Was there any obvious corrosion around one of the IC's that would lead you to believe that one or other was more likely to be bad? Was there a lot of corrosion near the LCD/Digitizer FPC connectors? Sometimes the whereabouts of the corrosion can give you clues as to what part of the board to focus on first.
Otherwise, I would start at the "end" of the circuit and work my way back before swapping out IC's. Remember that changing IC's requires massive amounts of heat; there's always a risk of secondary damage.
- Check the Digitizer and LCD FPC connectors on both the logic board and the screen. Some times there is a pin or two that get so corroded they do not make electrical contact anymore.
- The check the filters and capacitors on LCM_TO_AP_HIFA_BSYNC_CONN, TOUCH_TO_SAGE_VCM_IN_CONN, PP_SAGE_LX and PN_SAGE_TO_TOUCH_VCPL_FILT.
- Then check the voltage rails supplying the Touch circuitry PP5V1_GRAPE_VDDH, PP1V8_GRAPE, PN5V7_SAGE_AVDDN, PP5V7_SAGE_AVDDH and PP1V8
Once you have determined that there is no problems with the peripheral circuitry, I would start with Meson (Black) first, see if Touch returns and then try Cumulus (white).
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