The Yamaha SY85 is fitted with a good quality semi-weighted keybed with aftertouch capability. This makes it ideal for use as a master keyboard in the studio. The only flaw is that the note velocity range is somewhat limited.
The velocity range of the keyboard never reaches the full velocity value of 127 no matter how hard the keys are hit or what velocity curve is selected in the main setup menu. It will rarely register a velocity value above 115 during normal use.
When using the SY85 as a master keyboard this velocity limit can restrict the voices on slave units and soft-synths as they may need the higher velocity values to produce the full range of sound.
This modification to the SY85 will alter the keyboard velocity range to allow the player to reach the highest velocity value of 127.
If you choose to do this modification be aware that it is at your own risk – I take no responsibility for any damage you may do to your SY85. It is assumed that the person doing the modification is experienced with working with PCBs and that full antistatic precautions are followed.
Key Velocity Explained
The velocity value of a keypress is measured by two switches. When a key is pressed the first switch is activated near the start of the key travel. When the key is near the end of travel the second switch is activated. The time between the two switch activations represents the velocity value. If the key is pressed softly the time between the switch activations will be longer. If the key is hit hard the time between the switch activations will be shorter.
The SY85 has a dedicated CPU (called the PKS CPU) that measures the time between the switch activations and converts this into the velocity value of the keypress.
The PKS CPU uses an external 8Mhz clock signal as its master clock. If we were to use a slower clock signal then the velocity values it produces would be higher as the timing between the switch activations would appear faster to the CPU.
This modification disconnects the PKS CPU from the normal 8Mhz clock signal and connects it to a slightly slower signal supplied from an inexpensive clock IC. This IC will produce a 7.3728Mhz signal – about 92% of the normal clock speed. This speed difference is enough to get the velocity range up to the magical 127 value.
Show me the Mod!
If you have read down this far and are still interested in this modification then take a look at the full modification details as a PDF file using the link below:
This PDF file goes into more detail about the modification and gives the vendor and part code for the crystal.
After a quick look at the schematics for the SY77 and SY99 it appears that this type of modification could also be easily applied to these two units. As I do not own a ’77 or ’99 I cannot verify what the results would be and if there are any unwanted side-effects of the mod. In the time span since I modded my SY85 (2+ years) there have been no ill-effects at all 🙂