Yamaha PSS 140 Data Line Bend.

"The Data Line Bend"
For the Yamaha PSS-140 and other similar FM PortaSound keyboards.

You may have read on the internet about an interesting bend sometimes referred to 'data line bend', which can be applied to Yamaha keyboards which utilize FM synthesis as their sound source.

This circuit bend is quite different to most other bends which are found by random experimentation, but can be very rewarding by unlocking a whole massive range of interesting and useful sounds beyond the usual 100 presets of the cheap Yamaha PortaSound range.

Circuit bending is a process which usually involves creating connections between two points on a board which are not meant to be connected; 'short circuiting'. This bend is different in the sense that it involves 'removing connections', though not permanently.

Most FM synthesis PortaSound keyboards (such as the Yamaha PSS 140) contain two main chips on the circuit board. A sound chip; which contains the FM synthesis micro circuitry, and a CPU which is basically the brain of the keyboard.

Every time you change a sound on the keyboard, say for instance from "Honky Tonk Piano" to "Funky Marimba" the CPU sends data to the sound chip with details of how to create the new sound.

The image below shows the path (yellow arrows), taken by the data on a Yamaha PSS 140 as it leaves the CPU through the pins marked in red and arrives at the FM chip at the pins marked in blue. (This picture shows the main PCB flipped over, you will have to remove it from the chassis to see this).



The basic premise of this bend is that we cut these eight data lines from the CPU to the sound chip, and reconnect them on the other side of the board (the component side) with wires and toggle switches. When all the switches are switched "ON" the keyboard will respond and play as normal and none of the preset sounds will be affected. However, by changing the position of one or more switches to "OFF", that data line will be disconnected and whatever data the CPU sends to the FM chip down that line when you change a sound will not reach it, changing the sound of that preset until you reset all the switches to "ON" and restart the keyboard.

The resulting sounds can vary in quality, from a noisy mess to interesting and usable sounds perhaps deemed by the engineers at Yamaha to be too strange and twisted for inclusion on what is essentially a toy keyboard. It is also worth noting that playing with the switches will also affect the built in rhythms, demonstraton and drum pads in interesting and unpredictable ways.

Of course, just as often as you discover a new and interesting sound, you will discover that a certain switch combination and preset change result in the keyboard crashing, sometimes silently (no keys will respond) or very nosily (a large amount of high volume FM noise blasted from the speaker). This is of course easily recovered from, by simply resetting the toggle switches and then switching the keyboard off and then on. Seeing as the Yamaha PSS-140 has a mechanical slide switch which can easily wear out with constant switching, we will also place a 'push to break' toggle switch in line with the power supply to the keyboard so that power can be taken away and the keyboard's memory reset in an instant.

Step by step guide.

Of course, the first step will be to open up the keyboard. The PSS-140's main circuit boards are contained in the top half of the keyboard, so once the screws are out, place the keyboard on a flat surface, keys down and lift the bottom away.

Remove the screws from the main PCB (the big circuit board you can see in front of you) and flip it over. You should be able to locate the data lines as shown in the picture above, but don't cut them just yet, just be aware of where they are. Place the PCB back in its correct place now because its time to get soldering.

When soldering to small chips like the CPU and FM chip of the PSS-140, I prefer to solder straight onto the pins of the chip, as there is less risk (though the risk is still present) of allowing solder to spread to other pins of the chip causing undesired short circuits. My solder skills are very lacking and a more experienced solderer may well choose to solder the cables to bottom of the chip.



First of all, identify the CPU, this should not be difficult as it is the largest chip on the board and is quite clearly marked as being a "Yamaha XE56BO" . In the picture above, I have highlighted the eight data connections which you will be attaching the new wires to, and already began to solder three of the new cables to the pins of the chip, working left to right.

It is always important to remember never to apply heat to the pins of the chip for too long as this may cause damage to the internals of the chip and even melt the plastic casing. It is advisable to use a heat sink (an alligator clip will do) attached to the wire just above where you are soldering to divert heat away from the delicate internal workings of the chip.

My preferred way to solder to the pins of a chip is to first melt a little solder on the pin, then re-melt this solder and attach the wire, keeping it as steady as possible until is it set in place. Remember, only a little solder is needed (you don't want it seeping onto the adjacent pins) and to never apply heat for more than 4 or 5 seconds, just enough to get the solder flowing.

After you have soldered your 8 connections to the CPU, you should have something looking a bit like this (though yours will most likely be quite a bit tidier). It is worth noting that I have stripped far too much plastic from the wire, especially on the connections further to the right. This is a bad habit I have, and you should only strip off as much as you need to make connection with the pin to avoid having connections touching.




Now that you have eight wires stretching from the CPU and hanging waiting to be connected to their toggle switches, its time to the same thing for the FM chip, but before we continue, consider labeling each wire coming from the CPU with a little masking tape at the end. Label them 1 to 8, starting from RIGHT TO LEFT (this is important, you can see what I mean in the picture below), so that we can easily recognize them and connect them to the correct pins on the FM chip (via a toggle switch of course).

