Question Tablet motherboard troubleshooting Dell Venue Pro 5055

[sixt33n]

Hello!

I would like to ask for help troubleshooting a motherboard for a tablet that I have. The tablet is Dell Venue 10 Pro 5055 with windows 10.

DISCLAIMER: I am doing this for fun and learning. It's not a tablet exactly worth paying to repair as the cost of taking it to a professional would greatly exceed the tablet's own value.

For background, I first had a bootloop issue going on and I found instructions here to fix it. it involves shorting the bios chip (Winbond 25Q64FW1G). I was successful in getting out of the bootloop but I was not able to go to bios immediately after it booted so windows update began and I had not realized that the battery was already running low so it turned off after windows update. Then I charged it and attempted to turn it on again and thats when the problem I currently have started. LED's turn on, charging port led turns on, vibrates when turned on, but no power on the display. I do not have a micro-hdmi cable to test if it is really just the display having issues. I have researched that usually whenever, the display of a laptop doesn't power on, it is usually a broken capacitor near the display cable, however I have found a number capacitors that seems to have issues. I am guessing that there must have been a short somewhere possibly from loose solder, however I have tried using compressed air to try to clean it but to no avail.

So I pulled out a ohmmeter and started poking around using usb as the ground point. However, I don't really know how to interpret the measurements that I have taken so what I really need help with is for someone to look at the values that I have attained and maybe figure out the exact location of the problem.

In the image below are the resistance values that I got with the battery connected.



Just to be clear, the capacitors that I marked all have one side being grounded and the number value is the resistance from the other side. I am finding it odd that the resistance values of some of the capacitors are identical to some of the resistances on some of the pins on the battery connector. I am just not sure what to make of it. The up/down arrow are a set that I couldnt get a stable reading from and it kept going up and down.

Also not sure if resistance should be what I should be checking, advice on how to check welcome.

greatly appreciate the help!

-sixt33n

TLDR: Shorted bios chip with a wire to get out of bootloop -> Success -> missed entering bios -> windows update -> battery died -> charged and restarted -> turns on, but no power on display -> need help interpreting resistance values.

 

[Eximo]

You can't really measure in circuit resistances without a circuit diagram, you might be measuring more than you think. Resistance is best measured across components that have been removed from any capacitive or resistive system.

I imagine several of those are just snubbing or noise filter capacitors on ground or 5V+ so many of them might be the same measurement. Not 100% critical to operation probably.

Troubleshooting multi-layer boards requires complete schematics most of the time. Really you would want to be tracing known voltage points around the board to see if everything is powering up properly. Blown or shorted resistors and capacitors become obvious when you find out where voltages are bad.

On simpler circuits, logic probes can be helpful, but contemporary electronics are usually too fast. Then you need a pretty good oscilloscope.

 

[sixt33n]

Thanks for the reply Eximo. I'm not really that well verse with electronics and have just a basic understanding of how to use a multimeter. I've tried to search for the schematics for this motherboard but it probably isnt published in the web anywhere. I do not know how to use an oscilloscope nor do I have access to one, so I'm guessing there's no way to figure this out with just a multimeter.

One mistake I made on the picture I posted was was that the 34.11 reading is actually in megaohms. The rest are just in ohms. Not sure if that would make a difference in figuring this out. Kind of a shame if this tablet is junk now. Is it normal that the outside ends of the battery connector is reading 34.11 megaohms too? I thought that one of the end pins should be ground so wouldnt that mean I should have 0 resistance in that? I'll just blame windows update for causing this problem. Haha! I was so close too.

The other thing is the weird resistance readings for the capacitors Start from the set of capacitors(?) that fluctuate up and down, and to the right. The ones above it are reading just fine.

 

[Eximo]

34 Megaohms likely just indicates you are getting a little bleed through a semiconductor. Effectively that would be no connection to digital electronics. Not to mention that your multimeter will effect the measurement. Itself will have a 1M resistance or 10M for the nicer units. When you use it that puts it in parallel with whatever you are measuring. One of the reasons you can't just poke a multimeter into things as well as that actually will provide a path for current to flow, not much, but depending on what you are measuring it could do something.

Lithium batteries come with sophisticated charging and monitoring circuits. You might not get voltage out until some condition is met. To measure the battery voltage you would have to go all the way to the wires actually attached to the battery.

Schematics are really intellectual property of the companies that design these things. They aren't really intended to be user serviceable either. On popular devices people will sometimes leak or even reverse engineer everything on there.

With enough expertise you can identify and repair simple board problems, but it usually comes down to component replacement. Board damage can be bypassed with fine wires, but on multi-layer boards, this becomes an issue. Not to mention that the length and resistance of wires plays a factor in the internal timing on circuit boards. And wires draping across components can cause interference and crosstalk.

 

[sixt33n]

Ahh thank you for the detailed replies and the knowledge shared! I guess it is time to just go and give up on this thing. Just a shame that it seems to me that it is still a perfectly fine tablet cept for the motherboard.

I've watched quite a bit of troubleshooting motherboard videos on youtube. Most of which were Indian and have much larger components than the one I am dealing with. They were just using a multimeter to find the broken capacitor which had both ends grounded or using it in volts to measure other components that "seemed" fine to them. Working through experience I suppose. Was fun to poke around and tinker though.

Wish there were more posts that specifically deal with this kind of thing so I could read and learn more about it.

Anyways thanks again Eximo!