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ChromaTalk Archives: July 2008

SPSU testing

Luca Sasdelli [21010226]

R Hollman ha scritto:

Hello Luca Sasdelli,

I noticed at the end of your article on the replacement of the PSU for the Chroma, you show a "load" testing board (several load resistors and LED/driver circuit) in the "Test and Calibration" section. I was wondering if you might share the schematics or specifics of the construction of that board with me. I plan on building the SPSU/reset board from scratch and having a safer way to test the board (safer for the Chroma that is) is very desirable.

Hello Randall,

sure you can build a dummy load as per the one I use for SPSU testing; simply I had received a wrong batch of power resistors, so instead of spending more money to send them back, I've choosed to use them as a load for testing.

Here you'll find the PSU requirements: Upgrading the Rhodes Chroma Power Supply [by Don Tillman,].

Therefore, you can simply apply Ohm's law to fullfill the required currents:

  • +5V digital, 3.50A = 1.43 ohm (18W)
  • +12V 0.7A = 17 ohm (9W)
  • +5V analog, 0.25A = 20 ohm (1.25W)
  • -12V 0.7A = 17 ohm (9W)

then you can set several power resistors in parallel, achieving the required value. With same Ohm's law, you can simply divide the single resistor value by the number of resistors in parallel, e.g.: +5V digital = 1.43 ohm equal to 7 x 10-ohm resistors; 18W power divided by 10 resistors requires 2W each one +12V = 17 ohm equal to 3 x 47-ohm resistors; 9W power divided by 3 resistors requires 3W each one +5V analog = 20 ohm equal to 5 x 100-ohm resistors ... and so on.

Better to choose a higher wattage for the involved resistors. Murphy still waits for it.

The LED I've installed just for convenience are simply current limited at 50mA with suitable resistor, in respect with relevant voltage, e.g.:

  • 5V / 0.05 = 100 ohm
  • 12V / 0.05 = 240 ohm

The J3 connector has all required voltage pins; please see page at bottom.

If you really need a schematic, I can draw and post one.

Interface Manual

Chris Ryan [21030691]

The Interface Manual is now available in hypertext format at the site. It documents the computer interface, and includes sections on Command Descriptions, Software and Hardware requirements (along with schematics), and several appendices such as Parameter Information and Command Listing.

This is the 1982 edition that I obtained from the Canadian distributor in the mid-1980s. There was apparently a later revision; David Clarke [21030085++] has sent me some excerpts and I will publish them at a later date, or perhaps update the entire manual (does anyone have a page that might indicate the publication date of a later edition?). The additions appear to be mostly related to the Polaris.

The Interface Manual has since been updated to the 1986 edition.

All four major Chroma manuals are now online: Performance, Programming, Service, and Interface. As this is another significant addition to the site, I will consider it a "special summer double issue" and will not publish a new article for August (though I will still post the list archive for July and do other minor updates).

Chris Ryan [21030691]

This is the 1982 edition that I obtained from the Canadian distributor in the mid-1980s. There was apparently a later revision; David Clarke has sent me some excerpts and I will publish them at a later date, or perhaps update the entire manual (does anyone have a page that might indicate the publication date of a later edition?). The additions appear to be mostly related to the Polaris.

Here is a post by David detailing differences in the new version of the manual: Alternate (updated) version of the Computer Interface Manual (September 2004).

Dave Blees [21030552]

Chris - I have a "Chroma/Chroma Polaris Computer Interface Manual" dated 1986 here. (Fender part# 309004401, with Fender's Brea, CA contact address and such). It does reflect the changes noted by David Clarke on '04, and it was published as a plastic-comb-bound, grey-covered, 52-page manual. I recall ordering it when I attempted to decrypt the Polaris a bit in the late 80's. My copy has my own hand-written corrections to the erroneous last 5 parameters of the "POLARIS PARAMETER LIST" on page B-5.

