MK2.5 reel slot - error message 'coin in photo-optics'

[mctuerk]

When I switch the machine on, the error message 'coin in photo-optics' immediately appears. There is definitely no coin in the optics. I have the manual but it doesn't contain any schematics. Unfortunately, I only have one machine left and can't interchange the components. I think it's a faulty controller board. Can anyone tell me which components on the I/O board switch the coin handling or does anyone have the schematics? thanks for reading

[Sunrise Side]

Have you tried cleaning the optics?

[mctuerk]

Of course I cleaned and checked the optics. There are two identical optics in the machine, one for coin in and the other for cash box. I got one additional optics that is definitely working and swapped all three around. That's why I can rule out a fault in the optics.