Thats a good Idea. I didnt think of that. Its 120 volt. So I need a varible switch that can handle 120volts right? Im not exactly sure how to go about doing this.
Try a light dimmer. Not the solid-state ones, but a simple rheostat style. Can't guarantee it will work, but it's probably the best bet.
On a side note, I can't remember seeing a fan in a real arcade cabinet, probably because of the noise. Could you image 30 fans all whirring at the same time in a small room

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Most cabinets stay pretty cool inside using only convection cooling. For those that don't understand the principle of this, I'll try to explain it.
Hot air rises (obviously) so your cabinet will always be the warmest at the top. Which means the ideal placement for the components you want to stay cool will be at the bottom of the cabinet.
This also means you'll want to put a large vent at the top of the cabinet to let the heat escape. Most of the time in commercial cabinets, these vents are located at the very top portion of the back of the unit. This lets the heat out, but also keeps debris from falling inside, which could happen if the vent was in the top panel.
Now, the convection current is strong enough to bring the hot air to the top of the cabinet, but obviously not strong enough to create a vacuum. This means for proper convection cooling to take place, you need an equally large intake vent in the bottom of the cabinet. This should be placed on the bottom panel somewhere toward the front of the machine to make sure the cool air is drawn past the components you are trying to keep under control. You also need to make sure the cabinet is raised off the floor to allow air to flow underneath the cab. An inch and a half should be plenty if it's clean under there. Placing the intake vent in the very bottom portion of the back panel will work as well.
And if that doesn't work , just leave the back panel off and space it a couple inches from a wall

RandyT