Chroma Programmer

Leonardo Ascarrunz [no serial number]

After playing around with my new chroma i have found that programming it is quite a pain. I was wondering if anyone has attempted to make a dedicated programmer. I have seen the link on this site to the "chromanoff" but i could not find schematics or any diy info on the site.

Jesper Ödemark [21010135]

I'm sounding like a chorus in a pop song, but do consider a programmable "knob box". The CC+ along with one is a great tool.

Here's a pic of my solution, still awaiting some cherry end cheeks and new Chroma-styled graphics.

[TRADER] Great trader Mark L Smith

Rikard [21030448]

Hi List,

Just wanted to post a great trader post for list member Mark L Smith [2103-PT-002]. Bought a spare I/O Board from him and received it very fast and intact. Mark kept me informed through the whole sales process until the board left the US.

So, all in all Mark L Smith is a great trader and I would not hesitate to do buisness with him again.

NOS Chroma AC Socket w/Line Filter on eBay

Chris Ryan [21030691]

These have been popping up regularly for months; someone, please, put the seller out of his or her misery. :)

Item #130235418082, starting bid US$5, ends July 8.

Doug Terrebonne [21030114]

LOL... I have a "Chroma" power cord to go with it for $10... :)

Tom Klepacki [21030025]

I suspect that next week he or she will list a box of flute reeds!


AD 7541

Riccardo Grotto [21010249+]

Hi everybody,

Is there a reason for the AD 7541 to pass by (RIP) when replacing the SPSU with a new one?

It happened for two of my 4 units.

My fault, for sure. Would like to know why.

Leonardo Ascarrunz [no serial number]

The same chip blew on mine as well after replacing the power supply. Im not sure wether the chip blew or it was already broken. A good substitute is the LTC7541AKN.

Jimmy Moyer [21030184]

This happened to me.

The cause was a bad ground connection, the one to the channel motherboard, if I recall correctly.

Erik Vellinga [21010286]

Same here.... Not the SpSU kit but a similar PSU.

I think it happened because I forgot to connect the ground to the channel main board. Not 100% sure though... Besides, the IC's used are sensitive to electric change. So use a ground lead connected to you and the PCB you are working on as well as the chroma.


Interesting topic. I fried two AD7541's trying to resurrect my Chroma as well. The first time I used a SMPS, second time a lab PSU.

Chroma is still dead though, probable because the DAC circuitry and/or motherboard isn't 100% working.

Luca Sasdelli [21010226]

Hello all,

I think as every MOS-equipped circuitry, the Chroma needs to be serviced with proper grounding, and an important issue is the soldering iron, that must have its metal connected to earth as well (as normally happen for recent tools).

Should a working earth connection not available, I recommend to use an electrostatic discharge wrist link, connected to the instrument ground, or almost to repeatedly touch ground wires, to keep the body potential as close as possible to the one of the instrument.

To service the Chroma while connected to earth (e.g. via its power cable) WITHOUT a wrist link is NOT recommended, because the instrument would be at a very low potential, while the body could have a very high potential in respect to it. Moreover, to service the instrument with its power cable connected, would bring LIVE voltage in it, and it is dangerous.

Therefore, the rule of thumb is to have body and Chroma at same potential. Avoid to wear synthetic clothes during servicing, as well as too insulating shoes.

Leonardo Ascarrunz [no serial number]

I think the main problem is that the digital ground and analog ground are meant to be connected together at the power supply. The analog ground is connected directly to the chassis. Therefore you must connect the switching supply ground to the case as well.

Forgot to mention that if you plan on swapping the 7541's with the LTC7541AKN you should change the capacitor in the buffer circuit to 33pf.

James Coplin [16330036+]

Is this happening with the new power supply kits or with other supplies? Will I blow up the 7541's if I follow the installation instructions for the new kit? I was getting ready to install my new supply tonight but now I'm worried...

Åke Danielson [21010068]

Hi James,

I installed my new PSU and all went well. My Chroma has never been happier!

