If you follow me on Twitter, you’ve probably seen the picture of a CRT deflection yoke undergoing surgery. Well, it’s now installed in a mini Battlezone arcade cabinet that I built for Maker Faire this year.
It uses a 5-inch B&W CRT taken from a broken security camera monitor. I had to rewind the yoke to make the vertical windings fast enough to handle vector graphics. The horizontal windings were already pretty close, but they did need to be rewired to operate in series to reduce the drive current.
The CRT is a bit different than most of the old-style CRTs I work with. For the curious, it uses an EIA E7-91 style base which has an additional grid for acceleration purposes. The lower circuit board in the photo is a potentiometer resistor divider that develops a ~300V bias voltage to drive this additional grid. The upper board is a Cockcroft-Walton multiplier circuit to step up the 1KV power supply voltage to the 4KV necessary for the post-deflection acceleration (PDA) anode. I’m running this CRT a bit under voltage, and it works fine because I’m not trying to fill the whole screen with a solid raster.
The metal enclosure houses the power supplies–it is made out of soft steel with the idea of containing the stray magnetic fields generated by the power supply switching inductors. It works a bit, but I’ve had better luck in the past with mu-metal shields around the CRTs themselves. There are three power supplies: one steps 120V down to +12V, +5V, and -12V. Another steps +12V to +1KV, and the final one steps +12V up to +60V. These last two power supplies are detailed here. They are open-source hardware so you can build your own.
Outside the shield you can see the video amplifier board at the lower left, which is the same circuit used on my electrostatic deflection board but without the deflection amplifiers. At the bottom right is my new magnetic deflection amplifier board which drives the deflection yoke. It is based around an LM4765 audio amplifier IC. I’m not totally happy with the design but eventually I will release the board design. The green board in the upper middle is an STM32F4 Discovery board. The board to the right of it is an audio amplifier that drives the speaker.
The joysticks I machined out of small aluminum blocks. It took some experimenting to figure out how to arrange the springs to get the joystick to return to the center position. The joystick handle on the right is hollow to allow the wires for the fire button to pass through.
I also build a second unit with the same driving circuit but a different CRT–a 5-inch round 5AXP4. This CRT would originally have been used by a TV technician to look for problems with the chassis or the full size picture tube in a customer’s set. The glass is extra thick because the tube was designed to be handled more. This is the same tube that I started experimenting with last year.