Black Squares
Black Squares can be attributed to a number of processor / power supply problems. The most common seen is the power supply 5volt level is out of adjustment. This level must be maintained at 5.02 to 5.06 volts DC. We have determined that in most applications if this voltage lower than 4.8 volts DC black squares will appear. It is recommended that this voltage be checked utilizing a digital multimeter.
Depending on the processor board, we will check this voltage at one of the following points; on "White, or Coin" machines MTA1, on machines with 8088,8085 or 80186 processors either connector MTA31 or 1MTA31. The standard pin out is; pin 9 & 10 are DC ground, pin 3 = 5 vdc, pin +5 = +12 vdc, and pin 7 = -12 vdc, and is the same for all power connections.
The next most likely problem we encounter is a failed processor board, or software. This needs to be addressed at the same time, as we will typically replace both. It is difficult to determine which one has failed without having a spare set of components.
Once the new processor board is installed, the EPROM needs to be inserted to the socket. If this chip is not inserted correctly this will result in "Black Squares". Bent pins, inserted backwards, missed pins, will have the same result.
The last issue is a faulty display. In rare cases a display will not operate correctly, however the machine will run normally. To prove this select a known formula, than press start, does the machine run? If so, than simply change the display.
In conclusion we must first check the voltage to the processor, than attempt to run the machine. This should point us to the defective component.