Power Supply Assembly

The Power Supply, used to supply all voltages, is extremely tolerable to input line voltage variations. AII output source voltages are guaranteed to be stable for line voltages varying from 95V AC to 135V AC 60Hz. The regulated logic +5V DC level is rated at 6 amps maximum and includes over-voltage crowbar protection. Four LED's on the Power Supply Board indicate that the associated voltages at the outputs are present.

The +11.5V DC entering the Power Supply Board is regulated to +5V DC by Q11, Q12 and U11 and is adjustable by VR1. R15 and VR1 along with R16 divide the regulator output voltage to +2.5V DC at the reference pin of Ull, the programmable zener. As the output voltage rises, the voltage on the reference pin on U11 will rise. To compensate for the rising,output, Ull draws more current away from the base of Q12. This in turn turns off Qll which drops the output voltage. As the output voltage of the regulator falls, the reference pin voltage falls, turning off Ull. This will increase current flow through Q12 which in turn will increase current from the emitter to collector of Qll, raising the output voltage.

The over-voltage crowbar circuit mentioned earlier consists of C14, D12, R17, R18 and the SCR. The SCR requires 1.4V gate to cathode in order to turn on. The zener is rated at 5.6V. Therefore a voltage of 7V on the +5V DC line will trigger the SCR (7V-5.6 rated zener volts =1.4V). Once the SCR is on, the +5V DC line is shorted to ground, causing the fuse to open, preventing over-voltage damage to the TTL. R17 is a current limiting resistor for the SCR and R18 is the zener resistor. C14 filters out spikes that could trigger the SCR.

The 9V AC RMS is rectified and filtered and supplies the Audio Amplifier, LM2002, with unregulated +12V DC. The input Audio comes in on pin 3 of P7 and is 5V AC peak to peak (to keep a good signal to noise ratio). This signal is attenuated by R51 and R52 by fifty times and is fed to C52, the amplifiers input.

The 220 ohm R54 and 2.2 ohm R55 set the closed loop gain of the LM2002 by the equation

Av=l+R220/R2.2=101. Therefore the overall system gain is 101/52=1.94. R53, C55, R56 and C57 are compensation networks at 1 MHz and 10 MHz to keep the LM2002 unconditionally stable. C54 is the low frequency feed back capacitor. C58 is the output capacitor blocking the DC bias of the amplifier so that the output voltage on the speaker can swing above and below ground. The output apears at pin 3 of P6.

The center-tapped 15V AC RMS is utilized to supply three voltages. It is full wave rectified by D41 and D42 in order to supply the +20V DC average voltage used for the coin meters. It is also full wave rectified by D31 through D34 in order to supply the plus and minus 12V DC. The positive side of the bridge feeds the LM340K (U31) +12V DC regulator at 1 amp, while the negative side of the bridge is sent to the LM320 (U41) which regulates the -12V DC at 0.1 amp. D36 and D44 are used as protection diodes in the event that the output or the regulator exceeds the input.

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