So, your problem is in the upper left hand area? Maybe it took a few hits there when it fell on the floor, more than once.
Unlike in a guitar, and more like in an amp or effect, those pots would be soldered directly to the board, and the signal going through them comes off that board, back into it, and so on. So, I would think that it is loose solders on the board.
I'll try to show a pic with similar soldering and construction. A MOOG Opus 1. Pot on upper left like yours, and trim pots.
In older units, the skirted knobs might help prevent the force, or loosening on the inside if dropped. Check your solders.