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 127 velocity value 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 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 microprocessor (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 the 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 127 value.
Note: There is nothing special about the 7.3728Mhz value. I chose this as it is a very common clock speed used for Microcontrollers that use a serial port. Serial ports use common baud rates (1200 / 9600 / 19200 / 115200 etc) that need the 7.3728Mhz clock speed as a master clock so the baud rates are exact. Any clock speed from 7.0 to 7.5 Mhz should work for this modification. The clock IC must run from a 5V supply, and output a 0-to-5V square wave with a 50% duty cycle.
The Clock IC used for the modification was obtained from RS. The RS code is 767-5248
Mouser stock a similar Clock IC ECS-100AX-073. The link to this on the Mouser site is HERE
Opening the SY85
During this procedure all the internal voices, performances and multi-setups will be lost.
Backup your SY85 internal data before doing this modification!
The SY85 will need to have the bottom cover removed to allow access to the Main PCB.
!! Unplug the SY85 from the Mains Supply before removing the bottom cover !!
A detailed instruction for removing and refitting the bottom cover can be found in the Yamaha SY85 Battery Replacement Guide.
Although the Battery Replacement Guide shows the Main PCB being removed you do not need to do this for this modification. All you need to do is remove the bottom cover.
The photo above shows key locations for the modification on the Main PCB.
- The arrow on the big IC at the left is the clock input to the PKS CPU (IC12). This is Pin 3 on the PKS CPU. We will connect our new clock IC to this pin
- The arrow near the centre shows the trace we need to cut in order to isolate the PKS CPU from the original 8Mhz clock signal.
- The arrow on the far right shows the 8Mhz clock output pin on the Floppy Disk Controller. We do not need to do any modification to this IC.
Isolating the Memory Backup Battery
Isolate the Mamory Backup Battery before doing the modification.
The Memory Backup Battery supplies current to the memory circuits in the SY85.
When the backup battery is isolated the SY85 will lose all Voice, Performance, and Multi-Setup information. Make sure you backup all the internal data before isolating the battery.
Find the little black Jumper near the battery – this is CP1 in the photo. Remove this and set it on just one of the CP1 pins so as not to lose it.
Cut the PKS CPU Clock Input Trace
The photo above shows the location of the trace to be cut. Note the component references around the trace area to ensure you are in the correct location before cutting. This is best done with a sharp implement like a scalpel. Do not push too deep into the PCB – just enough to cut through the thin copper trace.
The photo shows the soldermask layer removed from the trace around the cut exposing the copper. You do not need to do this – just remove the small section of trace where the arrow is pointing.
Positioning the New Clock IC
This is the Clock IC that was used for the modification.
For this to operate properly it needs to tap into the SY85’s +5V power rail.
We need to locate the Clock IC somewhere near the PKS CPU where there is a +5V supply available. Luckily there is a location on the PCB that is ideal.
The Clock IC will need to be bonded to the PCB to keep it in place.
There must be an insulating layer between the metal body of the Clock IC and the PCB.
In this photo a layer of foam-based double-sided sticky tape was used to hold the Clock IC to the PCB and serves as the insulation layer.
This is the ideal position for locating the Clock IC.
Note the orientation of the device. It has one sharp corner and three rounded corners.
!! Very Important !!
The sharp corner of the Clock IC (Pin1) goes to the bottom left of the photo where “64” is printed on the PCB.
The arrows point to the PKS CPU Clock Input (Pin 3) and the Clock Output pin on the new Clock IC.
Connecting the New Clock IC
Now that the Clock IC is in position we need to make these connections:
|Clock IC Pin 1||No Connection|
|Clock IC Pin 14||+5V Input||Red Wire||Connect to Bottom Leg of C4|
|Clock IC Pin 8||Clock Output||Blue Wire||Connect to Pin 3 on PKS CPU|
|Clock IC Pin 7||Ground||Green Wire||Connect to Top Leg of C49|
!! DOUBLE-CHECK ALL THESE CONNECTIONS BEFORE CONTINUING !!
Ensure that all the connections to the Clock IC are correct and that there are no short-circuits or solder splashes around the modification area.
If you have a multimeter check that there are no shorts between the metal body of the Clock IC and Pins 8 & 14 on the Clock IC.
Re-connecting the Memory Backup Battery
Before replacing the bottom cover re-connect the Memory Backup Battery.
Place the little black Jumper back on both pins of CP1 as shown in the left of the photo above.
Replacing the Bottom Cover
Replacement of the bottom cover is detailed in the Yamaha SY85 Battery Replacement Guide.
Reloading the Factory Sounds
When the SY85 is powered up you may find all the Voices and Performances are lost or corrupt.
You can download the Factory Settings Disk & SYSEX data for the SY85 from the Sector101 Downloads page at:
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