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Old 12-08-2017, 09:00 AM   #1
m5b4
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Default Toshiba L50-B-245 GPU power issue

Hi,

I have a nonworking laptop Toshiba L50-B-245 with motherboard DABLIDMB8E0 REV E. When I plug in a charger, I get a charging light. When I press the power button, the power light blinks once and I also get a short fan spin. Sometimes both lights blink. I tried to search for short on the board, but I couldn't find any.

After many tries I managed to turn the laptop on, but I didn't get anything on screen. So I did some measurements and I found out that the GPU has no voltage. The GPU VRM is controlled by ISL62771 chip. This chip has an enable pin, which should be high to start the VRM. But there was 0V on that pin. So I turned the laptop off and it didn't want to start again.

When I press the power button I see on my multimeter that the voltage on the enable pin goes to 0.5V and then back to zero. So I guess that the chip gets 5V for a very short moment, but my multimeter isn't quick enough to sense that. I tried to supply 5V on that pin and I got 0.8V on the GPU. So I guess that the GPU VRM is OK, it just doesn't get enabled.

So now I'm stuck and I don't know how to continue the troubleshooting. My guess is that the chip generating this enable signal is faulty, but I don't know which chip it is. I tried to search for schematics but I couldn't find it. Does this signal come from the startup chip (or whatever it's called, on this board it's Nuvoton NPCE985LA0DX)? I tried measuring, but I couldn't find any pin on the startup chip directly wired to the enable pin.

I'm just a beginner, so I might be missing something obvious. But I'd like to learn. I tried to search this forum and other sites for similar issues, but I couldn't find any. Could someone please help me or just point me to a right direction? If you need more information, just ask and I will gladly provide it
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Old 12-08-2017, 09:41 AM   #2
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Default Re: Toshiba L50-B-245 GPU power issue

measure resistance GPU main power to ground
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Old 12-08-2017, 12:20 PM   #3
m5b4
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Default Re: Toshiba L50-B-245 GPU power issue

It's 33.5 ohms.
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Old 12-10-2017, 08:06 AM   #4
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Default Re: Toshiba L50-B-245 GPU power issue

I've been playing with it more and here are the results. The previous tests have been done without battery. Now I tried to put the battery back in and found out that it's much easier to turn the laptop on, probably like once in 10 tries. But still no picture on the laptop screen and also on the hdmi port. With the battery plugged in I also sometimes get five flashes of the power button led.

I was also thinking about the zero voltage issue. Could it be a normal behavior? The laptop has an integrated GPU in CPU (Core i5 5200U), so I thought that maybe the dedicated GPU is disabled during boot and I might be searching in a wrong part of the board. Could that be correct?
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Old 12-10-2017, 10:57 AM   #5
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Default Re: Toshiba L50-B-245 GPU power issue

yes, it is probable. so check what other voltage rails and signals are missing.
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Old 12-10-2017, 11:33 AM   #6
m5b4
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Default Re: Toshiba L50-B-245 GPU power issue

Thank you. I've checked other rails and I haven't found any issue. I measured on coils and when the laptop is powered off I get 3.3V and 5V. When I turn it on, I also get 1.35V (RAM), 1.05V (CPU) and 1.7V (I don't know what that's for). There is also one coil for battery which has 16.5V when battery is plugged and 2V otherwise. The only coil which doesn't have voltage is the one for GPU. So don't think any power rail is missing, except for GPU.

Last edited by m5b4; 12-10-2017 at 11:38 AM..
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