Thanks, this was very helpful!
I recently got my Dad's old 128K upgraded to a Plus, and was having the issue where the mouse wouldn't move left or right intermittently. This is the original 1984 M0100, with 2 screws, and no ICs on the circuit board. I remember when I was a kid that the clip that holds the wheel in place broke, and my Dad had glued a piece of plastic to keep it in place. That kind of worked, but I also discovered recently I had a bad IR emitter.
I figured out a way to test it without needing an oscilloscope. I connected ground and power to the mouse cable DB9 connector pins 1 and 2, then connected 2 LEDs between ground and pins 4 and 5. I removed the wheel from the mouse, and by blocking the sensor I was able to determine which side wasn't working. When I blocked the light from the emitter on the left side the test LED turns on when blocked and off when not blocked, as it should. But on the right side my test LED was always on, even if the beam is not blocked. I could get it to shut off if I shone a very bright light on it, so I assumed at first it was a bad OPL550 sensor. When I looked at the mouse with my phone camera, I was able to see that all 4 IR LEDs were working.
I ordered replacement OPL550A sensors and some OP240A IR emitters just in case. (The OPL550 was out of stock, so I ordered the OPL550A, which worked fine.) I desoldered the circuit board and replaced the OPL550 sensor, but I was still having the same issue. So I also replaced the IR LED emitter, which fixed the issue. So even though I could see the purple glow through my phone camera for the LED, it was either too dim or misaligned.
I also 3D printed a new clip to hold the wheel in place. It's now working as it should!
