Free-running the CPU to check address and select lines
Steps
- Remove the test EPROM and power the board: with no program the 6808 just steps through every address.
- The address lines are on U15 pins 9 through 25, skipping pin 21 (ground); they and should read about 2 V; pins 24 and 25 a little lower, about 1 V.
- A line that does not move has a short: bend that pin out of the socket, refit the chip and retest; if it moves now, cut the trace at points along its path to find the chip pulling it down.
- Select pulses: the PIA select pins 23, 24, 25, 35, 36 should show alternating signals around 3 V; with the test EPROM fitted, U27 pins 20 and 27 should read 2 to 3 V. A pin stuck at +5 V or ground points to the select logic; follow it on the schematic.
Systems
Related tips
- Running the CPU board on the bench
- Checking ground and +5 V on the CPU board
- Which System 11 CPU boards fit which games
- Checking the CPU clock signal
- CPU RAM at U25
- CPU board test switches SW1 and SW2
- What each 6821 PIA on the CPU board controls
- Checking PIA outputs with the test EPROM running
Source: Repairing Williams System 11 Pinball 1986 to 1990, Part Two, Clay Harrell (pinrepair.com)