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ChromaTalk Archives: February 2001

VCO Article

Chris Ryan [21030691]

I have posted an article, "The ARP/Rhodes Chroma VCO," written by Gene Stopp. It discusses some technical issues surrounding the Chroma's voltage controlled oscillator.

Originally part of a longer post to DIY or Analogue Heaven, the article was linked from the Chroma site's Links page until recently, when the page was removed. Gene gave me permission to add it to the site.

Expander - how rare?

Go to first message in thread (January 2001)

Dave Bradley [16330135]

Never did hear from anybody on this - surely some of our experts know how many Expanders were made!!!!

Paul Hackett-Evans [21010094]

Dave, I replied, tho not with numbers, did you get it?

Dave Bradley [16330135]

Yes, but I want numbers! At least ballpark figures, as in "under 100 made" or "less than 500".

I have seen the figure of 3000 tossed about as the total number of Chromas made. However it was on SonicState, which has some notorious bad info (probably lifted it from Mark Vail's book - don't get me started!)

Chris Ryan [21030691]

Just from looking at serial numbers (see the Registry pages at the site), it's pretty easy to guess that about 300 model 2101 Chromas were made, and about 900-1000 2103s. The single serial number posted for an Expander, 16330008 (the owner wasn't certain) doesn't help much but would obviously be the eighth produced. What's the serial number on your Expander?

There have since been a number of other Expanders [1631, 1633] added to the Registry, for a total of over 15; the highest confirmed serial number is 16330135. [June 2004] See also How many Chromas were manufactured? in the FAQ.

Dave Bradley [16330135]

I'd love to answer that question sir, but my Expander was shipped from LA last Friday evening, and is somewhere between Kansas City and Omaha as we speak <LOL>. It's hard to wait!

BTW, I have an old tape I recorded years ago of Herbie Hancock's Rocket Band on Saturday Night Live, hosted by Ringo Starr. Herbie's front line consisted of a Chroma, Expander, and Fairlight. There was a second keyboard player that had a Chroma too. They played 'Rocket' and some other song that was almost identical.

Think I'll drag that thing out and watch it tonight.

Mark R. David [21030170+]

Well, I can raise the ante a little. I've got 16330023.

Mark R. David [21030170+]

Herbie Hancock

Oh, and BTW, I remember seeing that SNL only a couple of months after I got my Chroma & Expander. I was VERY excited to see him playing "Rocket" on it - had to wake up my wife (then girlfriend) and show her what a great investment I'd just made!

Dave Bradley [16330135]

Was she impressed?

Hee hee hee!

Mark R. David [21030170+]

Naaah. But she married me anyway.

David Clarke [21030085++]

Well, I can raise the ante a little. I've got 16330023.

I'll see your '023, and raise you to '0034.

I guess we should all make sure our expander details are registered.

Dave Bradley [16330135]

Expander Arrived!

My long awaited Expander arrived today! My Chroma finally doesn't look lonely anymore. Aside from a horribly bad volume slider (which pops and goes dead if you even look at it wrong, much less fire off the thumper), everything seems to work. I haven't hooked up the Syntech box yet, which is external. I had to disassemble my Chroma to remove its internal Syntech so that I could just try it thru the Chroma Interface first.

To the experts, a couple of Q's:

I'm gonna need to refinish this thing cos the woodwork is rougher than my Chroma, and a little darker to boot. Does anyone know if the end panels are solid or veneered?

Also, the display on the Expander doesn't change when I change patches on the Chroma, even with the PANEL ENABLE on. I seem to vaguely recall this behavior when I had my old Expander, but am not sure. Is this normal? Another thing I noticed is that if I change patches on the Chroma (hence the Expander changes also), but I then attempt to edit the parameters on the Expander, nothing happens. It's as if the Expander is not bringing the patch that I'm actually hearing into the temp edit location. Does this make sense? When you send a patch change from the main Chroma, doesn't it copy the patch from the Expander's same location into it's temp buffer, where I should be able to edit it?

Would the fact that this Expander has the Syntech 4x memory mod make a difference?

