Lenovo NM-C121 board,Help remove shorting from burnt pad

[IamPritesh]

Hello friends,

I keep coming back for help and this community has never disappointed me. So I have this Lenovo NM-C121 board which has no display on screen. Works fine on external display though. Found one of the fuse (F3301) is burnt that supplies the power to the screen. (20 V)

After cleaning the area I can the damage already done. The area between the pads is burnt. What bothers me that it still shows short on the edp connector side even though there is not a single component on this side ( LVDS cable is not connected )

How can remove the short to ground when there is no component? The fuse directly connects to the edp connector. Any solutions

PS : Pics and schematic attached

[mon2]

Re: Lenovo NM-C121 board,Help remove shorting from burnt pad

Use IPA (alcohol) and q-tip (cotton swab) to remove the brown flux. Then take a clear picture and post it of the same area again.

[IamPritesh]

Re: Lenovo NM-C121 board,Help remove shorting from burnt pad

Use IPA (alcohol) and q-tip (cotton swab) to remove the brown flux. Then take a clear picture and post it of the same area again.

Thanks for the reply . Thats not brown flux but layer underneath. The top blue layer is gone.

[IamPritesh]

Re: Lenovo NM-C121 board,Help remove shorting from burnt pad

Is it internally short and no fix for this ?

[mon2]

Re: Lenovo NM-C121 board,Help remove shorting from burnt pad

What pins exactly on the EDP connector are showing as a short ?

How are you measuring this short condition ? Remove all power -> meter in resistance mode -> then short your meter probes to confirm the 'zero' resistance reading by your meter.

Then use on the EDP connector and post the pins so we can compare against the schematics.

[IamPritesh]

Re: Lenovo NM-C121 board,Help remove shorting from burnt pad

Meter in beep mode . Red probe to ground and black probe on edp connector PIN 1. Meter beeps and shows a reading of 002.X The fuse is already removed . Please advise

[mon2]

Re: Lenovo NM-C121 board,Help remove shorting from burnt pad

Remove all power. Meter in resistance mode.

Measure the resistance to ground of the same pins on the eDP connector. Polarity of the meter probes does not matter.

If the meter shows '1' or 'OL' then switch to the next higher meter scale.

Post the readings.

[IamPritesh]

Re: Lenovo NM-C121 board,Help remove shorting from burnt pad

Remove all power. Meter in resistance mode.

Measure the resistance to ground of the same pins on the eDP connector. Polarity of the meter probes does not matter.

If the meter shows '1' or 'OL' then switch to the next higher meter scale.

Post the readings.

Will do that as soon as I m at my bench.

[mcplslg123]

Re: Lenovo NM-C121 board,Help remove shorting from burnt pad

The edp socket is not burnt internally??

[IamPritesh]

Re: Lenovo NM-C121 board,Help remove shorting from burnt pad

Remove all power. Meter in resistance mode.

Measure the resistance to ground of the same pins on the eDP connector. Polarity of the meter probes does not matter.

If the meter shows '1' or 'OL' then switch to the next higher meter scale.

Post the readings.

Meter reading in resitance mode - 002.5

The edp socket is not burnt internally??

No damage is visible to edp socket . Please refer to the attached image.

[mon2]

Re: Lenovo NM-C121 board,Help remove shorting from burnt pad

So the resistance measurement is 2R5 = 2.5 ohms to ground?

What may be happening here is that the top pcb layer has been burnt away and the exposed layer is now layer #2 of this multilayer pcb. Often in the pcb layer stack up, the next layer (#2) will be the ground layer.

One of the fuse pcb pads may be touching the now exposed ground plane.

Continue in resistance mode on your meter and do the following:

1) short the meter probes. What is the resistance?

2) place one meter probe in the brown area of the pcb where the fuse was burnt. Other meter probe to ground on the logic board (metal shield or similar). What is the resistance?

3) test again with probe on metal shield and other on the fuse pad on the pcb. Resistance reading? Repeat with the other fuse pad. Resistance reading?

4) schematic shows 2 * capacitors on the same edp connector pins. Do you have these on your logic board? If yes, remove each capacitor and test again to see if the resistance changes.

[IamPritesh]

Re: Lenovo NM-C121 board,Help remove shorting from burnt pad

There are no capacitor on those lines. There are no installed on the board. Strange issue ..how to solve this. Can I cut that part of the board layer

[SMDFlea]

Re: Lenovo NM-C121 board,Help remove shorting from burnt pad

There are no capacitor on those lines. There are no installed on the board. Strange issue ..how to solve this. Can I cut that part of the board layer

You`ve only replied to 4) what about 1) , 2) , 3)

[IamPritesh]

Re: Lenovo NM-C121 board,Help remove shorting from burnt pad

You`ve only replied to 4) what about 1) , 2) , 3)

Sorry here they are:

1 resistance reading when both probes are connected to each other : is 0000
2 resistance of the brown area where the fuse was burnt
: 2.4K
3 resistance at other fuse pad : 6.5K

New Images attached

[IamPritesh]

Re: Lenovo NM-C121 board,Help remove shorting from burnt pad

Anyone ever encountered this kind of strange issue ?

[mcplslg123]

Re: Lenovo NM-C121 board,Help remove shorting from burnt pad

There is nothing strange in this issue. Its Ground layer exposed to pad of burnt fuse. Scrap with a sharp pointed knife unless the resistance is OL.