Skip Navigation.
Section 0

The Chroma CPU Plus (CC+)

by Sandro Sfregola [21010294] <>
and David Clarke [21030085++] <>

"The design and functionality [of the CC+] is simply incredibly beautiful. The Chroma was always one of my favorite synths, now it is without any doubt from another universe." – Christian Kleine [21030210]

"It is GIANT fun ... I only can recommend to everybody who did not already, to invest into this exciting board." – Werner Schöenenberger [21010114]

"I've been testing the MIDI in from various hardware sequencers and it seems like the timing and response is very tight." – Tom Moravansky [21030431]

Back in March 2005, with the presentation of the Chroma Expansion Board project we proposed a Do-It-Yourself (DIY) kind of upgrade for the original Chroma CPU board.

As Chroma users started to show interest and provide feedback, a reccurring comment was that there wasn't some kind of "kit" or at least a printed circuit board to help would-be users. It became clear that some people preferred to have a pre-built solution instead of a DIY kit.

In a later mail (about mid April) Dave raised the point that some components of the Chroma CPU had become difficult to find and proposed the development of a direct-replacement board for those cases where an original CPU board had been damaged or were no longer working properly.

As often happens, I was thinking about the same thing so we eventually decided to develop the board together. After a long rest in the summer, it took us about 3 months from the start of discussions to the completed, working samples you can see in the pictures. Work was co-ordinated on-line, with some items being done in Italy and some being done in Canada.

The original CPU board was designed in the early 80's with the digital technology and components available at that time. We designed the CC+ with current technology reducing the number of components and the power needed while augmenting the hardware capabilities with new and sometimes really "radical" features like the support of an alphanumeric display (patch and parameter names etc. like in later instruments).

We took into account the needs of a simple installation: in fact the new board is mechanically and electronically built to be a direct "drop-in" replacement for Chroma or Chroma Expanders.

The CC+ is not a DIY project: it is available pre-built and tested.

You will find the details below.

Inside the Chroma: full view of the installed CC+ board

Why Might You Want/Need the CC+?

Chroma owners had often reported difficulties with their existing CPU boards - generally due to acid damage caused by leaking memory back-up batteries. Acid damage can be quite insidious, as it can not only directly damage connections on the board but it can also affect the ability to later repair the card (as the acid affects the behaviour of the metal/solder on the board). In those cases where all the original acid was not fully removed - additional ongoing difficulties can result.

Some other users have encountered CPU-board problems that weren't related to battery damage. Many of the original parts - such as "S" series logic, and low power 512 byte SRAM chips - were common 20+ years ago but in the age of multi-gigabyte memory sticks are simply not stocked anymore. In many cases, the parts themselves are not even being manufactured and only New Old Stock (NOS) parts are available, often at unrealistically high prices. Users (and repair technicians) are finding it ever harder to locate the necessary replacement parts.

Still further yet, the CPU board appears to be a single point of failure for the keyboard with few (if any) spare CPU boards being made available. This has unfortunately meant that a Chroma with a dead (or missing) CPU board is often disassembled and sold for parts.

Even where the original CPU board is completely functional, there are still strong reasons you may wish to have a CC+. As outlined in the following section, the CC+ offers significant additional features that make it better and more flexible than the original CPU board.

How the CC+ is Better Than the Original

Having full control over the hardware and software (firmware) allowed us to ensure, by design, that the CC+ would improve on the original board.

The CC+ uses currently available parts and surpasses the original functionalities by also providing support for some of the most commonly used/desired 'add-ons' including MIDI and expanded program support.

The following highlights some of the specific improvements available via the CC+.

