hmmm is getting interesting!! lets go back to sqare one.
we now this is the switch that has had a mosfet replacement to make the joycons work isnt it? - can you check for shorts around that? (not likely but could be some power derivation.. im pretty much shooting to the sky with this one tho!)
if I had a console that does not display on handheld but it works on doc and touchscreen is responsive I would do the following:
get the board out and plug it into a known good assembly (with known good battery, lcd, ribbon cables etc...) and test if works there.. - if does, then take an LCD screen, plug the mobo in its own assembly and just use a separate LCD to see if isolating that may be the issue.
if that doesn't work then we know the mobo is bad.. so I would start checking for voltages on all of the known pads. I have attached the document I have with a big recollection of posts and pictures of the mobo including voltages and test pads description. I would see if all matches and try to isolate the problem to one "section" of the mobo.
I know this is going to sound repetitive.. but PI3USB and M92 chips have a big role to play in power and video delivery so I would try swap them and keep them anyway in case they were good.. power & voltage levels is controlled by M92T36 & PI3USB multiplexes the signals (ie switches the lines to do different things and I think one of those things may be switching voltage to the LCD connector)
I was having a look at one of the donor boards and I notice a bunch of what appears to be Panasonic EXC24CS inductors or equivalent (like the ones behind PI3USB) and I think these may play a similar role than the other ones behind PI3USB although those are used for the dock image.. these here are use maybe for the LCD image?.. could be that one of them is crossed or grounded?
other than this.. Im starting to run out of ideas! LOL
maybe is time to call the big boys !!!
@mattytrog are you around? can we summon you to enlighten us?
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OMG Im going to double post!! -- but need to add this... I just noticed some diodes behind the LCD connector.. check that they are working well and not become a resistor or a wire!!