Short detection - what am I doing wrong?

[Moosiv]

Hey guys/gals - I'm very new to electronics repairs. I only have a few months experience to be honest but I'm managing to replace hdmi ports / hdmi retimer chips on consoles, mosfets and ic on laptops etc. However, I keep having a repeating scenario and I can't figure out, I'm missing something and I've been trying to figure it out for weeks but I've now got the same scenario again and I dont want to give up or pass the job on this time so if anyone could push me in the right direction I'd really appreciate it.

This is the scenario, laptop board not switching on, I can confirm power into dc and first 2 mosfets. I can probe power rails around the board with mixed results but today the battery charging rail works (charges the battery orange to blue led) and 5v is there. I have a row of 6 coils not to far from the gpu (acer gaming laptop) and a row of 4 below cpu. all are shorted. I can't find any short at a mosfet. I'm injecting 1v at any coil and it will take any current I offer it - currently limited to 3a.

My issue is this - I don't get any heat spots to detect the issue such as a bad cap - I'm pretty sure what I'm doing is the equivalent of pushing the voltage straight into ground. I'll upload a photo of the board I'm currently working on. Components marked red are shorted. Can't find schematic. Not that that usually helps me with this injecting voltage to find shorted component.

If someone could prod me in the right direction I'd be very grateful.

[mon2]

Re: Short detection - what am I doing wrong?

A low resistance of the cpu or gpu rails does not indicate a short circuit. These rails are typically low voltage and respectively of low resistance so they are not a fault.

Post the measured resistance in ohms to ground for the rails of concern.

[piernov]

Re: Short detection - what am I doing wrong?

https://www.badcaps.net/forum/showpo...02&postcount=2

2.2. Don't suspect that something is shorted with no reason
Shorted components are not the only failure mode of a laptop. Additionally, some places will measure low resistance to ground under normal operation. In general, high power low voltage (<2V) power rails will measure less than a few hundred ohms to ground. CPU VCore will measure a few ohms only. GPU VCore with newer NVidia GPU can even measure less than 1 ohm. It's normal. If you suspect a short to ground, always report your exact resistance to ground measurements.

2.3. Never randomly inject voltage
Voltage injection is used to find a short to ground. Only consider it after you found that there is a short to ground and after doing a visual inspection trying to find a broken component.

[Moosiv]

Re: Short detection - what am I doing wrong?

Right OK - so basically I'm barking up the wrong tree - that actually really helps - thanks guys! At least I can put that to bed and move on. I have some photos of the two rails in ohms.

[mon2]

Re: Short detection - what am I doing wrong?

Where are you measuring the 0.5 ohms to ground ? Is it for the NVIDIA GPU rail ? That may be still ok.

Also, I believe schematics are available from paid sites. The attached screen grab appears to match your PCB markings.

[Moosiv]

Re: Short detection - what am I doing wrong?

yes the nvidia rail

[reformatt]

Re: Short detection - what am I doing wrong?

If you manage to get the schematic for what you are working on, they usually have a power sequence diagram. Laptops power is one big combinational logic circuit, you need certain states to be good prior to moving to the next state.

As a heads up, if there was an issue with a CPU or GPU being short, this is usually accompanied by a shorted FET on the buck converter feeding them. In that event, you'll see a dead short on the main power rail.