Jesper Ödemark [21010135]

I second that, so it's not the PSU that's the villain.

Luca Sasdelli [21010226]

Hi James,

it seems that this fault happen during Chroma service. It has nothing to deal with the SPSU kit, as some users experienced this fault by replacing the original PSU with a self-made one.

Anyway, I recommend to have all antistatic precautions as per my previous email.

Riccardo Grotto [21010249+]

I believe that there's nothing to worry installing the new SPSU kit, following carefully the instructions, step by step.

This was clear enough, I believe, in my original post when I wrote "my fault, for sure". By the way, the "riccardo" name on the's me. So I strongly believe in this work. I only wanted to understand where my fault is, stated that I mounted 4 kits, all in the same (or that seemed to me) way.

Two went flawless and two times the converter burned.

What I see is that it did not happen to myself only, but that are are few other cases. As Luca says, probably the grounding and the anti-static precautions are essential, along with "UN-PLUG, DISCONNECT THE MAIN AC POWER CORD".

Enrico Dibennardo [21030494]

I just touched the hydraulic metal tubes going from the sink in to the wall in my bathroom before touching the boards, wearing no gym shoes or other insulating footwear also. I think this was enough for me not to have troubles and alla went smooth.

Earth-Tone Chroma Identified?

Leonardo Ascarrunz [no serial number]

I have been reading some of the old chroma-talk archives. And came across this post.: Fwd: "Red Chroma's" [November 2003].

Seems like my chroma is the one that belonged to robert witt. I bought mine at a shop in denver and as he describes in the post my unit has no serial #.

Mark Smith [2103-PT-002]

Hi -

Rob was the original silk screener at Fender in Woburn MA who made the first earth-tone panels. He made mine, too.

Chroma For Sale on eBay

Chris Ryan [21030691]

Chroma 21030321, item #110268394640, "Buy It Now" price of US$3999 (!), ends July 16. From the description:

"With Syntech MIDI interface. The synth is 100% functional and has just been serviced. The power supply was completely rebuilt and everything was cleaned and calibrated as well. Cosmetically the synth is in nice shape. The worst cosmetic flaw is a scratch across the LCD screen and part of the synth as well."

This instrument was added to the registry a few months back as one of those whose registration information had been sent to Fender [see Registry Approaches 500 Instruments]. Other than that this is the first we've heard of it.

Paul Hackett-Evans [21010094]

Another Chroma on Ebay

Not mine!

Ebay item number 290246556420 Located in Sherman Oaks, California, United States

The description from the seller -

"This listing is for a vintage Rhodes Chroma analog synthesizer (Serial # 21030321) with Syntech MIDI interface. The synth is 100% functional and has just been serviced. The power supply was completely rebuilt and everything was cleaned and calibrated as well. Cosmetically the synth is in nice shape. The worst cosmetic flaw is a scratch across the LCD screen and part of the synth as well (check pics). This last synth designed by ARP and a great one to add to your collection. Feel free to email me any questions. Thanks! SORRY BUT NO INTERNATIONAL SHIPPING ON THIS ONE"

Priced at $3,999.00 as a fixed price.

The photos show a nasty scratch as listed.

Chris Ryan [21030691]

Re: Another Chroma on Ebay

This Chroma, 21030321, was again re-listed today as item #300245786061 at US$3799, $200 less than its previous two listings, and has apparently sold for that price. This would, I believe, make it the highest paid for a Chroma since the list started.

Leonardo Ascarrunz [no serial number]

Most vintage synthesizers seem to have been increasing in price very rapidly over the past few years. I am not sure if this is due to a increase in demand, or because most ebay sellers are artificially inflating the price. I bought a pristine jupiter 6 in 2004 for $900, now they are commonly listed for at least twice that.

removing expander front panel...

James Coplin [16330036+]

I decided to install my power supply and cc+ board tonight. I guess I never paid attention before when I had it opened messing with bad voice cards to notice that the main board is attached to the front panel. What is the best way to get at the front panel mounted boards? I wanted to make sure their wasn't some trick before I went off cavalierly pulling it apart. Thanks!

