The 3 V zener VR1 in the CPU low-voltage reset circuit is discoloured or burnt, sometimes with Q1 damaged too.

Reset section zener VR1 burnt on a System 80 CPU board

Cause: A design fault: VR1 tries to hold the base of PNP Q1 (MPSA70) at 3 V while Q1's emitter, fed from 5 V through CR35, pulls the base to about 3.8 V, so the two fight.

Checks

• Look for a darkened VR1 and test Q1 on diode test.

Fix

• Replace VR1 (3 V 500 mW zener) and Q1 (MPSA70 or 2N4403).
• On the solder side cut the trace from Q1's base to the nearby via.
• Solder a 1.5K resistor between Q1's base and that via, so it sits in series with VR1.

Systems

• Gottlieb System 80
• Gottlieb System 80A
• Gottlieb System 80B

Related tips

• How the control board reset circuit works
• Replace the System 80 reset circuit with a DS1811 chip
• What the System 80 reset (watchdog) board does
• Why a System 80 CPU stops running
• Dead CPU board: check the reset line first
• Game won't boot, or keeps rebooting, because of the reset board
• First power-up, step 4: add the CPU board one connector at a time
• Niwumpf replacement CPU board takes about 10 seconds to show scores

Source: Gottlieb System 80 Test EPROM V3.2 (February 2015), Pinitech