I have used several YouTube videos on how to clean the inkjet heads. Worked for the color ones. Black is still not working. This is a complete breakdown. https://youtu.be/KxJuTWGMWVU Here’s a partial cleaning, w/o disassembling: https://youtu.be/K76eANwoY64
I attempted this but didn’t work for me. Lots of precision and macro vision work. Tablet is very easy to damage the ribbon cables. Had to replace “rear camera” and “Finger Print Button”. Still had issues. Next step was to replace The Digitizer which I didn’t do. Ended up buying a new iPad9.