Adding the 4.7k base pull-up to playfield power transistors

Steps

• Optionally swap any 2N5875 for a 2N5879 or higher-rated part first.
• Locate a constant +24 V DC under the playfield: the white/red/red wire at the fuse block, or the same colour wire on most small relays. Confirm with a meter that it is present all the time.
• Run a wire from that +24 V point to each playfield power transistor.
• At each transistor, solder a 4.7k 1/4 W resistor to the base leg alongside the existing wire. The base is usually the leg with the thinner of the two wires.
• Solder the other resistor lead to the new +24 V wire and cover it with heat shrink.
• A 1/8 A fast fuse in the new +24 V wire is optional.

Systems

• Gottlieb System 80

Games

• Black Hole
• Haunted House
• Volcano

Related tips

• Coil stuck on that is driven through a playfield-mounted power transistor
• Replacements for the 2N5875 under-playfield power transistors
• Haunted House trap door won't open
• Building a replacement SRC pack from discrete parts
• Lower playfield pop bumpers lock on or blow their fuse: shorted driver transistors
• Coils pulse briefly at switch-on on Volcano, Black Hole and Haunted House
• Volcano playfield-mounted transistors
• Black Hole playfield-mounted transistors Q1 to Q5

Source: Repairing & Upgrading Gottlieb System 80 Pinballs from 1980 to 1989, Part Two, Clay Harrell (pinrepair.com)