RE: upping the serial number ante: I am the new champ! This Expander's serial is Y 0135. So we know that over 100 were made, at least. Here's a little history for the registry:

This Expander was purchased new by John Butler of Stonehenge Studio, who owned it until I bought it. He was an old time Syntech employee who was introduced to the Chroma by Ken Y himself! Small world, isn't it? I guess that's a good reason why it is tricked out with all the Syntech mods...

Dave Bradley [16330135]


Well I dragged out Herbie and watched him again. Boy, did that suck big time <LOL>! Totally inane music, a guy scratching records loudly drowning everybody else out, a goofy looking second keyboard player, and the sounds programmed on the Chromas weren't that hot, either.

OTOH, that Chroma + Expander sure looked sexy, even if Herbie's had duct tape stuck all over the sides.

I've never understood how he could play such sublime jazz, and then this stuff.

David Clarke [21030085++]

Expander Operation

Dave Bradley wrote:

a couple of Q's: [...]

the display on the Expander doesn't change when I change patches on the Chroma, even with the PANEL ENABLE on. I seem to vaguely recall this behavior when I had my old Expander, but am not sure. Is this normal?

I don't know if this is what was originally intended by the developers, but this appears to be 'correct' as to how the firmware works.

When a program button is hit, the Chroma will output a "Define Instrument 0" to the Expander (if the original program has a link, then a Define Instrument 1 will also be sent - if there was no link, then an "undefine instrument 1" will be sent).

Upon receipt of the "Define Instrument 0" command, the Expander will go on and internally define an instrument based on the program number provided. It will then assign voice boards to that instrument.

The instrument will be defined _directly_ from the stored instrument parameters - those parameters _are not_ first copied to the "program 0" or "safe buffer" location and none of the panel information is updated.

(There is no way to tell what program is currently 'active' on the Expander if you have chosen the program via the Chroma.)

Another thing I noticed is that if I change patches on the Chroma (hence the Expander changes also), but I then attempt to edit the parameters on the Expander, nothing happens. It's as if the Expander is not bringing the patch that I'm actually hearing into the temp edit location. Does this make sense?

When you move the slider, the Expander first looks at what it believes the "current" program is. It will then check a lookup table to see what instrument is assigned to the "current" program. It will then send the parameter changes (from the slider) to that instrument.

Unfortunately (as noted above), once you have used the Chroma to change the program on the Expander, the "current" (displayed) program on the Expander is no longer associated with the definition of any Instrument. This means that the parameter changes (on the Expander) will not actually be seen by the Expander's playing program.

The easiest way to get around this is to select the program (via a program button on the Expander) before using the slider. Alternatively - you can use the Chroma's slider to send changes to the Expander.

When you send a patch change from the main Chroma, doesn't it copy the patch from the Expander's same location into it's temp buffer, where I should be able to edit it?

A "patch change" won't end up in the temp buffer, but a "send program" will. For instance, if you are on program 43 on the Chroma, and do a Set-Split 20 (send program) then the information from Program 43 on the Chroma will end up in Program 0 (temp buffer) in the Expander. Any parameter slider changes on the Expander will then properly edit this sound (since it is indeed in Program 0 - the current program).

Would the fact that this Expander has the Syntech 4x memory mod make a difference?

I don't think so - I think what you're seeing are likely just the design 'features.'



Dave Bradley [16330135]

Re: Expander Operation

the display on the Expander doesn't change when I change patches on the Chroma, even with the PANEL ENABLE on. I seem to vaguely recall this behavior when I had my old Expander, but am not sure. Is this normal?

I don't know if this is what was originally intended by the developers, but this appears to be 'correct' as to how the firmware works.

Thanks for the explanation. It makes sense, I guess - I would have chosen slightly different operation.

Has anyone ever disassembled one of the sliders (e.g. Volume) to bring it back to life? Any tips?

See the thread snake oil that really works below for responses to Dave's question.

What's the difference...?

Paul Hackett-Evans [21010094]

I'm curious. What's the difference between Chromas with serial numbers starting with 2101and 2103? Were there 2102's as well?

