If you are talking about the trough emitter opto board that only has one capacitor, that likely the same as in Bally Truck Stop. It is a 10 uf axial tantalum. It's just on the 5V rail, I think safe with +16V on up for the working voltage.
https://www.ipdb.org/files/2667/Bally_1988_Truck_Stop_Manual.pdf