your guide to pinball in Belgium
Description of the PSU (power supply) board
The given voltages are: +6, +12, -5 volts and, in the same board the signals RST, -RST, INTR and SYNC are generated.
The 6 volts voltage is obtained from a full wave rectifier formed by two diodes of 3 amperes (BYS51), filter capacitor, power transistor (BD244), monolithic voltage regulator (LM341-6) and power resistors which with the diodes protect the power transistor in case of overload.
The voltage of 12 volts is obtained from a full wave doubler formed by a mo-nolithic bridge rectifier (VM18) and three capacitors, that circuit gets a voltage from 15 to 27 volts (according to the voltage of the primary) which is subsequently reduced and stabilized by mean of a monolithic voltage regulator of 0'5 amperes (LM341-12).
The voltage of -5 volts is obtained from a full wave rectifier formed by two diodes of 1 ampere (IN4OOl).
The signal of INTR is derived, from the two opposite voltages of supply, so thus a pulse whose rising edge is at the beginning of each half cycle of the a.c. line. That signal will be used to synchronize the shot of the thyristors that control the lights of the game, with the beginning of the half cycle of the a.c. line voltage. The same pulses are carried to a RC network which integrates them so thus their output will be high if one only half cycle is missing in some of both inputs of 11 volts a & B coming from the transformer. Amplified and inverted by means of a logic gate, this signal is used to charge, slowly through a resistor and discharge quickly through a diode, an electrolitic capacitor. This signal is once more amplified by two invert gates. Prom the first output it is obtained RST and from the se�cond output -RST.
When connecting the machine to the a.c. line RST are = 6 volts and -RST = 0 volts. After 0'5 seconds, when the capacitor has been charged RST are = 0 volts and -RST = 6 volts. Both signals are used to initialize the electronic system.
The signal SYNC is one of the sinusoidal inputs of the 11 volts A & B of the transformer converted into a square wave by means of a logic gate. This signal is used in the MPU to determine whether the game lights connec-ted to the 11 volts A or to the 11 volts B must be lit.