Each moving head features two optical incremental encoders. One for pan and one for tilt. 

Unfortunately they seem to be defective so I have to replace them. 

The actual encoders are very simple. It's a small PCB with two optical interrupters and a 4-pin JST connector. The optical interrupters are really the only thing here that could be fried, but they have no markings on the package so it's impossible to find an exact replacement. Instead I decided to go for two Omron EE-SX1041 photomicrosensors because of their almost identical form factor and pinout and because they're readily available and of decent quality. 

To make them easier to use with a microcontroller I decided to also replace the PCB with my own design which has onboard current limiting and pull-up resistors. So I started up KiCAD and came up with this: 

You might notice that the optical interrupters are not shown as I haven't gotten around to making the 3D model for them yet. I will be using OSHPark for fabricating the boards. I just have to triple-check that everything is gonna fit before I hit the order button. KiCAD source files, OSHPark project link and Bill of Materials will be uploaded in the next part. Meanwhile, here's a preview of the PCB: