I just googled using a multi-meter and it says and good example of DC power is a battery, and a good examplbe of AC power is current from the wall. So, based on that, you might think you'd be measuring boards using the AC settings. I'm assuming this isn't the case though because the transformer changes the AC power from the wall to DC, and that's why Tighe is measuring DC power?
A transformer steps voltage up or down... or not at all. A transformer is an iron core with two (or more) independant coils of wire around it. One coil is powered from an AC source, such as a wall socket. The other coil is electrically isolated from the first.
A current travelling through a wire will produce a magnetic field. You can do this yourself by winding some thin enameled wire around a large nail and wiring it up to a transistor radio battery. This produces a magnet which can pick up paper clips. Disconnect the battery, the magnetic field collapses, and the paper clips fall.
With AC, the voltage reverses at 60 hz (60 times per second). So, imagine the magnetic field collapsing and re-establishing itself 60 times per second. There are two coils of wire in a transformer, and as the magnetic field collapses, it induces a current through the core and into the other coil of wire. The induced voltage is proportional to the number of turns of wire that are wound around the core.
So, if you have a transformer with two coils, each of which are wound with 100 turns of copper wire, you will have a 1:1 transformer (an isolation transformer). The first coil induces a current in the second coil, through the breakdown of the magnetic field in the core. By altering the number of turns, you can step the voltage up or down. If you have 100 turns of wire on one side, and 10 turns on the other, the input voltage of 120v will be stepped down to only 12v.
The output of a transformer is still AC. It's phase shifted 180 degrees from the input sine wave, but it's still AC.
All digital logic runs on DC, however. Part of a game's power supply takes the AC from the transformer, rectifies it with diodes to create DC, and then smoothes out the remaining ripple with capacitors. So, the voltage you will measure ON a game board will always be (or at least, is supposed to be) clean DC. Most games have an offboard regulator that rectifies and filters and regulates the transformer's output voltage and turns it into clean DC. Robotron, Galaga, etc. This arrangement (transformer/rectifier/filter/regulator) is known as a linear power supply.
Some early games have the rectifiers and filtering/regulation circuitry on the game board. Games like Pac-Man, Breakout, and Galaxian are like this. The game board takes AC from the transformer directly, but converts it to the DC onboard. All the logic chips run on DC, of course - but the edge connector has AC on it.
Later games use switchmode power supplies. These are small integrated boxes that take AC from the wall socket and convert it into DC through a complex arrangement of chopper transistors, fast recovery diodes, high frequency transformers, magic smoke, and capacitors. They still work much the same way, although they use a physically tiny transformer, and operate at high frequencies to avoid the core saturation of standard 60hz.
So, the wall socket is AC. The transformer is AC. The game board is DC. The game's power supply has the necessary stuff to convert the AC to the proper voltage, then rectify and filter it into DC.
For the record, Donkey Kong uses a switching power supply.
-Ian
