with soldering ron,Hot air station is a must.

[Mirenia]

Thank you for clarification. At first sight it looks like it has tiny legs. But google gave me the answer: they're not.

So maybe then it is time to learn a new skill

I'll go and get a hot air station and practice a little.
Afterwards I'll post the results with this wson chip here

(And I really hope it is only a broken BIOS)

[diamon]

You can first start / try a bios recovery procedure. Remove external power adapter, press and hold CTRL + ALT on the keyboard of the laptop and then attach the power supply (still pressing those keys). When you see the dell logo, release the pressed keys. Before you start the above, download the biosfile.exe for your laptop and place this on a FAT32 USB stick.

Please let me know if this worked for your laptop

[Vesko356]

...press and hold CTRL + ALT...

Ctrl + Esc key

[Mirenia]

I already tried this. The USB stick then flickers only one time and no Dell Logo appears.

[Mirenia]

Nooo, I think I made it worse... To prepare for hot air desoldering, I measured the BIOS chip and I probably managed to create a short circuit somewhere.
Now I only have a red light as long as I keep the power button pressed. The light fades the longer I hold down the power button. And I hear a beeping sound on the board as soon as the power supply is connected.

To be honest, the schematics are so overwhelming that I don't even know where to start looking.

I do have experience with repairing circuit boards, but this is limited to older devices such as Atari, Commodore and co. A modern laptop is a whole different ball game.

Do you have any ideas where to start?