With the FM chip we will take a slightly different approach. Six of the connections will be made on the top side of the chip (the pins facing the CPU), but two connections (7 and 8) will be made to the two silver jumper cables to the right of the chip, as you can see below.



As you can see, connections seven and eight are not made directly to the chip, but to two silver jumper wires on the board. These jumper wires do lead to the correct pins on the chip and it is simply more convenient and easier to solder the connections straight to these. Also notice that connection seven is on the right of connection eight. That isn't a mistake! It is how you must connect it. Also, you may notice that I have used some cable ties to ensure the cables do not get tangled or snagged. Make sure you label up your wires like you did with the CPU (this time numbering them from left to right, as shown in the picture above.)

The next step is to solder up your connections to your toggle switches, but first, try turning on the keyboard (wait till everything has cooled down after soldering) and check everything works. We haven't actually cut any data lines yet or made any new connections (just the means to do so) If your keyboard exhibits strange behavior at this point, it may be worth checking all your solder work for any short circuits (connections between adjacent pins or jumper cables) which may be causing the chips to behave strangely.

If all is working correctly, its time to drill the holes for your switches (8 toggles and your one 'push to break' reset switch). I found the best place for the switches is to the left of the main circuit board where there is a wide open space of plastic. Mark carefully where you are going to drill so that the switches will be equally placed and there will be enough clearance for the switch on the inside when the casing is reassembled. I used 8mm switches, and began by drilling small pilot holes with the drill set to a low speed to stop the plastic from cracking, then using a larger bit to expand the holes and fit the switches in.




Soldering up your switches should be easy, I recommend using miniature SPST (single pole single throw) switches for simplicity. Solder up the correct corresponding connections to each switch (e.g. cable 2 from the CPU will go on a switch with cable 2 from the FM chip). This should be easy provided that you labeled them when you were soldering. Make sure you connect all the switches in the same way, i.e. all CPU connections go to the middle terminal of the switch, all FM connections to the outside connection. You can do it which ever way around you like, just make sure they're all the same, so that you can easily reset them all to "ON".



Here you can closely see my wiring for my 'reset' switch (the circular switch to the right of the square toggles). From the switch module, (the small circuit board in the top right of the picture) you will see a red and white double wire which you can split down the middle. Connecting the reset switch is as simple as cutting one side of the wire (I chose red) and connecting each side of your cut to the two terminals of the reset switch. When the reset switch is pushed, the connection the switch makes to provide power to the board will be broken momentarily and everything will be reset to normal (providing that all your toggles are in the "ON" position).



That's almost everything! Check everything's still working (remember, the traces on the bottom of the PCB are still in place so the switches will should not be having any effect yet!). If you are getting strange results already, make sure that all your connections are the right way around i.e. cable 3 from the CPU to cable 3 from the FM chip. Also make sure you check all your solder joints for short circuits.

On to the final step! Cutting the traces. Flip the circuit board over again (gently! you don't want to damage any of your solder connections!) and locate the data traces shown by the yellow arrows earlier.

Take a craft knife, or any sharp precise instrument and carefully cut across the black line shown in the picture below. You should be able to carefully scrape off the green coating and reveal the gold coloured traces below. You want to carefully scratch away at these traces until they are visibly broken and cannot make a connection anymore. If they are not cut correctly, you may find that some of your new toggle switches do not have any affect because the trace is still active on the board, making the connection you are trying to break. BE VERY CAREFUL! DO NOT CUT ANY OTHER TRACES! You only want to cut the eight lines from the CPU to the FM chip.



Once this is done, double check all your connections, ensure there are no shorts and tidy up your cabling (you may need to carefully route cables so that the casing fits back together correctly).

Here is my finished product, I guess the switches could have been a bit neater, but I'm happy with the outcome!





Set all the switches to the "ON" position and turn the keyboard on! If you are unsure which position is "ON", set all the switches to the same position, and switch the keyboard on. If no sound comes out when the keys are played, switch it off again and reset all the switches to the opposite position. If the keyboard still makes no sound, go back and check all your connections, read over the instructions again and make sure you haven't missed anything!

With a bit of luck, your keyboard should still be functioning as normal with all the switches set to "ON". Flip a few switches, and then use the keypad to change the preset sound. Hopefully, the data will be corrupted due to some of your switches being in the "OFF" position and you will discover a fantastic new sound. If the keyboard decided to crash into a noisy white noise mess, just hit your reset switch and start again!

The whole idea of this bend is that once it is applied, it accounts for hours of exciting experimentation and hopefully the discovery of some interesting new sounds. Be sure to keep a note pad handy to record your findings so that you can recreate them and don't forget to play with the demo song and rhythms too!

Yamaha PSS-140s were very popular keyboards and come up quite often on eBay so keep your eyes out, but bear in mind that this concept will work on most FM synthesis PortaSounds with a little adoptation, just locate the data lines and the correct pins on the chip and you should get similar if not exactly the same results!