How to Circuit Bend a Yamaha PSS 140

A guide showing you how to circuit bend a Yamaha PSS 140.

** Since writing this blog post I’ve made a free sample pack and a free Ableton Live pack which you can download here Free Ableton Live Pack #001 Circuit Bent Yamaha Pss 140 **

In this article I’ll show you how to circuit bend a PSS 140 Yamaha Keyboard to include: 

x Data line switches 
x Patchbay 
x pitch bend modification (lowers pitch). 

You will need:
Yamaha Keyboard PSS 140
Solder
Soldering Iron
Wire
SPST Switches
Banana Plugs

First a quick explanation of the Yamaha keyboard & the circuit bend, then on to the how to guide.

The Yamaha PSS140

The Yamaha PSS series offer us a range of cheap keyboards to start circuit bending. You should be able to pick up a PSS 140 for about £15 on eBay.

The Yamaha PSS140 features:

- 100 presets
- drum pads
- Auto accompaniment
- “rhythm section’

The rhythm section has a few beat types such as Waltz and Disco and the presets generally sound terrible! With some bending of the data lines we can create weird glicthy beats, odd FM breakdowns, blips and mayhem! Rather than creating a playable instrument this bend creates a complex sonic pallette ideal for sampling.

The Bend Explained

You may have read about this bend before? This bend is the “Data Line” bend, because we will be re-routing the data sent to the sound chip. We will remove the connections between two main chips on the circuit board, the FM synthesis chip and the CPU chip.

We do that by wiring them to switches. This breaks the synthesis data sent to the sound chip, creating weird and mis-synthesised sounds.

We will cut 8 of the data lines from the CPU to the sound chip and wire them to 8 SPDT switches and 8 Banana plugs. This allows us to turn the data flow on & off and re-route it.

When all the switches are “ON” and no wires patched then the keyboard will work as normal. The “OFF” settings create random alterations to the presets and rhythm sections. You can create timing changes to the beat, mixes of presets, glitches and FM “noise”. The overloading of the sound chip can create some very harsh noise and often crashes the keyboard (a LOT). Record every play about you have, as it is very hard to recreate the sounds!

As an extra feature to the data line bend, I have also included a pitch bend mod on this build. This allows you to pitch the keyboard down by around -1 octaves. Unfortunately this doesn’t work for the drums! This is done by connecting PIN 16 of the CPU to PIN 11 of the voice chip.

Yamaha PSS140 Circuit Bending Guide

OK, so on with how to do it. You will need:

1 x Yamaha PSS 140

9 x SPST Switches
8 x Banana plugs (2 different colours)
Coloured Wire ( 8 Different colours)

Soldering Equipment
Drill bit / Craft Knife
Electrical tape / Zippers

May need replacement screws
Multimeter (not essential)
About 1.5 hours?

Open up the Keyboard. Keep the screws in a small pot for reassembly!
You can detach the battery cable if you don’t plan on using batteries. I removed the cable in this build as the battery springs had corroded and I wasn’t going to replace them.

3. Once the PCB has been removed decide on a location to place your switches and banana plugs. I chose the top left of the keyboard.

4. Drill out 8 holes for your SPST (put these lower down so there is clearence space for them)

5. Drill out 1 hole for the pitch bend switch – I house this to the top right of the speaker.

6. Drill out 8 holes for the banana plugs.

7. House the switches & plugs. Ensure that the switches are all the same way and that you have two seperate colours for the patch bay.

8. Cut the Data line connection between the CPU and the FM chip. Use the drill bit / craft knife to cut the traces from CPU PINS 9 -16 to PINS 1-8 of the soundchip. The picture on the left marks the traces to cut in red. Make sure that the connection is completely broken. (You can check using your multimeter in continutity mode).

9. Now we are going to solder wire from CPU PINS 9 -16 to the SPST switches. Prepare 17 lengths of wire from 8 different colours. The wire will need to be long enough to reach from the CPU PIN to the SPST switches. We want:

8 x length varied colours
8 x length (same colours as above)
1 x length (any)

This will leave you with 8 pairs of coloured wire and one extra. Trim back to show a small amount of wire and pre solder a small amount of solder to one end. This will help create a better solder connection to the PCB.

10. Locate PINS 9 – 16 on the CPU, using the presoldered end, solder the wire to them. I would advise soldering on the underneath of the PCB rather than directly onto the components.

11. Gather the wires and tape / zip them together. You can now feed them to the other side of the PCB towards the switches. This helps tidy up the wiring!

12. Now we are going to wire up the Sound chip. Locate PINS 2 – 7 on the Sound chip. These are 6 of the 8 PINS we are going to solder. The other 2 connections are made to the chip via jumper wires. They are numbered 8 & 9 on the picture below. Grab the other 8 x lengths of coloured wire and solder as follows:

Sound Chip 9 (Jumper Wires)	Same Colour wire as CPU PIN 9
Sound Chip 8 (Jumper Wires)	Same Colour wire as CPU PIN 10
Sound Chip PIN 2	Same Colour wire as CPU PIN 11
Sound Chip PIN 3	Same Colour wire as CPU PIN 12
Sound Chip PIN 4	Same Colour wire as CPU PIN 13
Sound Chip PIN 5	Same Colour wire as CPU PIN 14
Sound Chip PIN 6	Same Colour wire as CPU PIN 15
Sound Chip PIN 7	Same Colour wire as CPU PIN 16

13. Solder the last wire to Sound Chip PIN 11 – this will be used for our pitch bend mod.
Tidy all these wires together and feed underneath the PCB. (Unfortunately the picture to the left doesn’t show the pitch bend mod wiring).

14. Now we solder the wires to our SPST switches and banana plugs. Before we do that, cut 8 lengths of wire for connecting the banana plugs to the SPST switches. We are going to solder the banana plugs & the SPST at the same time.

Take the pairs of coloured wires and connect them to the toggle switches – Don’t solder yet! Ensure that all the wires from the CPU go to the middle terminal of the switch & the corresoponding coloured wire from the sound chip connect to the outer terminal. This will make it easier to identify which switches are “ON” or “OFF” and easier to return the keyboard to its default state.

15. Now we connect the patch bay to the SPST switches. We will be creating an 4 x IN and 4 x OUT patchbay. One row of “IN” and one row of “OUT”. Take 4 x wires and attach them to a row of banana plugs. Connect the other end to the middle terminals of 4 SPST switches. (You can choose which SPST switch you want to connect to). This banana plug will now receive data from the CPU which can be patched via the “OUT” on the patch bay.

Now take the other 4 x wires and feed into the outer terminals of 4 SPST switches. Then link to the second row of banana plugs. This will be our “IN” on the patchbay as this sends the data to the Sound chip. When we insert patch cables into the banana plugs we complete the circuit and re-route the data. This will happen whether a SPST is switched “ON or “OFF”.

Before we solder these connections we need to connect CPU PIN 16 to the pitch bend togle switch. I did this by connecting an extra wire from the CPU PIN16 SPST terminal to the pitch bend SPST. The other terminal on the Pitch Bend switch is then connected to the SOUND CHIP 11 wire. Data will be re-routed the soundchip when the SPST is “ON”. This will result in the lowering of pitch.

Once you are happy all the wires are in place, solder everything.

Once the solder is set, turned all the switches to “ON” and check to see the keyboard works. Experiment with the switches to ensure all connections have been soldered correctly.

Re-assemble to the keyboard and start to experiment!

Happy Bending!!

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Free Ableton Live Pack #001 Circuit Bent Yamaha Pss 140