Mark Smith [2103-PT-002]

Hi -

I assume the back cover is off. Looking at the front of the instrument, undo the two screws to the far left and the two screws to the far right on the front panel. Now gently lift the front of the panel and swing it up and back. It will rotate to a stable position.

Riccardo Grotto [21010249+]

My expander also has two screw on the bottom side. They also hold the panel in place.

David Clarke [21030085++]

... What is the best way to get at the front panel mounted boards? I wanted to make sure their wasn't some trick before I went off cavalierly pulling it apart...

James - there's no specific trick. Following up to the suggestions from others, you first remove the back panel.

There's then 4 screws on the front panel itself, and two underneath. When the screws are removed, the panel will rotate upwards from the front.

The procedure, and location of the two screws on the Expander, is noted here: CPU Plus Installation: Removing the Old CPU Board.

Expander going rate?

Lars Johansson [21030632]

How much would one expect to pay for a serviced and working Chroma Expander?

Jesper Ödemark [21010135]

I think Eldkvarn-Claes offered me his at 19K (SEK) a year ago. Tempting, but I figured I could spend the cash more wisely.

Lars Johansson [21030632]

I've been offered one starting at 22.000 SEK which I'm sure is the going rate but I have a big hole in my wallet so.......

Jesper Ödemark [21010135]

Seems like that's the standard price in Sweden 2008 then. That "hole-issue" is extremely irritating...

Chroma Lives!

James Coplin [16330036+]

After about 3 years of not being used due to general flakiness, my Expander today has been revived with the help of a new power supply kit and CC+ board. Not to knock the power supply kit, but if you are putting this into an expander, you probably should expect to take the afternoon.

It was really tight going and I probably should have drilled a new set of holes but I was at the limits of my patience and emptying everything out of it to drill was more than I was willing to tolerate. Also, at least on my Expander, the supplied screws are too short and the heads too small. They would have easily pulled through if I had tightened them up much. I need about and extra 1/4" in length as the supplies screws just came up even to the surface.

Also, I don't remember a lag in the MIDI response with my ChromaCult interface but there is a definite delay with the CC+. Is it my imagination or did the ChromaCult have better response? Glad to have it back all in all!

Sandro Sfregola [21010294]

James - the CC+ has a very tight MIDI response due to its design, better than the "ChromaCult".

So check your Chroma and the external equipments, there is surely something wrong.

storing patches

Enrico Dibennardo [21030494]

Hello, I know this has been discussed recently but i couldn't find that message.

I've installed the new spsu kit and it works fine. (thanks) I've replaced the batteries and cheking voltage it says 4.95 so I'm assuming it is ok. I tried a sort of voice init holding down paramselect and then pushing each of the 50 parameters button (I wonder if there's a quicker way to get voice init by default) so i could finally get a sound from my chroma. Afterwards I tried to stre this basic patch pressing store twice. whenever I went bach to that memory location the patch was there but turning off the instrument and turning it on after a while I couldn't find the patch stored in memory. Again: batteries seem to work. Any help is of course appreciated

Luca Sasdelli [21010226]

Ciao Enrico,

did you check the memory lock switch on the back being unlocked? :-)

Enrico Dibennardo [21030494]

Thanks Luca

err... no i didn't check because I didn't know this selector existed at all, anyway, unfortunately I could see that it was in the unlock position already. In any case storing was possible, only after turning off the unit data wouldn't be kept in memory, similarly as to a battery failure but battery check says 4.95, so if I recall correctly any voltage above 2.50 should be fine.

Luca Sasdelli [21010226]


Definitely too much; I did read 3.20V max. Just try to replace the two AA-size batteries (1,5V each) on I/O board and see if something changes; also inspect the battery area, quite often polluted by battery acid leaks.

