Vintage Arcade used negative voltage to increase the volume to 11


When [Nicole Express] got his hands on the 1986 SNK arcade game motherboard Athena, it encountered a rather thorny problem: the card had to be supplied with negative five volts! [Nicole]Analysis of the problem and a brilliant solution are outlined in his well-written blog post.

[Nicole]S first task was to find out which devices need negative voltage. She discovered that the negative five volts were being fed by a capacitor at the ground pins of the Mitsubishi M151516L, an obscure 12W audio amplifier. After finding the datasheet, she realized something strange: l The amp was not calling negative voltage at all! A mystery was brewing.

To fully understand the problem, she considered a mid-1980s arcade and its cacophony of sounds. How would a manufacturer make their arcade game stand out? By doing it Stronger, obviously! And how did they make their game stronger than the others?

The answer lies in the requirement of minus five volts. The amplifier is always powered by a standard 12 V supply on its VCC pin. But with a ground set to -5 V, the voltage potential goes from 12 V to 17 V without overloading the chip. The result is a stronger game to attract more players and their new stacks of neighborhoods.

How was [Nicole Express] solve the problem? The ATX power supply stopped supplying negative voltage after the ISA slot of the PC disappeared, so it would not work. She could have bought an expensive arcade-style power supply, but that’s not her style. Instead, she used a wonderful little hack: a charge pump circuit. A charge pump works by applying a positive voltage to a capacitor. Then the capacitor is quickly disconnected from the power supply, and the input and earth are reversed, an equal but negative voltage is on its opposite plate. If this is done with a sufficiently high frequency, a constant voltage of -5 V can be obtained from an input of +5 V. [Nicole Express] found a voltage inverter integrated circuit (ICL7660) designed specifically for this purpose and put it to work.

The integrated circuit does not provide enough power to get 12W from the amplifier, so the resulting signal is passed to an external amplifier. Now [Nicole]the arcade game has sound and she can play Athena of the original arcade board, 1986 style!

Arcades are rare these days, but that doesn’t mean you can’t introduce your youngsters to the joys of dropping a quarter or two, or building a gorgeous Super Mario Bros cabinet made of oak with pixel art inlays. Do you have a favorite hack to share? Make sure you let us know via the Tip Line!

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