HP 82985A video interface for the Portable Plus - poor image quality

[Michael Fehlhammer]

A relatively rare accessory for the HP portable plus was the video interface that enabled power users to enjoy better image quality compared to the early ( non back lit) LCD. The monitors usually attached to the interface where the 82912A and 82913A, which also were the standard monitors for the HP 86 and the calculator video interfaces (HP-IL, 82163A, mountain computer ...).
Now I have got two of these interfaces and two portable plus computers that came with the necessary connector ( built into the battery door).
But when I look at the screen I can't understand why anyone should prefer THAT picture to the standard LCD...
The problem is inverse video: The DOS shell (PAM) makes excessive use of it, to mark key assignments (menu selections). The labels are hard to read, sometimes completely illegible.
Now I swapped interfaces, computers, monitors ... the result remained the same.
Next step was that a colleague replaced the usual suspects (electrolytic capacitors, f.e.), especially in a module denoted "sync mixer" which I assumed might be responsible for the mushy picture.
No success unfortunately, not even a slightly better image quality.
My colleague guesses that the image processor named "ILA -T08834" (can be found on the schematics created by Tony Duell) could be responsible and should be replaced next. But where to find such a very special IC? Any other ideas?
Does anyone have experienced the same problems with that video interface?

By the way, we found one strange difference between the schematics and the actual installed components: One capacitor in the "sync mixer" module actually is 200pF, instead of 10 pF in the schematics. Both my video interfaces (rev. A and A-4) have the 200 pF capacitor. The schematics are for the rev. A.