Feature Benefit
Lower Power The current design uses approximately 95% less power than the original design. Also the total AC input power of the Chroma is reduced by 30 % (with the original PSU).
More Reliable By using fewer parts in the design, the overall mean time between failure (MTBF) of the board is increased - making failures less likely. The decreased current draw of the board results in less voltage drop to the board, allowing components to work closer to their preferred voltage levels.
Cooler The lower power components allow the CPU board to run much cooler than the original, decreasing the overall heat created by the Chroma's electronics thereby allowing components to last longer.
Easier to Repair All integrated circuits on the CC+ are installed in sockets, allowing ease of repair should a component ever fail. Further, the use of sockets extends the life of the circuit board itself since all parts can be repaired/replaced without risking heat/solder damage to the cards or components.
Easier to Upgrade The full firmware of the CC+ is contained in one chip, thus any firmware changes, for instance to add new features, only requires the change of one chip (versus eight chips in the original Chroma CPU board).
Less Maintenance There are no batteries of any kind in the CC+, and hence there is no need to periodically check/replace the batteries.
No Acid Issues Since there are no batteries in the CC+, there is no possibility for failed batteries to cause damage to the system.
No Lost Patches The on-board non-volatile RAM has a 100-year data retention period, and so there will be no concerns over losing patches after a keyboard is stored for a long period of time.
Archived Configuration The full state of the machine (MIDI configuration, patch data, etc.) is stored in a single non-volatile memory chip. If desired, this chip can be removed and the full patch/configuration data will continue to be stored in the chip.
Multi-Bank Storage CC+ supports four banks of 50 patches each - for a total of 200 patches on-board. The additional banks can be selected through standard Chroma panel switches and can also be selected via MIDI.
More ROM Space The original program space (16KB) in the Chroma was full. There was no possibility for feature additions without removing other features. The CC+ doubles the amount of ROM space, providing the ability to add additional features.
More RAM Space Default storage capacity on-board has increased from 7 to 32KB.
Expandability Hardware elements have already been designed in to the board to assist in adding additional (future) hardware and software features without the need for further hardware changes to the CC+.
Service Mode A zero value is written on both the main and reference DAC, then the SYNC instruction is executed; the CPU stops, both address and data buses go into a high impedance state, minimizing radiated noise. This service assisting state allows for precise offset adjustment at the main DAC op-amp output and to check the offset at the reference DAC. (this is a switch-selectable feature).
Delayed Start-up To allow the electronics time to stabilize - a two second delay can be inserted during boot-up before calling auto-tune (this is a switch-selectable feature).
New Firmware features The CC++ has had additional features added to firmware to assist with operation (see section below).
MIDI Support CC+ natively supports MIDI.

The MIDI connections box (part of the optional MIDI interconnection kit)

When compared to the Original CPU plus a Syntech MIDI interface, the CC+ has the following improvements:

IN/OUT/THRU CC+ provides separate outputs for MIDI IN, OUT and THRU, allowing simultaneous use of the OUT and THRU ports (vs. other implementations which allow a single port to be used as either OUT or THRU).
Free Chroma Port The Chroma port remains unused - and so while MIDI is being used, the Chroma port can still be used to connect to an Expander, to an Apple Computer, or to a Knob Box.
Assignable CCs All controllers are accessible over MIDI, and each controller can be individually mapped. The mapping for the controllers can be edited directly on the Chroma, and can be sent via SYSEX for later retrieval and restore.
Activity LEDs MIDI activity is displayed by LEDs on the MIDI connector as well as via LEDs in the front panel display of the Chroma.
Local off The user can choose to configure the keyboard in a 'local off' mode.
Source Control The CC+ has full control over the MIDI code, so implementation changes are possible (if desired/needed).

The CC+ external connector (part of the optional MIDI interconnection kit)

Connection of the external MIDI box

Where it Can It Be Used/Who Can Use It

The CC+ was designed to be a drop-in replacement for the existing Chroma CPU board. No modifications are needed to the electronics or the hardware of the Chroma to use it.

The board mounts in the same location, with the same hardware and connects to the system with the same cabling. No soldering is required.

The CC+ will work in all Chromas and all Expanders, and maintains compatibility (i.e., full operation) with any/all existing MIDI interfaces which use the Chroma port (e.g., EES, JL Cooper, Syntech). Firmware has also maintained compatibility so that interfaces (such as the Syntech) will continue to operate properly, even alongside the built-in MIDI interface.

Newly Implemented and Future Firmware Features

The CC+ firmware implements all the features available on the previously released Chroma Expansion Board. The interface-related features include:

See also the discussion of MIDI-related features (and User Interface).

In addition to these pre-existing features, the CC+ firmware also provides multi-bank patch support, with on-board support for 4 banks. The banks are selectable from the front panel or from MIDI, and it is possible to copy patches into different banks.

In terms of future support, there is available RAM and ROM space provided so that the CC+ allows for unlimited ideas - only constrained by what the Chroma community might like.

A currently planned (future) upgrade is the implementation of patch names and the addition of a 20x2 alphanumeric display, to allow more useful display of parameter functions and choices.

Beyond that, the flexible infrastructure allows for additional items to be added. For instance, possible future enhancements could perhaps include:

How Can I Get a CC+?

After 10 years to continous CC+ builds, we're taking a break. There are no builds currently planned. That said if you need a CC+, please feel free to continue to e-mail us and we will periodically reassess when/if a new build may be performed. [April 2016]

CC+ Units can be created in Italy by Sandro Sfregola or in Canada by David Clarke.

