He is using the Texas Instruments TL783 voltage regulator. A far, far better solution than the LM317HVT that another person uses but the TL783 still not a good choice for voltages as high as what the Bally display's require.
I plan on using the TI part on my next version of Gottlieb System 1 and 80 power supplies because the Gottlieb voltages are well below the absolute maximum limits. But, to use them for the Bally, Stern or Williams high voltage section - wayyyy over the max limits.
Tom argues that the part specs are Vin to Vout limits of 125V - which is what the data sheet says. However, the Texas Instruments FAE laughed at using this part in that supply and said there are two major problems with that reasoning. 1 - If you have a shorted output then you have to dump a 300V differential between input to output which will kill the part rather quickly. 2 - At every powerup, you have a brief period where the input voltage goes to full value but the output is not yet brought up. Although it is a very brief period, it is still way over the limits. The FAE said that although it will take awhile, repetative surges such as this will eventually kill the part.
-- Ed