On the credit unit, there is one switch that opens when credits are at maximum. This can be adjusted by moving the pin that actuates the switch. Then there are the other rwo switches that open when credits are at zero. It is not adjustable. Also, there is a switch that actuates when a credit is added. That is driving the knocker coil.
Transformer voltage measurement should be done between the C terminal and the 6/24/high tap outputs, with your meter set at ACV. But remove the switch stack short (adjust the credit unit properly) before mesurements. You can measure the voltages even if the 24V or 6V fuses are not installed.
The coil voltage can be either 24V, or about 27V if the wire is moved from 24 to the high tap. I dont recommend using the high tap, since the game was designed for 220 volts input, and we are already at 230V. Cleaning playfield and all mechanisms should result in nice game play with no need to raise the voltage. High tap was really meant for cases of lower than normal line voltage.