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A transformer is two (or more) coils of wire wrapped around a ferrous core. The 'primary' coil (wall voltage) creates a magnetic field, which is coupled into (usually two) secondary coils. This produces lower voltages to power the game's lamps and solenoids. That is, the game's transformer takes AC wall voltage and steps it down to the appropriate voltages needed for the game. This usually includes 6 volts AC for the lamps, and 24 to 30 volts AC for the solenoids. An exception was Bally during the 1970s (50 volts), and Williams. Williams used 50 volts AC for coil voltage until 1962 (Friendship7), when Williams changed to 24 volts AC. The reason? Fifty volts is potentially lethal, so Williams felt it was better to use a lower voltage. Some manufacturers (Williams in 1972, and Bally in 1975, and Gottlieb in 1978) then convert the AC voltage to DC using a bridge rectifier for some coils. Genco also used DC voltage in the 1950s, by using big selenium rectifier disc plates mounted on the transformer to output about 18 volts DC for the solenoids.
Does a transformer ever go bad? Short version - NO. But I hear this all the time from inexperienced EM (and solid state!) repair people - "the transformer is bad." In fact, this is rarely the case. In all the EM games I have fixed (I fix about 200-300 per year), never has the transformer been bad. I have seen one bad transformer though, and it was so obvious it was bad (it was a melted pile of goo, since transformers are potted in wax, as they heat up and the windings burn, the wax melts and burns.)
Luckily EM transformers are very easy to test. The primary wall voltage (input) comes in on two lugs, and the transformer outputs usually two voltages: 6 volts (for the lights), and 25 to 50 volts for the solenoids. So there are usually three output lugs, with one 'common'. Put a DMM meter on AC voltages and one lead on 'common'. Put the other lead on one of the other lugs and check for 6 volts AC and 25 to 50 volts AC. Note 'ground' is not a reference point here, the common lug is used instead. There is also usually a "high tap" transformer lug, and this will be 2 or 3 volts higher than the normal solenoid voltage.
Transformer to a 1972 Bally Fireball.
Most people that try and measure voltage from the transformer are usually doing it wrong. Common mistakes include using "ground" as the voltage reference (there's no ground for these AC voltages). Also sometimes they are using the wrong transformer lug as the reference or common lug. Another mistake is using DC instead of AC on the meter
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