My incredible attempt to repair a T420 motherboard *WARNING: PICS*

[epik]

Hi!

I'm coming from the Lenovo forum where a user suggested me to post my question here. And here it is.

My little nephew is a very curious and smart boy but this time he caused a little trouble.

He was studing the internal of my old T420 but used a wrong screw to reassemble. The screw was longer than the right one, touched the motherboard and "remove" one component.

Now the PC is not powering on anymore.
Or better the LED on the power on button turns on but:
- on battery it stays on for a sec, then turns off for another sec, then turns on again for a sec and so on;
- on power it stays on for a sec than it turns off for a millisec, then on etc.

Well the PC is old and I can thrown it away but little nephew is now so heartbroken that I want to try the impossible (and become the best uncle in the world).

The microwelding is not difficult because the component is an isolated one and the tracks are still working even if they have suffered a bit. My problem now is to decide which SMD component to sold there (capacitor or resistor?).

So if anyone with an old T420 motherboard can do the measure for me, you're welcome, you'll become the best friend of the best uncle in the world

I attached some pictures to make you understand which component I'm talking about.

Thank you guys




The T420 MB damaged area location


The damaged component with the screw hole


The same with a little legend

[RealBlackStuff]

Welcome to the Forum.
The missing component is a Capacitor 22PF_50V
Size is 0402 (Imperial) and 1005 (Metric).

The "one that still works" should be (preferably) straightened out and (definitely) re-soldered.

Cincin!

[epik]

Thank you RealBlackStuff,
Can I ask you what your source is? Because in the meanwhile I found this hi-res picture of a T420 MB and the component seems to be a resistor, not a capacitor.
The image is very heavy so I only put the link here: https://thinkwiki.de/images/8/86/Mainbo ... exykuz.jpg

[RealBlackStuff]

I got the T420 schematics and thought the 'displayed' C647 was your missing item.
On second thought, that C647 is more likely the capacitor left of the letter C.
From that German picture, it looks indeed like there was a resistor on that spot.
Unfortunately without any indications printed on the circuit board, its value is anybody's guess.
(Unless someone has a boardview or teboview of that mobo).
All I can give you here is the resistor type: [unknown Ohm value]_1%_1005_1/16 w

[MikalE]

Wasn't there a post made within the last year or so that was offering schematics of most recent Thinkpads?

[RealBlackStuff]

The schematics are just that: schematics. They don't show you where an item is positioned on the mobo.
Without a boardview and/or an intimate knowledge of the circuit-layout, it's pure trial-and-error guesswork.

[epik]

Unless someone has got an old, even broken, T420 motherboard.
In that case he can measure the component in five secs since a resistor can be quite precisely measured even without desoldering it. The ohmmeter measures the less resistant route and quite always the less resistant route is that one passing through the resistor giving the right measure. Is this correct?
At least I'll have more chances than blindly guessing.

[MikalE]

Are you certain that spec of a resistor isn't still rattling around in the case or stuck under another component?

Are some components identified on the other side of the board?

I have never worked on equipment that didn't have the component number on the CB to compare to a list of components with values.

[badVista]

I guess the missing resistor is R142. The component numbers are printed left of the circuits.
To be sure, maybe measure some of the other components in that area, and compare ratings with schematics.

https://lnv.i.lithium.com/t5/image/serv ... 1E12617F12

[epik]

Hi MikalE,
Thanks to your cue, another user on the other forum find the resistor spec.
It was under the protective layer and I haven't seen by myself.
According to the layout, the resistor is R142 which is a 274K_1%_1005_1/16 w.
Thank you for your help guys. Really appreaciated.

[epik]

I guess the missing resistor is R142.

Simultaneous posts
Thanks to you too!

[RichardG867]

I managed to make the exact same mistake a while ago. Same screw, same resistor, same symptoms. I attempted to just drag the resistor back into place, but that didn't work, and it was eventually lost. Here's a very large picture of the damage, taken before I lost the resistor: https://i.redd.it/iulx3y5n1fo31.jpg

I ended up purchasing a parts machine with a good board, and storing the broken one until I can find someone who can actually fix it (took it to a shop and they just soldered on a random resistor taken from a random board which obviously didn't work) or get some microsoldering equipment.

