This section comprises hardware upgrades that you can obtain for your Chroma. Many of the MIDI retrofits have been discontinued, but are often found along with instruments for sale or auction.
A centrepiece of the community created and activity supported by this site, the incredible work of David Clarke [21030085++] and Sandro Sfregola  has after more than two decades resumed progress on the Chroma hardware and software platform. The CPU Plus (CC+) upgrade is a significant step forward in terms of capability and reliability for the instrument. Ordering information and user documentation is available here. The CC+ builds on earlier work on the Expansion Board project.
Luca Sasdelli  et al. offer a Switching Power Supply Unit Replacement Kit to address perhaps the Chroma's weakest design point. Chris Borman [21030194+] has begun production of a Pressure Sensor Kit,
Randel Osborne  put together a large 97-knob physical controller for the CPU Plus-equipped Chroma. On this site, see Chroma Enabler, February 2010; and Enabler Update, April 2010. Randel also has a dedicated page on his site.
The Chroma predated MIDI by only a year or so. The instrument probably died in part because it came out just prior to the Yamaha DX7, which in addition to being much cheaper also had MIDI built in. The Chroma's usefulness these days is greatly enhanced by a MIDI retrofit, although the cassette interface still works in a pinch on most of the units. This section provides information on all of the MIDI kits that are — or have been — available for the Chroma.
CPU Plus (CC+)
This custom CPU board upgrade mentioned above, features an optional Syntech-compatible MIDI interface with additional capabilities including a dedicated Thru socket; individually mapped custom controllers accessible over MIDI; local off mode; activity LEDs; and, as it is installed in the blank slot on the back panel, a freed-up Chroma port (for use with an Expander, Apple II computer, or knob box).
This is currently the only newly available MIDI interface for the Chroma. All details, along with order information and user documentation, are available here. This is the first new MIDI retrofit produced for the Chroma in twenty years, and is already proving popular with enthusiasts.
This has long been the most popular MIDI box for the Chroma. It is also the standard interface, its sysex format required for a lot of the software that supports the Chroma. Ken Ypparila, who ran Chroma Cult, continued to produce it after Syntech went out of business in the mid-1980s, and made another batch of Syntech kits as recently as 2001.
This was a MIDI box for the Chroma produced by JLCooper Electronics in the 1980s; it was relatively common and from time to time turns up with a Chroma for sale. Carter B. Horsley  writes that it "worked only for receiving MIDI data but I was told that it would also send MIDI if I got a new transistor." [February 1999] Christopher Now writes, "I had the JL Cooper and it never worked reliably." [March 1999]
The German company EES Technik für Musik produced this retrofit. They wrote, "The EES CHROMA MIDI CHROMA Interface is still available (but not sold very often since two years :-)). It is an external interface with MIDI In and Out, selectable for Mono-Mode and 1-2-3 channel Multimode. It supports most of Chroma data including memory dumps for single sounds and all Chroma data (SysEx). More details: Lever 2: Pitch; Lever 1: Modulation; Volume: Control 7; Switch Channel Offset: MIDI Channel 1-8 or 9-16." [March 1999]
This English company offered a MIDI interface for the Chroma. In June 2002, Kenton informed their US dealer that as of December 31, 2002, they would cease production of all MIDI retrofit kits. They kindly provided a copy of their self-install documentation for this site; the user manual is also available here. [January 2005]
A German company (jellinghaus.de) that apparently offers or offered a MIDI upgrade kit for the Chroma (I e-mailed them for more information but never received a reply). Thanks to Malte Rogacki  for this information. [February 1999] Ulrich Behrenbeck  writes, "I read something about [Syntech/Chroma Cult] <-> Jellinghaus problems" — presumably related to sysex incompatibility. [March 1999]