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Pressure Sensor Retrofit Kit

By Chris Borman [21030194+]

Polyphonic keyboard pressure or after touch, along with key velocity, offers the ultimate in keyboard synthesizer modulation, potential sonic nuance, and articulation. Monophonic after touch operates similar to a modulation lever or wheel, allowing you to play notes and modulate with one hand. Polyphonic after touch lets you modulate each note individually, similar to string players. I never understood why so few synthesizers have it, but now you can have it in your Rhodes Chroma!


Every Chroma has the polyphonic keyboard pressure sensor software and interface hardware, but few units have the actual factory pressure sensor installed. The CC+ CPU also supports the keyboard pressure option. This kit contains the hardware required to complete the Chroma keyboard pressure option. Installing the Chroma Pressure Sensor (CPS) is not difficult. Just align and peel-n-stick onto the bottom of the damper bar, then plug the CPS cable into the Chroma's I/O board J22 socket.

Order Info

Price for the CPS Kit for ChromaTalk members is USD$345 + $15 Shipping for the contiguous United States and $30 for international (shipping and handling is subject to change). Lead time is typically 3-4 weeks after order. These kits are built to order and do require a significant cash outlay to cover the initial FSR (force-sensing resistor) and PCB (printed circuit board) production cost. I keep a running wait list and provide status to the group from time to time for subsequent builds. Just send me an email to work out the details and I will invoice you through PayPal. Note: Lead time may be impacted by enough interested folks to cover pre-paid production runs of approximately ten.


Remove the AC power cable. Unscrew and remove the rear cover and unscrew and open the hinged Chroma control panel (see Service Manual: Disassembly). Remove enough connections to enable you to flip the control panel up and over to rest on a soft towel on the rear area of the Chroma to allow unobstructed access to the keyboard action damper bar area.

Use a fine indelible ink marker to uniquely and visibly mark every connector and mating plug with a matching identifier. I wrote the 'J' number on each mating part on the side facing me. This helps to re-install each cable in the correct location and orientation.

The CPS sensor array mounts on the bottom of the damper bar, centered at the contact point of the lead key weights. Rather then try to scrape off the original felt, It is easier to flip the damper bar over so the felt is on top, giving you a clean bottom surface for the CPS Peel-n-Stick Adhesive. Lightly sand this surface followed with a light alcohol wipe to ensure good adhesion. Also, since the damper bar has been flipped, check positioning and that the bracket screw holes line up!

Note: The Damper Bar, Brackets and Key Weights are all hand assembled components and may deviate positionally from unit to unit. My Chroma's action had a single key weight mounted 1/4" forward. It was a simple matter of repositioning it.

Note: Don't press the keys hard without the Damper Bar in place. Light playing is OK, but a hard press will push the end of the key action beyond the 2nd velocity leaf switch!

Temporarily slide the CPS Sensor array underneath the flipped damper bar and center it Left-Right on the keyboard action. Mark the damper bar at each end of the CPS sensor to indicate or frame this left/right centered position. Look where the Key Weights contact the sensor. Just the rear edge of each Key Weight contacts the sensor. Now position the CPS Sensor array fore/aft so the contact points are centered on the felt strip. Mark to frame this orientation as well.

Note: Dry fit it a couple times before exposing (removing the blue tape) and committing the sticky stuff!

With the Damper Bar/CPS assembly in-place, check vertical height relative to the Key weight contact point. The CPS Sensor itself requires less then .01" of travel for full deflection. Along with centering the key weight contact points on the Sensor Array, vertical positioning is the final critical adjustment. There is a fine line between limiting operation of the second velocity leaf switch and not getting enough travel for full deflection of the CPS Sensor. It should not be much different from the original position and just takes a little trial and error to get it just right!

Place the front panel back into the hinges and reconnect all the cables including the flat ribbon cable from the CPS Mux board to J22 of the Chroma I/O board. Double check all connections, especially the GND (Green wires)! Close but do not screw down the control panel.

Note: The Multiplexer Board is high enough where back edge of the front panel may hit it during opening/closing. Just lift up a bit on the Front panel during the hinge motion to clear it!

Power up the Chroma and check that the synthesizer is working OK. Pick a suitable patch and enable Pressure (Set Split 35) [e.g. in Sweep Rate Mod, Sweep Amplitude Modulation, Envelope 1 or 2 Attack Modulation, Envelope 1 or 2 Decay Modulation, Pitch Modulation, Wave shape Pulse Modulation, Filter Modulation, and/or Amplitude Modulation]. Check that each key provides a consistent pressure modulation. The potentiometer on the CPS Mux board adjusts the sensitivity of the pressure sensor to your playing preference. Crank it all the way up at first while you are adjusting CPS/Damper Bar height. Hinge open the front panel for easy access to the pot. (See previous note!)

The following photos show the approximate placement of components on the damper bar.

CPS on Damper Bar

CPS Front View

CPS Bottom View

CPS Installed

Mux Board Close-Up

Flat Ribbon Cable Orientation

Refer also to Luca Sasdelli's [21010226] CPS - An experience, and RJ Krohn's [21030455] fully complete w/ the pressure sensor; installation notes.

Special thanks to my friends Franklin Eventoff and his company Sensitronics LLC for providing the knowledge and assistance in producing the FSR sensor array for us and of course Chris Ryan [21030691] for developing and maintaining the Chroma website!

CPS Circuit Theory of Operation

The CPS consists of two identical hybrid FSR/PCB sensors. One assembled and oriented (flipped over) in a mirrored configuration. The two sensors are glued to a wood stiffener and join at the Multiplexer- Signal conditioning circuit board. The sensors are covered with a Felt layer to reduce key click, soften impact (just like the original Damper Bar felt) and improve sensor performance. On the the top layer of this sandwich is a pre-applied Peel-n-Stick Adhesive to secure the CPS assembly in place.

9 connections connections required. 5 data lines, the variable pressure voltage returned to the Chroma, +5V, +12V and GND. Chroma Data lines KS0-5 are used sequentially to select key pressure sensor 1 to 64 in unison with the currently pressed key(s). PRESS is the 0-5V pressure reading returned to the Chroma. Pressure on a key causes the current to increase though its sensor which is converted into voltage by a I-V Op Amp circuit with sensitivity adjustment that is read by the Chroma ADC for translation into local polyphonic modulation and MIDI polyphonic pressure commands.

U2 is used by Chroma Data KS4-5 to select 1 of 4 Multiplexer-Sensor Array pairs. Chroma Data KS0-3 is used by each multiplexer to select sensor 1 of 16 of the paired sensor array. Each sensor is supplied with 5V which is routed through the sensor and back through the selected multiplexer circuit to U1. U1 is a Current-to-Voltage converter with R1 as a sensitivity adjustment. The 12V line is supplied to the Op Amp circuit for improved PRESS I-V conversion.


CPS MUX Schematic

CPS Sensor Array PCB

CPS Sensor Array Schematic