Introduction:

Several of the Yamaha SY series are fitted with keyboards with Aftertouch capability. Aftertouch allows additional sound control by using finger pressure on the keyboard after the key(s) have been struck and held down. This pressure is translated into a ‘controller’ value that can be used to adjust Pitch / Modulation / Expression etc.

The type of Aftertouch used on the SY series is called Channel Aftertouch. This is the simplest type as it sums the pressure from all the keys held down to one single value. There is a more complex type called Polyphonic Aftertouch where each individual key has an Aftertouch value, but this is more expensive to implement.

As with most controllers the MIDI value generated will be between 0 and 127. Low pressure on the keys will generate the lowest value (0), and maximum pressure will generate the highest value (127). If you press down on a single key it will be more difficult to generate high values than pressing down several keys at the same time.

Although this article describes the Aftertouch circuit in the SY85 many other synths use a similar circuit inside the unit to detect Aftertouch.

Aftertouch Adjustment:

Disclaimer & Safety Notice:
If you choose to do this adjustment be aware that it is at your own risk – I take no responsibility for any damage you may do to your SY unit. It is assumed that the person doing the adjustment is experienced with electronic devices whilst under Mains Power.

The Aftertouch detection circuit is adjusted at the factory before the unit is shipped. For most people this adjustment is acceptable for everyday playing. There are no Aftertouch adjustment settings available in the synth’s utility menu so cannot easily adjust it.

Over time the Aftertouch strip beneath the keybed can age and wear causing the factory adjustment to drift. This can cause the Aftertouch to be over sensitive, not sensitive enough, or non-functioning unless the keys are pressed really hard.

Inside each SY unit there is a circuit board where the Aftertouch can be adjusted.
When you open your SY unit you will see a circuit board similar to the one in the photograph below:


This is the PCB from the SY85. On this PCB there are two adjustment trimmers:
Offset Adj
Gain Adj

The Offset Adj trimmer sets the Aftertouch dead zone. Ideally this will be set so there are no Aftertouch events being generated whilst the keys are idle. If the offset is set too low then Aftertouch events will not start to be generated until a lot of pressure is on the keys. If the offset is set too high then Aftertouch events will be generated without any pressure on the keys at all.

The Gain Adj trimmer sets the sensitivity of the Aftertouch. If the gain is set too low then a lot of key pressure will be needed to generate the high Aftertouch values. If the gain is set too high then the Aftertouch will max-out to 127 with very little key pressure at all.

Before adjusting either trimmer make a mark on the trimmer & PCB to indicate where it was originally set. This will allow you to undo any adjustment you make.

To monitor the Aftertouch values use a utility like MidiOX that can show Midi events in realtime.

The adjustment is now down to your personal taste. It is advised to try small adjustments from the Factory setting first.

Ideally the Offset Adjust should be set so Aftertouch events are only generated with a light touch on the keys.
The Gain Adj should be set to show the full 0 – 127 range of values using the key pressure you are comfortable with.

After the adjustment try playing some of your favourite patches to get a feel for the difference the adjustment made. You may wish to do this for a few days before closing the unit back up.

For reference here is a photo of the Aftertouch Circuit in the SY85:


//END