Technically, the best repair is to use the 3-terminal regulator, properly heatsinked. I have no idea how much currect the character generator chip draws, or indeed what the chip is. But I can't believe it's anything like that 1amp rating of the 7812. If you want, for historical accuracy, to use the resistor + zener, it's worth knowing how to work out the values of those components. What you need to realise is 1) The resistor is what actually drops the voltage. The voltage across it is the diffeernce between the supply voltage (nominally 16V on an S100 machine, but it could be highetr) and the load voltage (here, 12V) 2) In order to drop that voltage, the resistor needs to pass a certain current. Part of that current goes through the load (the character generator chip). The rest goes through the zener diode. The zener current will vary with the supply voltage (obviously, the higher the supply, the more voltage needs to be dropped across the resistor, therefore more current needs to be flowing through the resistor). 3) The zenre currnet also varies with the load current. Since, for a given supply voltage, the current through the resistor is constant, the zener current must increase if the load current decreases. How to calculatr things . 1) Take the worst case of minimum input voltage and maximum load current. Add one a suitable zener current (say 10mA) to get the current through the resistor. Calculator the voltage drop accross the resistor (diffeence between minimum input voltage and load voltage). Calculator the resistor value. Go down to the next lowest prefered value if you have to. 2) Now take the maximum possible input voltage and re-calculate the voltage drop across the resistor. Calculate the power dissipated in the resistor under those conditions (power = (voltage drop)^2 / R). Use a resistor of at least that power rating. 3) For the same (highest) input voltage, calculate the current through the resistor. Now assume that all that current goes through the zener, that the load is drawing nothing. Calculate the power dissipated in the zener (remember the voltage across it is the wanted load voltage, here 12V). Make sure you use a zener that can dissipate that amount of power. Let's work through an example. Suppose the character generator draws 50mA, and that the input voltage can vary between 14V and 20V 1) Minimum voltage drop across the resistor = 14V-12V = 2V Current = 50mA + 10mA (zener current) = 60mA = 0.06A Resistor = 2/0/06 = 33.33 Ohms. Use a 33 Ohm resistor 2) Maximum voltage drop across the resistor = 20-12 = 8V Power = 8*8/33 = 1.94W. Use a 2W resistor. 3) Currnet through the resistor under those conditions = V/R = 8/33 = 0.242A Assume all that passes through the zener, which therefore dissipates 0.242*12 = 2.91W. That's a big zener! current, but you get the idea -tony