MacBookPro 15" 2015 - Screen off / GMUX Issue ?

[wnt2fx]

Hello everybody, i could have some help.

I will note down the logic board number later if needed. Am away of the scope now and forgot it. It was the one schematics were never released for and so to substitute by a similar model.

The machine did fall a little less than half a meter. Plain, not on the edge. It looses lcd out on finishing the boot.

2 operating system tested. El Capitan and Catalina. To me it seems to be a gpu switching issue and I have no idea how to go on diagnosing.

On both OS it is avoidable by having an external display connected on boot. All normal. Output to LCD fine as well. No graphic glitches. No crashes on low and heavy usage.

Without external display the beaviour on finishing the boot is

Catalina:

Loosing video ( 3 seconds some stripes on the right and then black), backlight remains. Lid close / open or long press press or double tap power button or connecting external display brings back video out. Switching from integrated to dedicated and back to integrated is clearly visible. No more anomalies. No graphic glitches. No crashes on low and heavy usage.

El Capitan:

Loosing video. Loosing backlight. High fan spin. Plugging in external no effect / too late.......reboot...CPU panic report.

If I booted with external display and then disconnect -> LCD off, high fans, soon reboot. If I open e.g. cinebench and then disconnect the display all good. If I close cinebench -> you did guess it :-)

So the switch from dedicated to integrated seems to be the culprit here. I have no other display but to me it seems unlikely to be the reason !?

I thought about the famous CPU MOSFETS. Can we rule them out already ?

Help appreciated ! Do not want to replace all GMUX related chips on guessing.

best regards

[piernov]

Re: MacBookPro 15" 2015 - Screen off / GMUX Issue ?

820-00426 I guess.
There's an issue related to the iGPU for sure… Could be muxing circuit, power delivery, broken pads under CPU or PCH, etc. No real way to narrow it down that I know of, it's probably gonna be a waste of time like all boards after drop damage.

[wnt2fx]

Re: MacBookPro 15" 2015 - Screen off / GMUX Issue ?

yeah, I think so too. 00426 or a alike. Do not want to give up on that one yet. I believe it is something in the muxing circuitry. But can we really rule out the lcd ? It would have further unexpected behaviour is what I was thinking.

The iGPU itself works as perfect as the dedicated, straight integrated or dedicated boot, avoiding switching, showed that) Can we rule out the CPU FDMs ? I thought it would crash in catalina as well, at least in long term test on low usage / low voltage transitions. But all is stable. If I just would know the gmux circuitry better. If a slight boot graphic glitch ( lines on the side ) is present it is supposed to be one of the two main gmux ICs, right ? I will dig in the schematics to understand the gmux wiring better and probably reflow involved ICs. Maybe heating first. It feels like cracked solder pretty much. In my thinking cracked balls under CPU and PCH would lead to further anomalies, but there aren't.......ahhhh I forgot something. Now, just once, I've got an problem report with an efidump output. Did not yet dig into it. Anybody ever heard that components ( maybe their bonds) themselves get damaged by a device drop ? Or only solder is cracking ? Can EFi or SMC be hit ? I wouldn't expect to see such a well running system. Remember in catalina the only noticeable problem is on boot ! 3 seconds lines, video off, backlight on, no crash (also not if I wait 5 minutes) and then recatching the lcd by a brief lid close-open or power button. No crashes, no further anomalies, perfect sleep and wake up. So weird. If you / If the people think a short video and/or the el capitan panic report are useful I'll post that. wonderful sunday !