Need help - to understand Motherboard Symbols

[rxyzm]

Hello,

I am in the process of fixing my laptop.

I need some help to understand some symbols that are shown in schematic. I have attached a part of schematic doc here that sows them.

The + Symbol (one that looks like a double plus - one inside the other), appears on MB. I believe that represents a particular +ve bus (since they are numbered as FM2, FM4 etc.), but I am not sure. Also, I am bit rusty on these symbols used - particularly on MB.

The O symbol(a doughnut hole) also appears in schematic. I am find with the Ground/Earth presentation. But, the one where there is a small x at the end of line is a bit cryptic for me.

The same small x also appears on the + symbols. I am not sure whether it means that they are not connected to anything (float by itself?).

Will certainly help any guidance.

Warm Regards,

[khaahk]

Re: Need help - to understand Motherboard Symbols

This page on the schematic has no useful information to you if you plan to fix the laptop. Good starting point is here:
https://www.badcaps.net/forum/showthread.php?t=88758

[rxyzm]

Re: Need help - to understand Motherboard Symbols

Thank you KhaahK.

I have already gone through the forum that you have suggested.

What I want to know about + (hollow + symbol) is, is it used to indicate different + rails? There are identifying labels such as FM2 etc. Do these labels have any specific meaning or they change with the company?

Warm Regards,

[khaahk]

Re: Need help - to understand Motherboard Symbols

What is the platform name? The FMxx symbol should be represented on the schematic elsewhere if it had some importance in electrical meaning. You have the mainboard at hand, also i hope mutlimeter - so take measurements: are the + signs interconnected and what are they electrically connected to.
My opinion is that they have no importance to you, maybe internally to mainboard manufacturer or some meaning in emc domain.

[rxyzm]

Re: Need help - to understand Motherboard Symbols

What is the platform name? The FMxx symbol should be represented on the schematic elsewhere if it had some importance in electrical meaning. You have the mainboard at hand, also i hope mutlimeter - so take measurements: are the + signs interconnected and what are they electrically connected to.
My opinion is that they have no importance to you, maybe internally to mainboard manufacturer or some meaning in emc domain.

Hello Khaahk,

This is MSI MB - MSI-16P51.

On the MB, I do see the hollow + with a label such as FM2. But the exposed copper in the hollow + does not show continuity with other hollow + as much I have checked.

It is possible that the hollow + is covered with a protective material after or during board etch process.

There seems to be a short on 1.05 V bus near GPU and I am trying to find that. But I do not want to connect it to power (battery or Power pack) until I am reasonably sure that I have a location for short. Now for this, I need to know the supply rails (and there are many) including the 18V that is used to provide large current loads through local power management parts for GPU, CPU, Memory etc.

Warm Regards,

[piernov]

Re: Need help - to understand Motherboard Symbols

FM* are fiducial marks used during production for the calibration of the assembly line.

[rxyzm]

Re: Need help - to understand Motherboard Symbols

Hello Piernov,

Thank you for providing that insight. That could be another case.

I also went through your sticky - Beginner's Guidelines.

One point therein caught my eye as that's the condition that I am trying to understand.

It is mentioned there -
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.".

On my MSI MB - the GPU is nVidia N17E (in schematics it is mentioned that the chip is GTX 1070). I find 0 (Zero Ohm) resistance between the +ve supply plain and the -ve supply plane post the mosfets (87350D) that feed in to this rail. There are three driver chips (uP1909) and 6 mosfets.

I did the resistance check on the mosfet for highside and they seem to be fine. There is ample resistance between the gate and drain/source.

On the low side mosfet (for each mosfet) the source and drain show 0 (Zero) ohm resistance. But, this is expected because the source of lowside mosfet is connected to the output rail directly. The gate and source/drain show resistance depending on the probe polarity.

So, for me the resistance check is inconclusive in this case (can be right or wrong) if I consider point 2.2 above.

Also, in continuity mode, the probes may output more than 1 volt and that may impact the resistance reading too.

Problem I am trying to fix is - when I press power button, the power and caps lock leds blink alternatively. The exhaust fans do not switch on. No 1 V supply on nVidia power rail. And I think because there could be a power issue, the CPU supply may also not be switched on.

The issue occurred few times in the past, but the laptop would start when the power button was hit the next time.

Reason I have not connected the power supply to MB is, the 18V supply runs to all mosfets that provide large currents to GPU, CPU etc. Taking chances with key chips is something I would like to avoid.

Will appreciate any guiding pointers towards a plausible cause for this issue.

[rxyzm]

Re: Need help - to understand Motherboard Symbols

Well, After a long break (COVID and other interrupts), back on this topic.

I fixed the plastic body of the laptop. Fixing the Plastic Body was another experience.

On the MB side, I reassembled the laptop and started testing it module by module.

When first connected to power, it showed the original problem where the power light and caps lock LEDs blinked. But after 3 - 4 tries of hitting power button, suddenly, the laptop responded by turning on and then started working.

Now, for the very first time when connected to power source, it does not boot on the very first press of power button. The power led comes on and then goes off. After about 3 - 4 tries, the machine responds and then boots on charger as well as battery on the press of power button. But what causes this behavior? Now that's the question.

Well, something to work further with...