Mega Drive 1 - Switch less Mod (Continued)
The more difficult one!
The other Mega Drive I had was different and had me scratching my head for a bit.
The reset button worked by sending a +5v signal to a chip causing the console to
reset. This is no good for Sebs mod which deals with GND signals to detect
the reset button being pressed and to actually reset the console. This
console had IC BD M5 PAL VA4 written on the board.
I tried to use a 7404 Hex Inverter that I had spare, I thought I could use it to swap the +5v for GND but I just couldn't get that to work. I could however get it to swap a GND signal to a +5v signal so I decided to isolate the reset switch, bypassing it so that all it does is send a GND signal to the 16F630 when pressed (exactly what the 16F630 is expecting). I started with a multimeter and studied what's connected to what and what happens when it's pressed.
The photo below shows the +5v source for the reset button, cut the trace as shown and the button no longer gets any power at all.
When I looked on the underside of the motherboard it was obvious that the +5v that's going through the button also goes off and feeds something else. So we should link that up to the original +5v source - that's the red circles with a dotted link in-between them. I just soldered them up using some Kynar wire.
The next thing to do is cut off the reset switch on the underside of the motherboard, the 2 necessary cuts are shown in red above. I use a multimeter to find a source for GND and linked it up to the reset button - that's the black circles with a dotted line. Now the reset button behaves how the 16F630 expects, when the button is pressed the GND signal that's being fed to the points highlighted in purple is sent to the points highlighted in blue.
Solder a wire to either of the 2 contacts of the button that are highlighted in blue above (my purple wire below) this goes to the button point on the 16F630 board. The red wire below is soldered to the single point highlighted in blue above, it's soldered to the left side of R2, this is the wire that will eventually deliver the +5v that will reset the console. You can also see the thin black Kynar wire which is bypassing the reset button. The thicker black is the GND link to the reset button. (Ignore the black and yellow wires, they are explained later on)
The Yellow and Black wires that are on the photo above are a +5v (Yellow) and GND (Black) that I'm going to use to power a Hex Inverter. I use the Inverter to take the GND reset signal from the 16F630 and change it to a +5v signal that this Mega Drive expects. For some reason I chose to get these signals from one of the Voltage Regulators that are attached to the bit metal heatsink on the left-hand side of the motherboard.
Right, I'm assuming you've got the 16F630 board ready and soldered in place to the JP1 etc Jumpers. You now need to make a simple board with a 7404 Hex Inverter on it. It's very very basic, the photo below shows the hex inverter in place on the board, it expects a +5v source on Pin 14 and GND on Pin 7. I've tried to explain on the photo which way the copper tracks are going in relation to the 7404, and the red line through the middle is my attempt at showing you I made a cut through the copper tracks separating one side of the inverter from the other.
(Because we are only inverting one signal, it would be possible to use a simple NPN Transistor Inverter - a single NOT gate, here's some info on what to do. But I had a spare 7404 which is just multiple NOT gates in an IC)
Pin 9 has a black wire going to it, this is the GND Reset signal that the 16F630 is sending out to reset the console. The Hex Inverter swaps converts this GND signal to +5v and sends it out of Pin 8, this other end of the Red wire a couple of photos up, the one that is soldered to the left side of R2. The purple wire in the photo doesn't attach to the Hex Inverter at all, it's just linking up the reset button to the 16F630 circuit board.
Here's the finished mod, all linked up. Even when I had all this in place, the machine was stuck in JPN mode, there was a Capacitor on JP1, I tried it without the Capacitor and now it selects between ENG and JPN modes as expected. Does the capacitor need to be there? I don't really know to be honest but it's been functioning fine without it.
TheYellow and Black wires coming from the Jumpers the the mod are the +5v and GND, the Purple wire is the video mode select wire, and the Green one is the language select.
I bought some diffused dual colour Red/Green 3 legged LEDs to use for the Mega Drives, with the RF shielding in place there's a handy hole to feed the wires though.
Now you have to remove the original Red LED from the upper casing. I used some side cutters and a scalpel to cut it out (some Mega Drives let you remove the coloured panel that's holding the LED in, this make is a bit easier).
I cut those extra grooves to accommodate the wider legs of the new LED, here's a test fitting and then it's hot glued into place.
This is based on Sebs mod and the MD1 switches guide and cobbled together by me after many hours of experimentation.
