Preparing a ribbon cable

In the past, I’ve done several bends where each bend gets a wire to the pin. Instead, I’m going to attach a ribbon cable to each side of the sound chip. Conveniently, ribbon cables are available for cheap – I’m using a cable for a floppy drive.

First, cut off the connector. Then, count out twice as many wires as you have pins (I have 12 pins per side, so I’ll be using 24 strands of the cable) and tear off the extra. It should tear easily.

Next, strip the odd-numbered wires (counting the red-marked wire as 1) and cut the even-numbered wires shorter. We won’t be using the even-numbered wires; they’re just there to act as a spacer. Don’t strip off too much insulator; you only need a couple millimeters of bare wire to solder a joint and keeping it short will reduce the chance of (unwanted) short circuits.

Now the cable should line up with the chip, with one bare wire for each pin. Solder each wire to a pin, starting with the wire on each end. Doing them in this order will help keep the entire ribbon aligned as you work.

Do the same for the other row of pins on the chip.

It’s quite likely you’ll be attaching wires to pins you don’t intend to bend with, or maybe aren’t even used by the chip. Attach them anyway, as it will strengthen the overall connection. Later, you can add a blob of hot glue to strengthen it further and reduce the chances of a short.

At this point, it’s a good idea to re-checked the bends you found earlier, to ensure that your soldering job was good. It’s much easier to do this now than try to find where something went wrong later.

Connecting the breadboard

For a breadboard, I’ll be using a perforated circuit board with copper pads around the holes. These are fairly easy to use and cheap. For a good tutorial on building circuits on a perfboard, check out “Soldering a Perf Board”.

To connect the ribbon to the perfboard, strip or cut the same wires you stripped and cut on the other end of the cable. Twist the bare wire and tin it (put a tiny bit of solder on it to hold it together) and then insert it through the bare side of the board. You may need to separate the wires a bit more on this side to get all of the wires to fit. A little solder holds it in place.

A view of the perfboard and ribbon cable from below.

Adding switches/knobs/etc

Now I’m going to add a switch to control a bend I found earlier. Pins 16 and 17, when shorted, create a wonderfully crunchy overdrive effect.

Here, I’m taking a double-strand of speaker wire, and running the ends into the holes in the perfboard, next to the wires coming from pins 16 and 17.

On the copper side of the board, I bent each wire over and soldered it to the corresponding wire from the chip. Here’s one wire finished.

Then I soldered the wires to a toggle switch.

Since I might have 50-something switches, it’s a good idea to label them.

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.

Types of Bends

It’s a good idea to take notes during this process. A 24-pin chip has 276 possible bends.

There are six types of bend effects found on the sound chip:

1. Pause – these prevent sound output while shorted, but allow sound to resume normally when released.
2. Crash – these cause the chip to hang, refusing further input. Some crashes will be silent, others will play a sound until manually reset.
3. Reset – These will reset either the chip or the entire device.
4. Control mods – these result from messing with the control pins of the chip. Often these will be like pressing a key or button, but may also result in unpredictable sounds.
5. Data mods – The main source of stable sound mods, these often result in totally new sounds that can be played normally.
6. Random mods – Unpredictable results every time you make this connection. Ghazala used these to make his Aleatrons.”

Most of these can come in two flavors – momentary and permanent.

Momentary effects only happen while the short is made. For these, you would want to use an on-off switch (such as a rocker switch or a toggle) to turn the effect on and off. Permanent effects stick around after the short is released. Many of the ‘pause’ shorts also impart permanent effects. Often, the effect can only be removed by resetting the device, though some effects can be turned off again by making another short (either the same one or a different one).

Adding a Reset Switch

Adding a reset switch.

Early on, I discovered a reset bend. Shorting the VSS and GND pins (top left and bottom left) on the sound chip resulted in the entire device resetting. During testing, it is often useful to have a quick way to reset the device, so I attached a pushbutton switch (momentary on) to act as a reset button.

If you don’t find a similar reset bend, you can instead install a switch between the batteries and the circuit board, or, if wall-powered, use a surge protector switch.

This is basically the same method used to add a control to an effect – a pushbutton switch in particular is useful for controlling a permanent effect.

Experimenting with a Resisted Short

Here I’m testing a crash bend by adding a variable resistor (potentiometer) to search for any interesting effects near the crash threshold.

At first, it may seem that pause or crash bends are useless. However, each of these may only be useless with 0 resistance. You can try each one with a potentiometer in series with the clips, to see if anything interesting happens with more resistance. Often there will be a very narrow range of resistances in which random effects happen; above this range, the bend has no effect, and below this range the crash/pause happens. If you find a range like this, it is very useful to have a multimeter handy!

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.

Identifying chips and key areas of the board

Removing some more screws, I obtained access to the top of the board. This was cumbersome, as the ribbon cables to the keyboard/button panels were not detachable.

Fortunately, we only need to have a quick look to see where the chips (Integrated Circuits) are (on the left) and where the power-related areas are (on the right). There’s even a chip with a heat sync, which I removed temporarily to discover it was a powered pre-amp. For the most part, any power-related area is off-limits for bending. While some bends can be made here with careful use of resistors and luck, the chances of frying a chip are too great.

The chips of interest, however, are all easy to see on this side of the board.

The CPU.

This is responsible for taking input from the keys and buttons, keeping track of the state of the keyboard, and talking to the other components.

Bends here are highly varied.

In fact, the model of CPU may vary from the owner’s manual, as is the case here.

The sound chip, a YM3812 by Yamaha, was so popular that it appeared in dozens of keyboards and sound cards including the AdLib, the SoundBlaster, and even merits a Wikipedia page.

A significant number of bends will be made here.

Some other chips (a D-Latch and possibly a demux).

These are mostly related to temporarily holding data from button pushes and translating hundreds of buttons and LEDs into addresses so the CPU can perform input/output to each without needing a pin for each.

There are rarely bends here, and most short circuits here will behave as though you pressed one or more buttons/keys.