Basically, what I am doing is adding an audio out jack and also a bigger, sturdier telescoping antenna. This is a great improvement on the original.
Step 1: Go Get Stuff
Gakken Theremin Kit (from Make)
A power drill
A mini screwdriver set
A mono jack with switch
Something with a telescoping antenna
A solder setup
Some extra wire
Step 2: Make the Kit
Alright, let's now assume that you do.
Step 3: Open It Back Up
First thing is first, we need to open that theremin back up.
Remove the screw in the bottom and take the casing apart.
Step 4: Drill and Install
Once you have your hole drilled, mount and fasten the jack in place.
Step 5: Wire It Up
First wire SP- (the wire on the right) to both the audio signal tab on the jack and the minus tab on the speaker (where it was just connected). Or, to put another way, wire the audio signal tab on the jack to SP- and the minus tab on the speaker.
Then wire the SP+ wire (the wire on the left) to the tab that connects to the big rounded metal bit on the back of the jack. Wire the + terminal on the speaker to the only other free jack (the tab that connects to the the little metal switch on the inside of the jack). This will turn off the speaker (break the connection) when someone plugs in an audio cable.
Step 6: Get a New Antenna
If it does not have a wire and piece of hardware on the end that will clamp nicely under a screw, you may want to attach one.
Step 7: Attach the Antenna to the Case
I drilled a small hole in the side and held it in place with a spacer and a machine screw.
Step 8: Wire the Antenna
Step 9: Close the Case
Also, chop off any part of the plastic inside the case that may interfere with it closing.
Once you have done all of this, close the case back up and reassemble it by inserting the single screw back into the bottom, putting in some batteries and closing it back up.