Testing the 6821 PIAs in circuit with a meter

Steps

• Game off, meter on diode test.
• Red probe on PIA pin 1 (ground). Touch the black probe to pins 2-9 (PA0-PA7) and 10-17 (PB0-PB7) in turn.
• Expect 0.5 to 0.6 V on each. U51 pin 9 reads zero in circuit, which is normal.
• A zero reading on any other pin from 2 to 17 means a shorted PIA: replace it.
• Check all six PIAs (U10, U38, U41, U42, U51, U54), plus the sound PIA U9 on boards before 11C. This finds shorts but not every failure.

Systems

• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C

Related tips

• What each 6821 PIA on the CPU board controls
• Checking PIA outputs with the test EPROM running
• SRC packs cannot be tested in circuit
• Find out how the board died: failures cascade to the PIAs
• Leon test EPROM does not run: check the 6808 and its pins
• Wrong coils fire and drop target dead: bad PIAs, shorted coil diode and Q14
• System 11 CPU board won't boot after cleaning: worn ROM socket contacts
• System 11 NVRAM upgrade not working: fit it at U25, not U23 (sound RAM)

Source: Repairing Williams System 11 Pinball 1986 to 1990, Part Two, Clay Harrell (pinrepair.com)