I'll be interested if you figure this out. I just bought a Super Sprint and also have an APB that I think uses the same type of opto boards. I am not sure about the condition of either so I could see ending up in the same boat when I get around to working on them.
A point of possible interest. The LED is infrared. I am guessing that you don't have an issue with this since you can turn one direction but who knows. But I learned something just yeaterday. I have a remote control for something that I was questioning whether it was outputting anything and was looking up how I could tell. I discovered that if I turned on my cell phones camera and looked at the area where the IR came out of the remote, I could see the IR light as if it was a regular light. I tried to show my wife later last night and discovered that her phone, an iphone 5, would not do this. So possibly this could help you at least quickly verify that the IR LED is doing its job if you have a phone that does this.