Enrico Dibennardo [21030494]

Of course, two AA batteries can deliver no more than 3.00 right. Well it seems there's a issue since I even totally removed the batteries and it still displays cell 4.92, it is stuck to that value so I don't know if it could be a value generated by a short in that area. There's some acid on the contacts of the battery container but nothing can be noticed on the board actually. where do you think I should start first and I'd be corious to know if a working chroma would display cell 0.00 with no batteries or any other unpredictable value. I know I can not ask people to check just removing the batteries from their instrument.

James Coplin [16330036+]

Make sure you check the underside of the board. I had a battery leak and everything looked fine from on top, just isolated to the battery clips. When I turned the board over, the acid had run down through and etched part of the bottom. Luckily, it was easily fixed and not too serious, but it could be.

David Clarke [21030085++]

... where do you think I should start first...

Enrico - the circuitry involved in the memory backup is nicely contained in one area on the CPU board. The items of interest are schematically shown in the "CMOS RAM POWER SWITCHING" section in the upper left-hand corner of the Computer Board Schematic.

The items are physically in one area on the board too. A picture of the circuitry, as well as some general debug ideas, can be found here: My Chroma is not happy [September 2003].

Based on your functional observations I would expect that you're not going to see a voltage at "Point C" when the Chroma is off (thus allowing the contents of the memory to go away).

From there it is a matter of tracing back and determining what has been affected.

Enrico Dibennardo [21030494]

storing patches and batteries

I wanted to thank all those people who gave opinions and help. I checked and could see it was rather easy to fix since corrosion had broken the link between the two batteries in the battery holder, no other damage so i was really lucky and now it stores patches flawlessly. I'm having odd problems with voice boards now. As i read on this list I can confirm that the routine for checking boards errors has an odd behaviour. I.E. I find err 2 3 and this is correct since if I put those two boards in other locations the error shifts to thode new locations, but... it sometimes says err 7 whatever board is in that slot so I understand it is not the board but the slot and this happens intermittedly. But these are not the odd things, what I find strange is that, although i don't get err 0 board number 0 is always skipped even with set 31. I know this since I recognize the same out of tune voice (from another board) every 14 notes and never every 16. Board 0 plays only if it is the only board inserted, otherwise it is checked, it is considered regularly working but it is skipped while playing and while checking with set 31. Another board is considered perfectly working but only one oscillator can be heard so I'm afrais tha machine has something wrong wjth its adiagnostic system and it isn't of much help by itself. One of the boards i much older it is arp and it is of a different colour but it works well

One more time

Mark Smith [2103-PT-002]

I've got one more voice board to sell and this one works. Here is the url - [eBay item #260261083588]

Good luck!

Graphics panels for sale

Mark Smith [2103-PT-002]

Here is the only set of gold and white graphics top panels for the Chroma. There is no switch layer, it's just the graphics panels. Good luck - [eBay item #260262115268]

Picture from the auction:

SPSU kits still available

Luca Sasdelli [21010226]

Hello all,

this just to inform that I still have enough items to build 6 Chroma SPSU boards and 11 MeanWell SPSUs, with mounting brackets, should anyone need it.

Matt Thomas [21010021]

I'm just about to start installation of the SPSU kit and my Chroma has the Kenton interface installed – has anyone else performed an SPSU installation inside a Kenton'd Chroma? Any issues?

I've followed the wiring from the Kenton to all its many destinations and curiously none are connected to the PSU board. This seems to be reflected in the wiring diagram from the installation manual on the Chroma site, so maybe that's OK?. Again – anyone know if this is correct?

Anders Elo [21010104]

Hi Matt, yes I've done that procedure, the kenton retrofit needs no wires to the SPSU. If my memory doesn't fail me, all the wires from the kenton interface are soldered to various chips and transistors on the io-board.

Power One Power Supplies

Chris Ryan [21030691]

Uro [21030401], not currently subscribed to the list, asks, "is the MAP 80-4000 or MAP 80-4000C the correct replacement power supply, because Power One makes both."