Chris Ryan [21030691]

Dave Clarke wrote a detailed comparison of the two models [ChromaTalk July 1999].

I don't think there was a 2102; though one owner reported a serial number starting with 2102 [21020003], he wasn't certain of it.

Rhodes electronic piano 3363

Eirikur Hallgrimsson [21030467+]

There's an unusual item on ebay at the moment, one of the rare (I've never seen one) all-electronic Fender-built Rhodes pianos. I did some dredging of the net and the dating is about 1983, which suggests that it is related to the ARP 16 (4) Voice Piano technology, possibly acquired along with the Chroma design.

Anyone know much about that beast? Heard it?

Dave Bradley [16330135]

I'm pretty sure you're correct. If so, I played the Arp and it wasn't very memorable sounding. In addition, it was horribly unreliable, with the kbd membrane switches failing at a high rate. I'd steer clear if you are seeking a working instrument.

End Panels

Paul Hackett-Evans [21010094]

Dave, the end panels are DEFINITELY veneered!!!

My Chroma is currently in a couple of dozen pieces being restored. Thanks to foam rot, I've had to strip off all the varnish, fill the dings suffered over the years and sand it all smooth ready to revarnish. I'm not intending to varnish it until I get the power supply back from repair and I'll reassemble & test the electronics first. No point in refinishing it if the instrument has to be packed & transported 200 miles to a workbench. Roy the engineer is awaiting the BIG power capacitor from the States before he can finish the repair.

Anyway, when I disassembled my Chroma, I discovered a knot in the surface of the wood of one end cheek,which was normally invisible inside the instrument. I can clearly see through the knot that the end cheek is chipboard with a cherry veneer on it. Furthermore, it isn't a very good veneer, as mine has two distinct different coloured bands across one cheek, where the veneer was formed from two different logs at the timberyard. This was only visible once I'd stripped off the varnish, it looked fine before that due to the varsnish used (see below). The trim strips running across the width of the Chroma top panel appear to be solid wood pieces, they are too thin to be veneered. But the square section piece running across the front edge in front of the keys MAY be veneered too. I'm not sure of this but I'd be careful of sanding it too deeply at the corners just in case. (Does the expander have an equivalent piece?)

I had to rebuild the front corner of one endcheek where it had been crunched against something by a roadie. If it had been solid wood, I'd have just reshaped & rounded off both sides to match. As it's veneered, that was impossible so I used resin "liquid wood" mixed with the sanded off sawdust from that endcheek to help colour-match it, built up in layers then sanded down to a smooth matching shape.

All of the wooden pieces of my Chroma are different colours, but all were thickly varnished with coloured varnish then lacquered to give a pleasing uniform finish when the instrument was built. One challenge for me in restoring it is to use the same approach to getting a good finish on the wood. This could mean that the colours are different on all of them and you could have problems getting the two to match. If you're going to get the wood finish on your expander to match your Chroma, you may have to strip BOTH instruments and refinish them to achieve the same finish on both.

You can strip off the foam-rot (if there is any) with mineral spirits to leave the varnish reasonable underneath. You can remove the varnish completely to start again using a quality wood stripper like Nitro-Mors. I did this as a 2-stage process and sanded very lightly all over. The wood came out just nicely, albeit with its true colours revealed!

I guess I could have just got a carpenter to cut & finish new pieces to replace the originals, but it's part of the romance of restoring my old friend to see if I can keep the original parts. It's also more rewarding to do it myself. Anyway, if it doesn't work out, then I can still get new ends made if I have to, they just screw on.

I'm going to send Chris some step by step photos of the restoration when I get round to it. He'll post them on the website. They may help you.

I still wish I had that expander!

Best wishes

Dave Bradley [16330135]

This is interesting. My Chroma end panels are a medium cherry color, and the finish is transparent enough to show a nice grain. The wood (veneer or not) is clearly one piece, not strips.

