When a game starts a score relay stays pulled in, its reel stops scoring and the reel coil may start to smell burnt.

Score relay stuck on, reel does not score

Cause: A playfield switch held closed (old rubber can press one shut), a stuck feature relay switch (such as the 50 point relay), a dirty or welded reel EOS switch, two solder lugs bent together, a stray wire strand bridging lugs, a damping blade touching the other blade, or pitted contacts driving the reel.

Checks

• Look for any closed playfield switch.
• Check feature relays that feed the score relay.
• Check the reel EOS switch opens.
• Look closely at playfield switch damping blades.

Fix

• Free or re-gap the offending switch, separate shorting lugs, trim stray strands.

Systems

• Electro-mechanical

Related tips

• A chime, bell or score reel coil is stuck on
• 1,000-point reels reset but never score for any player: failed 1,000-point relay coil
• Clean the score reel relays
• Bally score reel relays
• Find out if score reels are stopping the start-up
• Hundreds reel keeps stepping after reset: a stuck playfield scoring switch
• Reset stalls with the player unit stuck at step 20: broken wire to a score reel
• EM score reel will not reset to zero

Source: Repairing Electro-Mechanical (EM) Coin Operated Games to 1978, Part One, Clay Harrell (pinrepair.com)