Sony PCG-21313M Backlight Short

[KvnTM]

Hey Guys.

While working on the above mentioned netbook i accidently shorted the Backlight power rail which burned one inductor off the board. It was still running after shorting it but the backlight was obviously dead. After rebooting it was completely dead. The CPU gets warm to the touch but not near as hot while normal operation. Maybe there is a short on a power rail? According to the schematic there is no feedback from the Backlight power rail to any controller and it is powered directly from the 5V rail via some mosfets. It feels like it has burned the first layer of the pcb but didnt go through to the next layer which seems to be a ground layer anyways.

I had no time to measure anything yet but i have the same inductor as a replacement. I want your opinion if it is worth a try. I dont need backlight control on this machine since it works as an interactive pictureframe on a wall. So setting it to a fixed brightness wouldnt bother me.

It was running fine after shorting it out (without BL) so i guess there shouldnt be that much damage?..

I attached a photo of the burnt area. The blown inductor is L14 on the schematic.

We do mistakes.. am i right?.. *sigh*

[KvnTM]

Re: Sony PCG-21313M Backlight Short

After reworking the damaged area it is still dead.

+LCDVCC = 0V
+LED_VCC = 0V

+5VALW = 5V
+1_8VSUS = 1.8V
+3VALW = 3.3V

VHCORE = 0.18V Seems to be NOT Ok.

I attached the Schematic. I hope somebody can help me...

[piernov]

Re: Sony PCG-21313M Backlight Short

Indeed, make sure you probe correctly VHCORE (on PL8 for example), this is the CPU VCore voltage, it should be between 0.8V and 1.2V approximatly. If it's not present, check voltage on IMVP_VR_ON.
If IMVP_VR_ON is present, check resistance to ground on VHCORE (on PL8 for example). It will be low, that's not necessarily a short to ground.

[KvnTM]

Re: Sony PCG-21313M Backlight Short

Indeed, make sure you probe correctly VHCORE (on PL8 for example), this is the CPU VCore voltage, it should be between 0.8V and 1.2V approximatly. If it's not present, check voltage on IMVP_VR_ON.
If IMVP_VR_ON is present, check resistance to ground on VHCORE (on PL8 for example). It will be low, that's not necessarily a short to ground.

IMVP_VR_ON = 3.2V
Resistance on both pads of PL8 to ground = 27 Ohm.

It Looks like a shorted cap somewhere to me.
Maybe i should try Louis's method of inducing the expected Voltage and looking at the current draw and components that get hot except for the cpu itself?

Before that i'll check the Voltage on PL8 with a scope to see if there are any Spikes. If so i'm pretty sure theres a short somewhere.

[KvnTM]

Re: Sony PCG-21313M Backlight Short

Sorry for multiple posts..

I had a random thought.. does it matter if the lcd is conncted via lvds? Because im working on the bare board without anything connected except for ram ofc.

[piernov]

Re: Sony PCG-21313M Backlight Short

27 ohms to ground on CPU VCore is usually fine, the issue should be elsewhere. IMVP_VR_ON being high means that the CPU VCore regulator should turn on though.

Check voltage on +5VRUN and +3VRUN.

The board should still generate VCore even if the screen is not connected. But it may need the screen to generate graphics related voltage.

[KvnTM]

Re: Sony PCG-21313M Backlight Short

Both 5VRUN and 3VRUN are fine. VCore is still about 0.2V.

[piernov]

Re: Sony PCG-21313M Backlight Short

Then I think you're good to check the voltage on all the pins of the CPU VCore regulator PU7 then to try to figure out what's wrong.

[KvnTM]

Re: Sony PCG-21313M Backlight Short

OK I will do that. Thank you so far!