Trough opto numbering is backwards

Only three of the four optos on each board are used. Q1, the far-left one, is unused and can be robbed as a spare. Q2 is trough switch 1 (left), Q3 switch 2 (middle) and Q4, nearest the shooter, switch 3 (right). The LEDs are numbered the same way.

Systems

• Bally 6803

Games

• Blackwater 100
• Dungeons & Dragons

Related tips

• Dungeons & Dragons and Blackwater 100 use trough optos
• "Ball missing" with all balls in the trough
• Ball missing or balls stacking in the shooter lane
• Stop trough optos from breaking with vibration
• Test trough emitters and receivers
• Fitting new trough optos on D&D and Blackwater 100
• Diagnostic test numbers

Source: Repairing Bally 6803 Pinball Games from 1985 to 1989, Clay Harrell (pinrepair.com)