Testing a Bridge Rectifier.
The following test will check if a bridge has an open circuit or a short.
Turn the game off.
Put the DMM on diode setting.
Put the black lead of the DMM on the "+" (positive) terminal of the bridge.
Put the red lead of the DMM on either AC bridge terminal. Between .4 and .6 volts should be seen. Switch the red DMM lead to the other AC bridge terminal, and again .4 to .6 volts should be seen.
Put the red lead of the DMM on the "-" (negative terminal of the bridge.
Put the black lead of the DMM on either AC bridge terminal. Between .4 and .6 volts should be seen. Switch the black DMM lead to the other AC bridge terminal, and again .4 to .6 volts should be seen.
Couple other things i found:
Reset Line (First Step in a Non-Booting MPU board).
Before the CPU chip will start to run, the reset line on the Z-80 CPU chip U11 pin 26 must be high (5 volts). The reset circuit of the MPU board controls the reset; Upon power-on, U11 pin 26 is held low (0 volts) for a short period of time (50 milliseconds or so). This allows the 5 volt power supply to stablize. Then the reset circuit makes U11 pin 26 go to 5 volts. This is done using four transistor QA,QB,QC (2N3904) and QD (2N4403), a 1N4738 8.2 volt zener diode, and some resistors and caps. Unfortunately the reset circuit is right below the battery, so battery corrosion often ruins this circuit. If the reset circuit of the MPU board is not working, the MPU will *not* start to boot.
With the power to the MPU board on, use a DMM and check U11 pin 26. It should be 4.5 to 5.1 volts DC. With the power off and the DMM on and connected to U11 pin 26, turn the power on to the MPU board. The DMM should show 0 volts for just a brief moment, and then jump up to 4.7 to 5.1 volts. if it does this, the reset circuit is working. If U11 pin 26 stays at less than 4 volts, the reset circuit is faulty. Start by replacing the four transistor QA,QB,QC (2N3904) and QD (2N4403). Also it's a good idea to replace the 1N4738 8.2 volt zener diode (the Gameplan parts list calls or a 1N959B diode, but use a 1N4738 instead).
Zero Cross Circuit.
The MPU board requires 24 volts DC (+/- 6 volts) solenoid voltage to boot. This is used for a zero cross circuit. Even though the 24 volts is DC, it is not filtered with a filter capacitor. The voltage comes from the transformer and goes through a bridge rectifier on the power supply board. The bridge "full wave rectifies" the AC transformer voltage into rough DC. This rough DC is required for the zero cross circuit - if a filter capacitor was used to smooth the 24 volts, the zero cross circuit would not work!