On a 1950s Genco, coils are weak: the bell plunger rises but barely rings the bell, and score reels or continuous units reset sluggishly.

Genco coils weak: failing selenium rectifier

Cause: Genco powered its coils from about 18 V DC made by a selenium rectifier. These rectifiers gradually build up internal resistance, drop more voltage and run hot, and finally burn out.

Checks

• Measure the DC coil voltage while coils fire.
• Smell for the sharp odour of an overheating selenium stack.

Fix

• Replace the selenium rectifier with a silicon bridge rectifier (see the conversion procedure).

A burning selenium rectifier gives off toxic fumes and can start a fire.

Systems

• Electro-mechanical

Related tips

• Bridge rectifier for 1970s Bally and Williams DC coils

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