Question CPU high side Mosfet Gate reads only 0.5 ohm when Mosfet is removed ?

[Tim042]

Hi i have a weird reading if anyone can help me on the CPU high side mosfet but normal 5.1V on 2 other low sides (2 +2) if you can help me nail this down? 9/10 times the screen stays blank but ALL times when i turn on, lights, fan, 5, 3.3, 1.8, .8V ect ON all over the board so good and i already flashed the Bios although the ME region needs cleaning is a cause?.

Anyway i get 0.5 OHM ON THE GATE and 19V available on the drain at all times. I removed the high mosfet and the half ohm is STILL ON THE m/b PADS?? and the High mosfet has millions on g,s,d so that is not shorted. When i can get the screen to come on after many on cycles, all 4 mosfets seem normal readings and the Coil into the CPU is 1.6V from memory and is now hot and running, and i can read the bios screen and move the cursor and its stable,

BUT when i try Ubuntu on a usb, it will load to the Ubuntu menu and when i try to start temp install it, a few seconds later it resets and i lose the gate V ect THEN a few secs later the Ubuntu Menu comes back again as voltages come back. So what is causing the 0.5 OHM and reset voltage crash when i LOAD the system? From a close schematic for this MB, i understand the Gate goes straight to the VRM or with a small resistor? so how can it have 0.5 ohm unless shorted, but then why does it work sometimes normally unless the VRM or other chip is faulty? Thats my guess so far.

Any help would be much appreciated. I love learning board repair and this is a 2007 Vintage Laptop and i really want to save it and learning board repair. Already swapped CPU's and ram and using a external monitor plus when screen is not coming on, 3.3V is MISSING from the Wifi slot 22 pin so it cant be the GPU? as its not being switched on. Interesting how the 2 Low side mosfets have 5.1V on them waiting for the High sides Gates to get voltage so they can regulate to earth and send through 1.6 V to the two coils and CPU. 2 sets of high and low mosfets for this single core CPU. Thats as much as i understand so far.

 

[Ralston18]

Start by reverse engineering it.

Take the applicable schematic(s) from a working build and pretend that you want to cause or create that 0.5 ohm condition.

What would be necessary? How could it be done?

Breadboard the motherboard etc. to test and try.

 

[Tim042]

Hi, In or our of circuit, GtoS is still half a ohm and on the other high side one too which sort of rules out ONE shorted high mosfet as they not connected directly. BUT i removed it anyway and it tested ok as expected and the half ohm is still there on the M/B and may be normal for this design i dont know. I checked another diff M/B at that place and got tons of resistance so i assumed that ALL would be the same?. The low resistance may be normal and fact it does run sometimes and seem all voltages steady makes me think that half ohm is not the problem. i just asked so a experienced tech could say NO, thats NEVER normal or Yes it can be and your problem is elsewhere.

Still have to buy a thermal camera and maybe that will show a warmish chip or part that is failing 9/10 times or i could inject a volt and see if anything is warmish. Last night i finally got a reading on the coil of 1.2 V but the screen didnt come on and the mosfets were also low telling me it all came on but TOO low to start the CPU and GPU? so something is out somewhere. Could still be the CPU feed backing its failing and the chips are switching off/Low but i worked on this a year or two ago and thought i swapped in a known good CPU and no change.

So i need to find that one and confirm or that CPU isnt recognized but this bios maybe? and this CPU is actually bad. I might just buy SAME exact CPU for a few bucks and swap it in and rule it out. Could also simply need the ME region of the bios cleaned but thats a whole messy procedure of files and this and that to do.

An Indian repair channel said it will be likely either the Clock crystals not working- NO, checked that ok OR ME region in BIOS damaged and thus wont trigger all the chips ect to turn on. Fact it sometimes does but dies when loaded , not sure if its the bios chip, BUT i asking as dont know. If no other feedback that works, i will buy same CPU for $10 but i doubt as tried another CPU and no go. I hate throwing parts bin at things as that shows you dont know, and i want to learn to diagnose and NOT GUESS. Mechanics do that and charge you for it and techs too, they swap chips and not check for signals ect to narrow down real reason and thats what i want to learn. Know, not guess!