Luca Sasdelli [21010226]

Some time ago I asked Sandro Sfregola [21010294] about the PowerOne (I've got one), and he said that the MAP-80 is perhaps too powerful for a Chroma, especially if equipped with CC+ because of its very low consumption; a switched-mode PSU needs to operate within its typical range in order to work stable and reliable. Therefore a MeanWell T60 or equivalent is recommended.

Jimmy Moyer [21030184]

Some switching power supplies like the MAP 80 have minimum current requirements. There was some question as to whether a Chroma with a CC+ provided enough load.

I put a MAP 80-4000 in my Chroma at first with the original CPU board and later with the CC+. It works fine in either case for me.

I can't see any reference to the "C" version at Powerone, so I don't know the difference. Link?

Old PSU units......

Riccardo Grotto [21010249+]

So now I have five of them on the desk. Are they worth something, some components which might be useful......? Electronic memorabilia?

Luca Sasdelli [21010226]

Hi Riccardo,

indeed your desk should be quite robust, I guess :-)

Membrane switch panels

Ron Joseph [21030042+]

Tech savy list members:

I recently purchased the gold colored overlay w/o switches (Thanks Mark). As the layout and text are similar to the Halloween set [see for instance Chroma 21030620] I'm assuming that a normal membrane switch set wouldn't line up.

I vaguely recall the chap in Australia [Greg Hutchings] mentioned something similar during his sale of his halloween overlays. My question is how hard would it be to manufacture a membrane set? Anyone able to give a general overview on how they work? Simple on/off switches etc. Also, there's a mistake on the overlay, two adjacent parameters are switched; would it be possible to wire these to the right points on the connector thereby rectifying the mistake? Thanks for the bandwidth.....Ron

Mark Smith [2103-PT-002]

Ron -

There's no way to correct the swapped text on 48 and 49. The earth tone switches are the same way. I don't notice it when I use my Chroma

Leonardo Ascarrunz [no serial number]

You can have membrane switches manufactured to spec. See Pannam Imaging.

Ron Joseph [21030042+]

Excellent, Thanks for that.

Ron Joseph [21030042+]

Membrane Panels

So I picked up a set of the Halloween membrane panels w/ switches. I disassembled the Chroma's original membrane panel, replaced it with the "H" membrane panel and tested it. Most of the parameters worked.

The parameters on the left side of the panel (1-5) (31-35?) didn't work or were intermittent; for example parameter 3 came up as 31 or something like that. I trimmed a little bit on each ribbon connector (was I an idiot to do this?) as there was too much extra ribbon and I thought that maybe the ribbon was being pulled thus creating an intermittent connection. Now none of the parameter buttons work.

I think I may have trimmed the exposed contacts on the ribbon too far; is there a way to reexpose some more of the ribbon. Is there a setup/calibration I should do before expected the panel to work?

I looked in the service manual but frankly it's greek to me....Thnx for the bandwidth and peoples time.....Ron

CC + upgrade question

Leonardo Ascarrunz [no serial number]runz [[no serial number]runz">no serial number]

I was wondering if the CC+ source code was available. I plan on doing some hardware modifications and would need the source code to implement them.

David Clarke [21030085++]

Leo - the original Chroma source is available (xref: "Disassembled Chroma (Main Firmware) Source"), but the CC+ source is not (xref: "PLEASE READ CAREFULLY BEFORE DOWNLOADING".)

Chroma earth tone panels for sale on eBay

Mark Smith [2103-PT-002]

Hi all -

I'm selling my last set of top panels with switches for the Chroma. Here's the URL - [item #260266507550]

Good luck!

Pictures from the auction:

My SPSU replacement / Chroma re-animation

Matt Thomas [21010021]


Just thought I'd share my progress installing one of Luca's SPSU kits in my Chroma.

