820-2915-B , chimes but no signal & no backlight

[premtec]

I have two same 820-2915-B [with 216-0809000] that have same problem, they chime and even start OS [I can hear volume up/down with 10.9] but they give no signal at display and no backlight.
I have disconnected LVDS and tried with display port but without luck.
The first one had lines at display when brought in and I replaced gpu - after that the problem occured. So I reballed the vrams and reflowed the gmux also with no luck.
Next I placed back the old gpu but still nothing happened. The used new gpu had been installed at another one and worked ok [I have a good stock of them] so someting else is happening here.
The second 820-2915-B was received with the exact same issue and neither me or anyone else has meshed with it yet.
About some measurements,

PPVCORE_GPU 1,10 V
LCD_PWR_EN 0V
PPVCORE_S0_AXG 0,45 V

I have to mention that I had before one same case with 216-0810005 gpu but I had to return it as I thought that it wasn't common issue and didn't worth to spend time.

 

[dukefawks]

The first board was probably a badly soldered GPU, but since you messed with the GMUX that will now probably need reballing/replacement.

The second one has not been messed with yet?

 

[premtec]

I do a lot of bga rework so it would hardly be bad soldering.
Yes, for the second one I haven't spent any time by replacing the chip of anything else as I'm sure there's something else happening here.
The only thing I got some measurements that match with the first board.
Any hints where to start?

 

[dukefawks]

First see if the second board works on external display. I assume you have heated up the GPU to make sure it is not just a simple bad chip.

 

[premtec]

I had it tested with external display but I hadn't heated it.I did some heating and display came alive! Now I'm replacing the chip.
I didn't have any similar issue with no display because of gpu at this model so it wasn't my practice to heat them.
Any hope for the first board also or not?

 

[premtec]

Second worked ok.
Leave the first one, too much time waisted here, I'm returning it.