The Expander is darker, more opaque, less figure in the wood - the finish just doesn't come off as nice. I'd guess that my Chroma is much earlier than the Expander, and that the later ones cut corners in the cabinetry. I do remember that my original Expander finish matched the Chroma much more closely.

snake oil that reall works

Eirikur Hallgrimsson [21030467+]

Has anyone ever disassembled one of the sliders (e.g. Volume) to bring it back to life? Any tips? !

My first shot at pot/switch problems is to give the problem item a good shot of Cramolin, which is now called DeOxit. Actually, I just checked the site, and the relevant product is now called R5 Power Booster. This stuff is NOT the standard "cleaner" spray sold at Radio Shack. It's pretty close to magic. It really does disolve oxides and 'heal' contacts. The R5 is a 5% "Cramolin" solution in a new Dupont (ozone friendly, I think) solvent spray. That's the best for getting into controls that you can't or don't want to disassemble. Anybody who deals with electronic hardware should have some of this stuff. It's safe for all the plastics you'll find in an old synth, but I would try to keep it off displays just for the sake of not pushing your luck. I have sprayed it around liberally at times and never had a problem.

Personally, I have and use just about all of the related products, including the full-strength liquids and the paste with copper particles (for really nasty corrosion cases like the dome-light switches in car doors).

It looks expensive, but you only use tiny amounts of it. I still have the sampler kit from about 7-8 years and two product name changes ago.

I learned about this stuff from high-end audiophiles. You can pay even more for it under fanciful names at boutique audio salons.

I can't recommend this stuff enough. I'm not affliliated with the company in any way, BTW.


Dave Bradley [16330135]

Thanks for the recommendation. The very first thing I tried was to shoot it with DeOxit. It's going to require a more radical solution than that - I have to actually pull out on the slider to get it to make contact. Since I don't want to go through life with a wad of paper jammed behind the knob, I want to disassemble the slider (or even replace it, if you can still get the part).

Chroma in Germany

Helmut Segeth

Hi, my Chroma needs some repairing (broken PowerSupply). Is there any known contact address for technical service in Germany?

Erik Vellinga [21010286]


I live in the Netherlands. I have had a lot of trouble with the power supply. After a long time I finally have replaced it by a HP ECG monitor power supply.....and it has never let me down....

Unfortionally I have dumped the original supply, I did not want to see it again in my life, so I don't have parts.

I am an electronic engineer, can I help ??

Paul Hackett-Evans [21010094]

Power supply repair

I know the power supply is a heavy item and expensive to ship around the world, but Roy Paynter, the engineer who is servicing my Chroma power supply, could also take a look at yours, Helmut. Roy is one of the original Chroma trained engineers, and he has a Chroma which he uses as a testbed for repair units, swapping voiceboards and components, etc to test and calibrate them. He can take individual sections of a Chroma and service them for you if you need. (Like the power supply from my Chroma, much cheaper than shipping the whole instrument to him.) Roy has sets of leads to connect parts of his testbed Chroma to corresponding parts of repair Chromas to make this easier and more accurate. He does an excellent job. If you don't find anyone qualified nearer to you, I can supply you with Roy's contact details. Please tell me if you need them.

Roy is currently having trouble sourcing the very large capacitor for my power supply. They are still being manufactured by Mullard in the USA. He ordered one from Mullard a few weeks ago. Mullard accepted the order, then later wrote to Roy saying that their minimum order quantity is for ten of them. Roy is trying to get them to honour the original order of one capacitor, especially since it is an expensive item.

If there are any Chroma Service Agents in other countries who can help supply one new identical capacitor, please contact me, just in case Mullard don't help.


Erst Eimer [21010299]

Hello Helmut,
I haven't needed a technician for my Chroma so far. But I know that the previous owner of my synth had it repaired once. He sent it to TBS in Nuremberg.

The address is:
Touched By Sound GmbH
Sigmundstraße 110
90431 Nürnberg

Greetings, Ernst

See Service: Europe for more European options for Chroma repair.

