During some research for a project I stumbled upon a snippet of information in a Sharp Electronics datasheet that I have never seen before in the datasheets of any other brand BUT I'll bet the same applies to them all.
Optos are constructed from materials that degrade over time. That's probably not too surprising, BUT in the case of the Sharp datasheet advice, they 'can' lose 50% efficiency in only 5 years! WOW!
The sheet advises engineers to make allowances in their design for the falling efficiency of the device. I truly wonder if this advice was given to pinball designers 20 years ago or if they had any idea of how much the performance of these devices would degrade? I very much doubt it.
This could explain the myriad of 'mystery' problems we are seeing in pinball machines that are over 20 years old.….
My advice is to pay close attention to the opto devices in your machine if you start having 'weird' problems and if in doubt - replace them.
Here is the part from the Sharp datasheet (a company I trust):