A1398 820-3332-A no GPUVCORE

[marigx]

This machine was going to a blank/dead screen while in use, more and more often. Eventually it got to the point where it would chime on re-boot, but the lcd would show no image.

Tried re-soldering/replacing U8900, then when that didn't help, realized I needed to stop guessing and start analyzing.

First thing I noticed was that I knocked off several components around U8900 (I know, rookie mistake - didn't have the air turned down enough) After I replaced those, I checked for the GPU power rails from sheet 88 of the schematic and measured the following:

P3V3GPU - 1.3 millivolts
P1V8GPU - 1.805 volts
GPUVCORE - 0.0 millivolts
P1V35GPUFB - 0.0 millivolts
P1V05_S0GPU - 0.0 millivolts

I could use a clue to understand how to troubleshoot the different power rails.

With the board out of the chassis, connecting the power adapter, the light comes on, fans spin up and over the course of 2 minutes or so, gradually go from low speed to high speed, and pretty loud. Not sure if that is normal.

 

[marigx]

Ok, after I thought about a while, that P1V8GPU measurement I took must have been wrong. I measured at pin 1 of R1600, which I see now is associated with the CPU power rail, not for GPU.

I did go and check the following enable signals for GPU at the resistors around U9100 - and the signals are all not there. Wouldn't these be present when the bare board is powered up? Or are they just momentary after the board first starts?

P3V3GPU_EN at R9121 - 1.0 millivolts
P1V8GPU_EN at R9122 - 1.0 millivolts
GPUVCORE_EN at R9123 - 1.0 millivolts
P1V35GPUFB_EN at R9124 - 1.0 millivolts
P1V05_S0GPU_EN at R9125 - 1.1 millivolts

I also confirmed USB activity on a bootable flash drive.

 

[2informaticos]

First of all, welcome to the forum!

Be sure U8900 is properly soldered and check all traces and surrounding components.
Post voltage for pins 16, 17, 25, 38.

 

[marigx]

Appreciate the welcome! Thanks!

I have again checked U8900 and surrounding components. I have made diode-mode measurements for each pin, and compared to a known good board. I previously found a bad R8900 this way, and replaced it. I have a few other maybe-bad/maybe-not-bad readings I'm not sure what to make of, if it would help.

pin 16 - 5.00
pin 17 - 12.53
pin 25 - 5.00
pin 38 - 0.001 (my MM reads 1.0 millivolts)

 

[2informaticos]

GPU is probably bad and not worth to solve this problem.
Is better to disable GPU, not too complicated on this board; hardware mod already posted on the forum.

 

[marigx]

Ahh, thanks - that's a shame. I found the hardware mod you referenced, so I may try that.