For Sale: Chroma + Expander and Many Extras

Chris Ryan [21030691]

Chroma (with Anvil case--original owner) and Expander (no case--ARP plate on back) with matching wood finish, improved power supply cooling mod, and latest revs on proms, working great and in cosmetically fine condition. Dual pedal assembly with spare parts; Rhodes footswitch; two Syntech (Chroma Cult) MIDI interfaces; RS 232 Chroma cable; original sales and marketing materials; original owner's info and parameter chart; Opcode Galaxy patch librarian software for Mac OS (includes Chroma librarian module); original cassette programs and extras; Panasonic 2107A cassette deck (minus Chroma cable).

More details on the site's Ads page. [Of course this one has long since sold!]

Windows interface for Chroma

Mark R. David [21030170+]

I'm not done, but I got pretty excited when I got this far, so I thought I'd publish a teaser. Attached is a small glimpse of the patch programming interface. What's exciting (to me, anyway) is that I've made this completely skinnable; i.e., visually customizable by any user without touching the source code. For a full-size picture and some explanation, see [obsolete URL removed].

For other posts on the topic by Mark, see Windows Editors & Librarians: Windows Interface for Chroma.

Dave Bradley [16330135]

You, sir, are a madman! Well done! But make my rug Turkish, please!

Mark R. David [21030170+]

I know, I think I've gone off the deep end.

Actually, every different patch type (Simple, Independent, Parallel, Series and Variable Mix) gets a different rug. I can't entirely recall, but I think one of those is Turkish.

Knob styles?

Dave Bradley [16330135]

Glancing through the Chroma brochure at the web site, I see a lot of pics of sliders with smaller knobs that look, well, Arpish. Every actual Chroma I've ever seen has the larger knobs that are identical to the ones used on the levers.

Does anybody know the story? Did any of those smaller knobs ever make it to production?

Eirikur Hallgrimsson [21030467+]

I've only seen the matching knobs myself, and I think I have touched about 5 Chromas.

eproms for rhodes chroma

Andi Beit [21030620]

How can I get eprom bin files for my crashed rhodes chroma. Is there anybody, who can help me ?

Erik Vellinga [21010286]

I have read my EPROMS and put them on floppy, as a back-up. If you want the files I can send them by email to you. I do not know what firmware it is, but I can look inside my Chroma for the revision number, if you want them.

By the way, some one on this list asked me for the Polaris schematics. I have lost your email adress !!

Andi Beit [21030620]

Hi Erik!
Thank You very much for Your great offer! I think I have REV14. But that is not so important, because it would be better to change my whole eproms. It would be nice, when You send me all bin.files, I could burn them all!

Lots of thanks again!

See David Clarke's EPROM Code for downloadable binary copies of Chroma chip contents.

Harold Rhodes, Alan R. Pearlman

Chris Ryan [21030691]

Just noticed that Harold Rhodes passed away in December.

While the Chroma was, through a convoluted set of circumstances, branded with his name, I wonder what happened to Alan R. Pearlman. The last mention I heard of him was "The Rise and Fall of ARP" article (Keyboard, 1983), which said he was "attempting to recover face, fortune, and his faith in free enterprise with Selva Systems, Inc., a microcomputer software company" in Wellesley, Massachusetts. A quick web search didn't turn up any current information.

Chroma Items on Ebay

David Clarke [21030085++]

Hi all,

Some of you may have already seen these - but just in case you haven't, there are some voice boards, replacement membrane panel parts, midi interface, etc. avail. on Ebay.

Here are the links ( ... no, the items aren't mine ...)

Item 1410296596 (Midi converter) [link removed]

Item 1410294370 (other parts) [link removed]

BTW, I have asked the person listing the items to put some info in the Chroma Registry and/or to share some of his other info with us.

Bill Lacey

Uh, wonder why he had such a quick auction (1 day)? Probably could have gotten more for it. The same two people bid. I would have bid on it had I known. Oh well:(

R14 Eprom files ???

