As promised, here is a GetLoFi take on modifying GameBoy DMG-01 with pitch control via the LTC1799 Precision Module from the GetLoFi Shop. The process to replace a fixed 4.19 MHz crystal with a variable digital clock is pretty strait forward, however we will not be responsible for any damage to your hardware, so please do this at your own risk!
First you will need some assembled LTC1799 modules from the shop. You will also require an original Nintendo Gameboy, note that it is possible to modify Gameboy Color and probably Pocket however Gameboy Advance and SP currently do not work.
Open up the Gameboy by removing 6 screws and gently remove the top half by unplugging the ribbon cable. To make room for the potentiometer take out the speaker by clipping yellow wires. Carefully drill an appropriate size hold in the speaker grill from the inside. The legs of the potentiometer may need to be bent upwards, DO NOT use a hard surface to do that, the entire assembly may snap off! Use pliers to gently bend them. Attach one wire to the center and one to an outer leg for connecting to the pads on the LTC board. The value of the potentiometer may vary depending on your application. For drone-like sounds a 500k or larger may be good, but if you just want ~50% BPM adjustment try a 50k.
To wire up the bottom half start by cutting the lower trace of the crystal, no sense in desoldering it, this also leaves the option to restore the connection. Take a closer look at the diagram before doing this. DO NOT cut any other traces around and check to make sure its completely cut with a continuity tester. Connect the output of the LTC Module to the upper leg of the crystal as indicated in the diagram. This is where the clock signal will be injected into the CPU. Use thin wires cut to length, if they are too long or too thick closing the case will be a problem.
Next connect to the Ground point to the LTC Ground pad, then Positive point on the DC Jack to the +5 volt pad on the module. Refer to the above diagram once again. The final step is to install a Trim resistor on the LTC board. Cut the trace between two unoccupied square pads and solder in a 10K resistor. This will prevent the Gameboy from crashing when the CPU frequency is too high. Reattach the top half of the case via the ribbon cable and turn the system ON, because the speaker was removed you will not hear the tone unless its plugged into an amp. Adjust the potentiometer to make sure the clock modification is working.
Once everything checks out, neatly arrange wires and slide the module into the groove on the right side of the battery case. Make sure there are no wires right under the potentiometer or the system will not close. Reinstall the screws and test the system, at this point you are done!
More modifications will be posted very soon including: Pro Sound and Backlight. Please Stay Tuned!