The CPU board will not boot the game. With a test ROM it completes its tests, but as soon as the watchdog is reconnected the CPU LED comes back on, pointing at the watchdog circuit.

Gottlieb System 3 CPU passes test ROM, then watchdog trips: torn trace and wrong parts

Cause: A trace under the board between C21 and C6 had been torn away (it still looked like a faint mark), and one of the resistors fitted in the watchdog circuit during an earlier repair was the wrong value (680 ohm instead of the correct high-value part).

Checks

• Meter every trace in the watchdog circuit end to end, especially under capacitors and around earlier repairs.
• Check the value of every part replaced earlier against the parts list.
• After fitting a socket, meter from each socket pin to where its trace goes.

Fix

• Repair the torn trace with a wire jumper.
• Fit resistors of the value the parts list calls for.
• The poster found a 74HC123N would not work at U11 on this board and recommends the 74HC123AN.

Systems

• Gottlieb System 3

Related tips

• Logic chips worth stocking
• U11 must be a 74HC123AN
• CPU board will not boot, or keeps rebooting
• Game resets every two seconds
• Suddenly won't boot, CPU and DMD LEDs solid, display dark: reseat playfield ground plugs
• Low battery / Bad U3 or U6 error and beep loop after a coin cell swap
• Where the boards sit in a System 3 dot matrix backbox
• Low battery error comes back a month after a new coin cell: the new cell was bad

Source: Pinball Repair help group!! (Facebook group)