Murray Macdonald [21030276+]

Hello fellow Chroma freaks and freakettes,

I am looking to repair my rhodes chroma. The eproms oozed this white gunk/corrosion on about half the eproms and it stopped working. I have an eprom programmer and can easily burn new eproms, but I need a file to copy from. I understand from talking with a fellow that worked at the chroma factory that V14 was the lastest software version, and that there were no required hardware upgrades to support the latest software, just plug in the chips. Does anyone out there have the ability to send me binary file(s) that represents the V14 eproms? Any help or direction would be much appreciated.



Jerry Leonard [21030100++]

I saw that someone had them on this forum. As a backup, CAE Sound in Palo Alto also has the Rev 14 EPROM files.

See David Clarke's EPROM Code for downloadable binary copies of Chroma chip contents.

When I enter the patch no3 the Chroma stops working


I have a problem with a Chroma: sometimes, when I enter the patch n°3 and I play a note in a further time, the Chroma stops working or resets himself. Then I turn off and on again and the Chroma works as normal, even in the patch n°3. Someone could help me?

Eirikur Hallgrimsson [21030467+]

This sounds like bad RAM. Sometimes it doesn't read right, and crashes the Chroma. I don't remember if there is a memory test. You could remove the patch storage ram chips and put them back in in reverse order to see if this moves the problem away from patch 3. If it does, you could just replaced all the chips, or do more work to figure out which one it is.

David Clarke [21030085++]

Is there anything special about Patch no. 3? Does it maybe contain a "all channel-mono" program? If you store Patch no. 3 in another location (like Patch no. 26, or something like that) does the problem follow the patch data?

Don Tillman

Did you try the obvious physical stuff?

  • Open up the beast.
  • Vacuum out the dust.
  • Use contact cleaner (DeOxit) on all board connectors, ribbon cable
  • connectors, IC sockets, slide pots, etc.
  • Make sure all the boards, connectors, and ICs are seated tightly.

Did you check that the power supply voltages are accurate? And adjust them if they're not?

Robert Shanks [21030301]

On the Rev. 13 Eproms I programmed a patch that would crash the Chroma. It was a sorta "circular saw meets mega distortion electric guitar with a little bit o' world blowing up" sorta patch - Play one note, the cpu would go off in the weeds, and the pitch would just drift upward until you turned it off. Great fun - I use Rev. 14's now so I kinda miss that. :)

I never did figure out what exactly did it - probably just one modulation too many - I assume it would get tided up doing mods and not make it back in time to do something. I don't know if that makes since on an interupt level - but sure was fun.

Cheers Robert Shanks/Synthlab

pops at autotune

Dave Bradley [16330135]

Hey all,

My new Expander pops quite loudly when the audio mutes and unmutes when autotune starts and ends. Any solutions / places to check?

David Clarke [21030085++]

A couple things you can try to try and isolate the source of the pop:

  1. Do you also hear the pop on the XLR output?
  2. Disconnect all external cables (except power) and hook up a pair of headphones where you'd normally have line-out connected. With just power and head phones connected, do you still hear the pop?
  3. Does the pop change in volume (depending on the position of the volume slider)?

Dave Bradley [16330135]

I don't have an easy way to test the XLR output, my mixer is strictly line in. The pop is pre-volume control, and is present on Output 0 as well as Low and High level mono outs, provided the volume slider is up. I have schematics if that helps.

David Clarke [21030085++]

If you haven't done it yet, I would do the check with the headphones (with no other external connections on the Chroma) just to rule out signal-ground related differences between the mixer and the chassis ground on the Chroma.

Assuming the pop is still there with the 'phones, it is then just a matter of tracing the signal backwards to see where the pop starts. You indicated that the pop is pre-volume control - so you can start just before that. The volume is controlled with the 3360's shown on the EQ Board Schematic. The first place to look would be the input to the 3360 for OUT 0 (pin 6).

Then go back to the output of the output summing amps on the Channel Mother Board Schematic - look at pin 1 of Z18A (output amp for OUT 0).

If I had to make a wild guess, I'd say that Z16 (the actual analog switch) might be a good guess for the pop, or maybe Z18 - but your probing should be able to show you which device is causing the pop.