SEGA Master System II

What is the device power input?

 * +9V DC ± 10%, 0.5A.
 * 5.5mm x 2.1mm barrel plug.
 * Center-pin_negative.png Center-pin negative polarity (tip: +9V DC, sleeve: -9V DC).

Where I can find on the board?
The ground plane is present all around the board as a large copper via (see picture).

Procedure

 * Measure the input voltage VI. Voltage must be  on this pin. Wait a minute! Why not +9V DC here? VIN goes through a diode D2 before reaching IC0/IC8. D2 is a silicon diode rectifier, and they always have a forward voltage drop (&asymp; 0.55V at 0.5A for a 1SR35-100). stock IC0/IC8 can operate with a voltage as low as +7V DC, so we're good.
 * Measure the output voltage VO. Voltage must be.

Replacement
Virtually any 7805 (TO-220) will do the trick, but if you want to pick your own regulator:
 * You may want to choose a TO-220 package if you want a drop-in replacement (same pins and pitch).
 * Make sure the pinout is correct.
 * The regulator must provide a +5V DC nominal output voltage.
 * And accept a +9V DC input voltage  (check the minimum and maximum input voltages too).
 * It's advisable to double-check the dropout voltage  (The difference between the input and output voltages for the voltage regulator to work). We have a +4V DC difference between   and   in our context, so we need to make sure the dropout voltage is lower than +4V DC.
 * IC0/IC8 will provide a critical reference voltage: keep the tolerance as low as possible &le;5% (the tolerance is the output voltage accuracy).
 * Keep the junction-to-air thermal resistance (usually named ) as low as possible.