The kapton tape arrived and I got out the hot air gun again. The sheild still seemed reluctant to budge, but when I managed to lift a corner it became evident that there was just a frame soldered to the board, and the shield was simply a lid clipped onto the frame.
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So I was able to try reflowing those two chips, but with no effect. But really, I think those chips are only performing power management not video processing. They’re next to the battery connector, and if they were involved in video processing the signals from the CPU would have to double back on themselves to get to the LCD connector. Besides, those chips seem too small to be doing anything complex.
I’m not saying necessarily that that’s not where the fault lies. Your Level 3 Repair doc gives a few voltages that I tried to measure. Not easy as some of the components are barely larger than a grain of salt, but the voltages didn’t look right. The backlight supply might well be derived there but that doesn’t seem to be the fault. Maybe a secial voltage is required for LVDS LCD drivers, but I’m not sure tablets use LVDS. One of the chips I identified as SM5703 - a battery charger chip, apparently prone to failure, but the battery seems to charge. The other ship only bears the marking HYGKA which doesn’t mean anything to Google.
I also tried reflowing what I took to be the processor, but with no effect. I think the time has come to offer it on eBay for spares or repair.