A switch (other than a flipper EOS or score motor home switch) arcs heavily and burns.

A switch throws a big blue spark

Cause: A low-resistance (often shorted) coil in that circuit drawing excessive current. When any coil is switched off its collapsing field kicks back about twice the supply voltage, and the lower the coil's ohms, the bigger the arc. EMs, being AC with no CPU, have no suppression diodes on coils.

Checks

• Measure the resistance of each coil the switch feeds and compare with the same coil elsewhere or a known value.

Fix

• Replace the low-ohm coil, then dress the burnt switch contacts.

Systems

• Electro-mechanical

Related tips

• Ball release hold coil won't release, big spark on its switches

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