According to the schematics, this resistor is part of the circuit that generates VCCGFXCORE_I which is the integrated GPU voltage rail, not to be confused with VCCGFXCORE_D which is for the Nvidia GPU. My board is an Intel just like OP's.

[epik]

I managed to make the exact same mistake a while ago. Same screw, same resistor, same symptoms.

Well this means only a thing: that screw hole must be limited in some way. It's a flaw in the circuit internal design and it's very strange because the Thinkpad notebook are usually perfectly designed and assembled... Lenovo, nock if you are there...

Anyway I have bad news for you.
As I reported in the other forum where everything has started, I successfully soldered the two resistors but the PC doesn't start the same.
I fear the screw short-circuits something and damages some other components. It becomes really difficult to understand what, so I gave up.

Well I go a bit OT now.

If you want to try the microsoldering, it's very easy and cheap to start. I'm not speaking about professional tools that could be very expensive but a cheap USB microscope and a solder station with hot air and soldering iron cost no more than 50€/55$. Then you need a whole set of micro tools such as flux, tin in paste and wire, tweezers, brushes, cleaning liquid, syringes, cutter, copper braid... but you can calculate no more than 20€/22$.
It's a funny and cheap hobby and, usually, it gives you great satisfactions.
And it's almost Christmas time so it can be a great present to ask to Santa

[RealBlackStuff]

At this stage I would look for another motherboard.

[epik]

At this stage I would look for another motherboard.

Well since it's an old PC and I was no longer using it (BTW my current one is a T450) I probably leave as it is and use it as a HW teaching platform for my nephew and a replacement parts source.

[thinkpadcollection]

Guys,

You all are not only one, but worse especially phones.

Especially iphones. Iphones has limited PCB real estate and uses pieces of flat thin metal held in place for to keep flexible PCB (tracks for electrical connections) tapes with connectors in place for reliability by very tiny screws. Well, there is one screws or more that is longer than other few with same thread diameter. Problem is the soldered screw stand offs is threaded through completely and there's no open hole for butt end of screw to extend through. Results in damaged PCB with buried incredibly fine traces are broken.

Repair is careful scraping till broken traces come to view and are jumpered. Expensive job and requires expensive tools, small enough and done under microscope. I have current job repairing cellphones including iphones.

PS: The other poster is correct, this is not a capacitor since traces is not connected to ground. Comparing the other boards (another one) and measuring and replace with correct components from any parts board.

https://www.youtube.com/watch?v=H-JEhsqtdAc

Cheers, thinkpadcollection

[RealBlackStuff]

You all are not only one, but worse especially phones.

Explain this poor English, please.

One has to admire that iPhone guy's tenacity, but I can't see myself (or many others) try and do the same, even if I do have a (200x) USB microscope.
I used that initially to do my SATA-mods, but they were literally superficial (= occurring on the surface) and never damaged any PCB-layers.
Like I said before, at this stage (i.e. with such damage as above) I'd look for another motherboard.

[epik]

Wow what a really interesting video for a sucking designed circuit.

I think that the screw with no limiter in the T420 represents only a little bug in an otherwise perfectly engineered and repairable circuit.

But I also think that the other manufacturers create their products with the clear intention of making them not repairable.
They want us to always buy another one, even if the damage is fixable, and it's bad.
And I'm not only speaking about PCs and smartphones but also about TV sets, electronic watches, domestic appliances...
I'm asking if it is possible that I can't even open my coffee pod machine because they secure it with round-head bolts...
BTW this is the key to open them

and BTW I opened them without the f******** key and replaced the little o-ring causing the water spill for the astonishing amount of 2€. 2€ to have again a perfectly working machine.

The reasons why I'm on the Thinkpad world are the repairability, the availability of the hardware maintenance manuals for every machine, and the modularity of their PCs.
I'm here and for sure I won't leave it.