To place an order, simply get in touch with either Sandro or David (see e-mail addresses at the top of this article).

To lessen overseas shipping and payment logistics, it is generally expected that people from North America might want to receive the items from Canada, with those in Europe receiving the items from Italy.

It is possible to purchase just the CC+ CPU board alone or to order the CC+ CPU board with an optional MIDI Interconnection kit.

The standalone CC+ might be ordered if you only wanted to access to the additional sound banks, or just replace a failed CPU board - without wanting to use the built-in MIDI features.

The optional MIDI Interconnection kit (consisting of the internal and external cabling to allow connection of MIDI IN/OUT/THRU signals) also allows access to the MIDI features inside the CC+.

CC+ CPU Board without the interconnection kit is USD$319/€265. The interconnection kit is an additional USD$70/€59.

These prices include all necessary mounting hardware, packing materials and installation/usage instructions.

We will also be planning to make any software updates/fixes available on-line so that the EPROMs on the boards can be updated by the owners at no additional cost.

Shipping is not included in these prices. The method (and hence cost) of shipping can be arranged when the order is placed.

Because this is a build-on-demand project for which there are only expected to be a small number of would-be purchasers, units will only get built upon receipt of an order. There are two implications to this. First, we will need to spend money to build the item, and there will be some delay between when the order is placed and when the order can be shipped.

To allow the purchasing of parts (as well as to help ensure that would-be purchasers don't back out after we have already spent money to procure the raw materials), we will need the payment in advance (after confirmation of the order).

All attempts have been made to choose components which can be procured quickly; however, there will be a non-zero amount of time in order to place the orders, receive the components and to perform the assembly and test.

In order to ensure we can provide board at the costs above, we need to order items in batches, assuming at least a quantity of 10 boards are being produced at any one time. When you get in touch, we'll certainly do our best to inform you of the expected timelines for creation.

It is estimated that the initial run of boards (based on an initial number of circuit boards we have on hand) could be made available within a month of order placement.

It is important to remember though that this is not being done as a commercial enterprise. We (Sandro and David) are doing this as individuals for the group. As individuals we may be away from our e-mail for a few days here and there, we may go on vacation or have other duties to take care of (i.e., our families, our paying day-jobs, etc.) We will contact anyone interested in the CC+ as soon as possible - but some delays at some times should be expected.


Some may wonder why there are apparently a few missing components on the board; the reason is that the CC+ was designed not only to be appropriate for the Chroma - but also for a new instrument, the C2 module. (Additional information on C2 will be presented on the Chroma site at a later date.)

A "side by side" technical comparison of the original Chroma CPU vs. the new one:

  Original CPU Board CC+
Technology Mainly Nmos/TTL + 3KB Cmos RAM Fully Cmos; Use of CPLD (Complex Programmable Logic Device) and flash memory
Microprocessor Nmos Motorola 68B09 Cmos Hitachi 63C09 Can run in native mode with added registers and faster execution time (+30% typical)
Program ROM 16KB EPROM (8X2KB Nmos 2716 IC) 32KB EPROM (1X32KB Cmos 27C256 IC)
RAM 4KB static Nmos RAM (8X512B 2114 IC); 3KB static Cmos RAM (6X512B 4334 IC with battery backup 2X1.5 volt cells) 32KB Cmos static Non Volatile RAM (1X STK14C88 IC)
MIDI Interface N/A 1 MIDI interface (IN, OUT, THRU) with hardware data buffering (16C550)
Alphanumeric display interface N/A 1 dedicated serial I/O
Other optional I/O N/A 1 I/O expansion connector
1 24 I/O lines connector
Number of IC 33 10 including MIDI interface
Supply current/power 1080 mA (1.08 Amp)/5.4 Watt 58 mA (0.058 Amp)/0.29 Watt

Additional Pictures

Click for larger views.

Production Update: June 2006

Since the original posting of this article in December of 2005, additional fit-tests suggested that some of the internal MIDI connections should be moved - to allow for a better installation in the Rhodes Chroma Expander.

While updating the circuit board for those changes, additional internal modifications were made to improve the board's overall electrical behaviour.

As well, in an effort to keep the costs down (to allow the purchase price to be lower), excess circuit board space has been removed. This prompted the use of a series of inexpensive mounting rails for the CPU board.

We have also created a set of standard cables and a rear panel connector to make the addition of the MIDI interface as easy as possible.