Apparently the newest Kensington Expert “Mouse” trackballs have a hack re-orienting the scroll ring quadrature detector. The picture from my original writeup shows the previous situation:
The quadrature detector, the black block on the left, is oriented with its lens (and, thus, the actual detectors) pointed away from the IR emitter. I thought it might be an assembly screwup, but it’s actually worse: the PCB layout is wrong.
A note from Tristan in NZ explains the situation:
So I have a later model than yours. It has a 2nd PCB chunk between where the legs normally would be. Just a floating piece with two holes for the legs, holding the legs from the board […] to the main board.It is also pointing the correct way (with the lens towards the three leg emitter).
The new quad detector has only three pins and no convex lens, but the active area now faces the emitter across the gap.
Because the interposer PCB occupies the space previously devoted to the emitter & detector leads, Kensington apparently soldered the new parts directly to the top surface without any clearance:
It’s like they failed to put through-vias to the rear or didn’t route them to the bottom another way, hence the solder is under the component
Tristan managed to wreck the detector while attempting to re-solder the intermittent joints, a situation I’m painfully familiar with. He replaced it with a quad detector harvested from a mid-90s optical mouse and it’s back in operation.
So I think the correct “fix” for the old-style PCBs (without the new interposer) is to unsolder the detector, rotate it so the lens faces the emitter, then somehow rewire the pins to the original pads. This won’t be easy and definitely won’t be pretty, but as long as it’s pointed in the right general direction it should work:
mine works off axis quite a bit
Should either of my Expert Mouse trackballs fail, now I know what to do
Many thanks to Tristan for reporting his findings!
Update: A note from Alan brings more data to the discussion
The Smell of Molten Projects in the Morning 
Maybe I’m missing something… if the lens is pointed the wrong way, how did it ever work at all and escape the factory?
Nobody has a good answer!
Apparently, even the reversed detector worked well enough, at first, to allow shipping the PCB with the wrong layout. Then it becomes a tech support problem.
Afterward, as the LEDs age, the trackball gets knocked around, and fuzz collects in the apertures, the low-margin detectors stop working and tech support goes nuts.
Uncountable tens of thousands of tech support hassles later, they issue a “patch PCB” to continue using the unmodified base PCBs.
I’m astonished they still sell the same trackball more than a decade later: it must be popular!
Interesting. Thanks for the info. I’ve been using the Expert Mouse since they were beige. Unfortunately, the build quality has gone down quite a bit since they were acquired by ACCO Brands. The little plastic BBs in the well of the trackball used to be actual steel ball bearings and inner part of the ring that passes between the optical sensors you mention in this post used to be metal.
Unfortunately, no one else makes a trackball with this large of a ball anymore. That’s why they’re so popular still after 35 years in the business.
There seems no urge stronger than that of cost-reducing a good product, ending with a piece of crap bearing the same name & SKU. Verily, they don’t make ’em like they used to.
I have a spare on the shelf, but this one keeps on keepin’ on. My wrist can’t live without it!