Finding Bends

Traditionally, bends have been found by using a pair of jeweler’s screwdrivers connected with a wire. Holding both screwdrivers and playing the instrument often requires more hands than I have. Instead, a pair of insulated alligator clips connected with a wire allows hands-free connections.

To start, I’ll be focusing mainly on the sound chip, a Yamaha YM3812.
An integrated circuit (IC, or ‘chip’) is like a miniature computer. It interfaces with the rest of the circuit board with a series of wires or other electrical contacts (called pins). Pins are numbered counter clockwise from the top left of the chip. Pins are generally dedicated to one of three purposes:

Power. ICs need power just like the rest of the circuit board. There will be at least two pins that carry power to the chip (one ground and one ‘high’ – 3 or 5 volts.) In datasheets, these are usually denoted as GND and VSS, V5, V3, V+, etc. In general, shorting a power pin to another pin will result in a crash or at least cancel the output of that pin. According to the YM3812 data sheet, the top left pin is VSS, the bottom left pin is GND.

Control signals. Often broken up into Clock, Interrupt (IRQ), and Command. These are used to synchronize the chip with another chip, or pass commands between them. Shorting these usually results in highly unpredictable behavior, including crashes and partial lockups.

Data. Usually in groups of 8, data pins are used to transmit digital information in and out of the chip. In many cases, the same pins can be used for both input and output, and the YM3812 makes use of this. On this chip, pins 10,11 and 13-18 are data pins. Shorting data pins is the most common way to get relatively stable sound modifications.

An easy way to try all combinations of pins
Start at the top left pin. Attach one clip to this pin. This is the ‘fixed’ clip. Using the other clip, try all the other pins in sequence.
Move the clip from the top left pin to the next counter-clockwise pin. Then try every other pin, leaving out the top-left pin, since you’ve already tried 1 shorted to 2.
As you progress, you can skip every pin the ‘fixed’ clip has already tried.

Up top, we have the battery holder (will cut and re-attach the wires later).

On the left and right are the speakers.

Between the speakers are a pair of circuit boards, one brownish and one green, one mounted above the other with standoffs and screws.

At the bottom is the backside of the keys, conveniently colored brown as well.

In fact, both brown boards are just key/button mounts (one for the keyboard, one for the control panel) with pullup resistors and ribbon cables.

This leaves the main green board, which is the one most interesting to us.

Gaining access to the main board

First, we have some shielding/padding to get through.

This is here to prevent interference from below the device from affecting the main circuit board.

For now, I’m removing it, but will add it back when I’m done.