In the target bank, the M target only registers when it is the first target knocked down; hit after any other target it isn't counted and the bank doesn't reset. Switch test shows the same, and the connectors have already been repinned.

One drop target only registers if it is the first one down: reseat and clean the PIAs

Cause: Poor contact at the MPU's PIA chips (U10 and U11) in their sockets.

Checks

• Reproduce it in switch test by dropping the targets in different orders.
• Pull the PIAs at U10 and U11 and look at their legs and sockets for tarnish.

Fix

• Clean the PIA legs (a pencil eraser works), swap the two chips' positions and reseat them firmly.

Systems

• Stern MPU-200

Games

• Meteor

Related tips

• Switch reads closed even with switch connectors unplugged: shorted resistor on the MPU
• MPU switch connector pins J2 and J3
• Bally switch matrix: 5 strobes by 8 returns, numbered 1 to 40
• Odd switch problems from corrosion at the DIP switches
• Player 1 display shows credits and ball in play
• Switch 3 shows closed and scoring goes haywire: check the coin switches
• One target also scores its neighbour: a stray switch jumper elsewhere
• Fourth drop target won't count when lit, but works in switch test: MPU fault

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