Let's start mounting our parts!!
The Game Gear is significantly curved all around, which makes mounting parts flush with the case a pain. What we're going to do, is cut out perfboard in the shape of the two small PCBs we removed earlier. We'll use these to mount some of our floating components.
Trace both of the small PCBs on the perfboard. Mark the screw mounting holes, and any other voids. Drill equivalent sized holes in the perfboard. It helps a lot to clamp down the perfboard while drilling (or use a drill press), as the perfboard holes can cause the drill bit to jump to the wrong spot.
Use the screws that originally held in the small PCBs to attach the new blank perfboard pieces, and check to make sure all of the clearances are correct.
Next, we'll mount the capacitive sensors. These are mounted on the inner part of the case, right next to the cartridge slot plastic. You'll have to remove the perfboard pieces to access the area. The inner case of the Game Gear is entirely coated with conductive material. In order for the capacitive pads to be mounted flush with the case, we need to scrape all of the material completely off.
First, we measure the distance from one of the mounting holes, to roughly the center of the sensor pad. This comes out to roughly 10.8mm, according to the trusty calipers.
Next, we have to drill out two of the mounting holes (only two are really neccecary), and a hole in the center so that the button can be pressed easily! Finally, use your rotary tool to grind off the conductive coating. It needs to be completely gone! Any stray bits can cause lots of issues. Use your 2x8mm bolts and nuts to secure the sensor. Repeat this process for the opposite side.
Before these sensors become buried under the rest of the components, it's a good idea to solder on wires to them. We need three signals, GND, OUT and VDD. LEDA is an optional pin which could be used for effects. Keep the wire colors consistient for each module, and give yourself a couple inches of working wire to come back to.
You may have to drill a small hole in your perfboard cutouts to accommodate for the bolts holding the sensors in place.
Here is the mounting plan for the rear case. The power switch is in the recessed area where the volume slider used to be, and the USB Mini-B breakout ends up where the original barrel jack connector was for power input. The headphone jack will remain in it's original location. A USB-A connector will be where the EXT/Link port resided on a small riser. The MAX98306 and PowerBoost are mounted on top of the cartridge slot, and the LiPoly Backpack is on the right perfboard with the Mini-B.
For the MAX98306 amplifier board and the Powerboost 1000, we need to isolate them from the conductive material on the Game Gear case. Use your rotary tool to grind off the conductive paint from the top of the cartridge slot, where we're going to mount those boards. To be extra safe, you can put a layer of electrical tape on the underside of each board.
For the USB A riser, cut a small strip of perfboard and mount the USB A connector that came with your PowerBoost. Bend the two shield tabs firmly against the perfboard for a strong physical connection. Add a dab of hotglue for additional strength. Drill a hole in the base perfboard, and the strip of perfboard, mount the connector on a standoff. You should be left with this handsome little USB port.
For the headphone jack, you can use the Game Gear's old jack, if it's still working. It's possible to desolder the jack, or you can simply cut the PCB around the jack until it's free. Position it deadbug where it previously resided, and apply hotglue to the underside.
The power switch leads should be bent into the perfboard. Add a dab of hot glue to improve mechanical strength. It helps at this point to solder your power switch leads onto a two pin header, so the perfboard will not have to be removed again. A small diagram is provided below.
Go ahead and drill the holes to mount your parts. The Game Gear plastic is thick so use 2x8mm bolts to secure each board. Anything that passes through the Game Gear case plastic will need at least 2x8mm, but 2x5mm will be enough to get through the perfboard. The Raspberry Pi needs about 6.5mm, as does the USB A Riser, so try and get something close to that for the standoffs to mount them. When it's all done, the back will look something like this.