Some update: I made some test with aluminum foil but the result was similar to paper - not enough suction.
I also washed the platen as well as the high voltage PCB with a PCB cleaner and Isopropanol to make sure no dirt conducts my valuable electricity away.
Mine is the "C" versions of this plotter (9872/7720) which has a different schematics that the "A" model shown by Rik.
I am not sure what I should expect at points (A), (B), (C).
I could not find a replacement type for U102, the HP number does not appear in my X-Ref lists.
Here is the relevant part (with my blue scribbling on top, check marks indicate that I desoldered one side of the component to test it):