LA-6901P no display, cpu_core 0.5v and vgfx_core 0v

[teknosrp]

Hello

I was trying to fix a laptop with LA-6901P board (schematic available in many sites). It was totally dead and found a problem with the battery charging circuitry. Bypassed it and now it powers on from the ac adapter, power led stays lit, fan spins a few seconds but shows no image. It does the same powering it from a already charged battery

I started checking voltage rails and found all correct except cpu_core, which was at about 0.480v, too low for me, and vgfx_core, which remained at 0v. Removing the ram makes no difference, and removing the cpu makes vcore rise up to about 4v and board powers off itself after some seconds. Is this 0.480v vcore value normal before fully booting? What can be the problem?

If someone wants to know, problem with battery charging circuitry was a shorted low-side mosfet at the charging buck regulator and an open resistor at the charging IC (ISL6251) 5v regulator input. IC was shorted internally and I had not replacement, so I removed the charging IC, fed 5v externally to pin 1 and tied to ground the ac adapter plugged in signal (ACPRN) in order to bypass it.

Thanks to anyone reading this and willing to help.

[mcplslg123]

Re: LA-6901P no display, cpu_core 0.5v and vgfx_core 0v

0.48V as Vcore is too low. However, i think the reason could be charging IC bypass. You should get the requisite parts and replace them before diagnosing further. Charging ic is connected to SIO-so it may misbehave with some inputs/feedback missing.

[teknosrp]

Re: LA-6901P no display, cpu_core 0.5v and vgfx_core 0v

0.48V as Vcore is too low. However, i think the reason could be charging IC bypass. You should get the requisite parts and replace them before diagnosing further. Charging ic is connected to SIO-so it may misbehave with some inputs/feedback missing.

Replacing the IC surely is the best option, but as it takes too long to arrive from china and this IC seems to have few feedback pins, I decided to try first without it.

The IC is an ISL6251AHAZ. By reading the datasheet and looking at the board schematic I interpret that it is active only during ac adapter operation, and running the laptop from a already charged battery does not change the laptop behavior. So I doubt this is the problem.

[mcplslg123]

Re: LA-6901P no display, cpu_core 0.5v and vgfx_core 0v

Charging Ic communicates with SIO thru SMBUS.

[teknosrp]

Re: LA-6901P no display, cpu_core 0.5v and vgfx_core 0v

Hello

Thinking about the problem I have another hypothesis. The CPU VRM has two phases, and the GFX core one. Measuring the resistance between the high side mosfet gate of each phase to ground I obtain these values:
- CPU phase 1: 40 ohm
- CPU phase 2: 1.1k
- GFX unique phase: 1.1K

Maybe phase 1 is not working properly and hence I have an vcore of 0.5v, just a half of what it should be? But in this case, the feedback circuitry should not detect the voltage mismatch and reach the correct vcore voltage by using phase 2? Also measuring these resistance values just after turning off the board give different results that measuring them after leaving it to cool down for some time. The values indicated before correspond to this last situation.

Charging Ic communicates with SIO thru SMBUS.

This charging IC does not have SMBUS capabilities. It is a pretty isolated subsystem with few and simple control signals, this is why I decided trying bypassing it.

[teknosrp]

Re: LA-6901P no display, cpu_core 0.5v and vgfx_core 0v

Hello

Thinking about the problem I have another hypothesis. The CPU VRM has two phases, and the GFX core one. Measuring the resistance between the high side mosfet gate of each phase to ground I obtain these values:
- CPU phase 1: 40 ohm
- CPU phase 2: 1.1k
- GFX unique phase: 1.1K

Maybe phase 1 is not working properly and hence I have an vcore of 0.5v, just a half of what it should be? But in this case, the feedback circuitry should not detect the voltage mismatch and reach the correct vcore voltage by using phase 2? Also measuring these resistance values just after turning off the board give different results that measuring them after leaving it to cool down for some time. The values indicated before correspond to this last situation.

Today I have tried removing the resistor (PR251) between the driver chip (ISL95831, PU12) and phase 1 high side mosfet (PQ61). After doing this, I still have a 40 ohm resistance between driver output to ground, but a high resistance between PQ61 gate and ground. This means that the ISL driver chip is gone?