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How to Check the +5V on the Game Board

The +5V is the main logic voltage. Keep it within the range of 4.9V to 5.1V measured on the game board. See Multimeter "A", you can reach the Jamma connector of the game board and this is a good place to check the voltage (*see note). Remember it is not good enough to check the voltage on the power supply terminals only. The wires and the connectors providing the connection to the game board present resistance to the current flow which results in a loss of power (voltage drop). Note Multimeter "B" which measures the voltage drop across the Ground wire. Lets assume that the same voltage drop is measured across the +5V connection. The total voltage drop would be then 0.3V. This really means that if you adjust exactly +5V on the power supply terminals you get 0.3 volts less (4.7V) on the game board.

The voltage drop is proportional to the resistance of the connection and the current taken by the game board. Therefore the voltage drop will vary from machine to machine. Increasing current and resistance will result in an increasing voltage drop. 

It is always a good idea to check the +5V on the game board and on the power supply as well. You may find that in some older machines the connection is so miserable that the difference between the voltage measured on the power supply and the game board may be as big as 1V. You would really need to adjust the power supply to 6V in order to get 5V on the game board and that would be a terrible thing to do. Not only that the over-voltage protection may shut down the power supply but there would be also a great risk that the voltage on the game board will be badly fluctuating, causing all sorts of intermittent problems or even damage to the board if  momentarily rising above 5.25V . 

What is the right thing to do? If the difference between the voltage measured on the power supply and the game board is greater than 0.3V fix the connection first and than adjust the power supply. (Keep in mind that after you fix the connection the power supply voltage may need to be turned down before the power is applied to the game board). 

 * In machines where you cannot reach with your multimeter leads directly the game board, try to measure on the closest connector to the game board. You may adjust 5.1 V allowing for 0.1V voltage drop.