Add a reset button to your Master System II

The Master System II is a dumbed down version of its older brother, the Master System: to save costs SEGA removed the expansion port (probably used only in kiosks to attach multiple cart devices), the card port, the A/V out connector (except for the French version, i think) and the Reset button.
Of all these features, at least two can be restored: the A/V port and the reset button. As for the A/V port I'll write something in the future, now I want to explain how to put the reset functionality back into place.

SMS2 with reset button added back

First of all, I want to say I took some informations from this forum topic at smspower.org .

The Z80 inside your Master System receives the reset interrupt when a certain pin of the I/O chip is shorted to ground (low), while normally this pin is connected to +5v (high logic level). In the first Master System version, this pin is connected to +5v through a resistor, and to ground through a button. When the button is pushed the pin is shorted and the reset signal is sent. The resistor avoids creating a short-circuit between ground and positive, and this resistor is missing from the SMS2, which has the pin wired directly to 5v: we need to restore this circuit.

The reset circuit after the mod

As You can see from the picture, the mod is quite simple: You need to lift pin 33 of the I/O chip (marked as 315-5237 on my console) so it's disconnected from the circuit board, and then put a resistor (1k5ohm, or 1500ohm works OK) between the lifted leg and 5v, and then connect a button between a ground point and the leg (BEFORE the resistor, naturally).

Lifting the leg can prove to be challenging. Here is how i did it: using a solder pump i removed ALL the solder on pin 33 from the underside of the board. The pin is ready when it can move freely inside the hole: don't try to do anything unless it clean! Also, avoid heating up the pin too much: it could burn the chip or melt the trace.

Circled in red is pin 33

After this step, You can start pushing gently the pin through the hole using a toothpick or a needle. Be careful! When the pin starts to bend and stick out, You can run a wire behind it and then pull from the wire instead of pushing through the hole. When the pin is completely out, You can try to straighten it and make it parallel to the circuit board using a pair of tweezers.
When the pin is disconnected, You can solder your 1k5ohm resistor on the now empty hole (as it provides +5v) on one end, and the other end to the free pin.

Pin connected to 5v through a resistor

Ok. Now You need to pick one point to take ground from. I choose a capacitor near the connector ports.

Ignore the green and the orange/white wires coming
from the VDP chip (left of the I/O chip): they're not
related to this mod

Now You can connect a wire from ground to a button, and from the other pin of this button to pin 33 of the I/O chip (as shown on the scheme above).

After this is done and You checked that it actually works ok, You can search for a spot on the case where You want your button to stick out: I went for  the area on top of the pause button, heated a screwdriver and made an hole through the plastic, then screwed the button in place. It doesn't look too bad!

Ok, why would You want to go through all this just to fit a reset button on your console? First of all, it saves your console some stress each time you have to restart a game by cutting the power and turning it back on (which is what the ON/OFF switch does), the reset button does a software-driven "reboot" instead.
Also, in some games the reset button is required to perform some cheats ;-)

DISCLAIMER: I'm not to be held responsible for anything that happens to You or your console while doing this modification!