The replacement was prompted by a state of more or less permanent 'E01234567' that began about two years ago and has led to me pretty much giving up on my Chroma since then. In all other respects it'd always worked well, so having read the site it seemed that a power supply replacement was worth a try. It's worth mentioning that I hardly ever open up synths except to replace fuses or clean sliders etc. I don't do electronics.

It's taken me two sessions of a couple of hours each to very cautiously remove the old board and install the new one. I then powered up the Chroma and was instantly hit by a blast of horrible sound (although no keys were pressed). Random stepped pitches, rising and falling resonant gurgling, squealing and whining. Playing the keys yielded very little effect except to introduce a slight arpeggiated nuance to some of the noises.

I turned the power on and off a couple of times. On one attempt when I played the keys I heard what sounded like a familiar patch underneath all the cacophony, but only a few notes. I then powered down and pulled all the voice boards except 0 and powered up. Instant success! No horrible noises, just one monophonic Chroma voice behaving as expected. I then moved the same voice card from slot to slot – still behaving just fine. I then tried each of the other voice cards in slot 0 in turn and scored four good cards and four bad cards.

Next I replaced all the voice cards in their original slots and rather confusingly I now have five good voice cards. And I must say I mean good :) They are all very closely tuned to each other and across the range of the keyboard. At this point I had to call it a day so tomorrow I'll be trying the diagnostic routines on the remaining reluctant voice cards.

This brings me to question of the day – the 5.05V setting from the SPSU. How important is it that this is checked? I ask because it looks like buying a multimeter with that degree of accuracy is going to run me £150+, and like I said, I don't do electronics. I can't really justify such an expensive purchase for something I'm not going to get any further use from. Given that Luca tested the board before shipping is it really likely to be that far out of whack?

Leonardo Ascarrunz [no serial number]runz [[no serial number]runz">no serial number]

You will need a digital multimeter to adjust the Digital to Analog Converter. Which i recommend you do if you changed the PSU and still have tuning problems.

Paul Tillotson [21030218]

I second that: you're just randomly poking around without correcting the voltages from the new SPSU and to the Analog/ digital board. My multimeter cost me less than $20 American, I believe, and did the job. Luca's board, complete and easy as it is, cannot fine tune all voltages within your machine.

Good luck.

Luca Sasdelli [21010226]

Hi Matt,

the Chroma, as all analog-based synthesizers, requires at least a basic meter to service it properly; the items to be checked are the power voltages and the I/O board zero point. You don't need an expensive instrument to do it; as an example, just see Mini Digital Multimeters + Voltage Detector (Amazon), it has 0,5% precision at around US$50.

Jesper Ödemark [21010135]

This brings me to question of the day – the 5.05V setting from the SPSU. How important is it that this is checked? I ask because it looks like buying a multimeter with that degree of accuracy is going to run me £150+, and like I said, I don't do electronics. I can't really justify such an expensive purchase for something I'm not going to get any further use from. Given that Luca tested the board before shipping is it really likely to be that far out of whack?

I used a cheap digital one with just two decimals and that worked splendid. I payed 80 SEK for it, that's less that 7 GBP or ca 12 USD.

good luck.

Luca Sasdelli [21010226]

Hi Jesper,

did you find it in the soap box? :-)

BTW: indeed, although a high-precision meter isn't required, it has to be stable in respect to battery voltage and ambient temperature.

Jesper Ödemark [21010135]

Luca Sasdelli skrev:

did you find it in the soap box? :-)

Almost, in a box of cereals ;)

BTW: indeed, although a high-precision meter isn't required, it has to be stable in respect to battery voltage and ambient temperature.

It's served me well in fact. I've used it a lot. It's just recently that I've started using an analogue one to supplement it with...

Enrico Dibennardo [21030494]

When I installed mine and checked it was already set at 5.05, so I think Luca did test and tune it before shipping, of course voltage must be checked but, was I just lucky or in most cases it keeps correctly set whatever unit you are going to install it in?

Matt Thomas [21010021]


Today's update on the patient -

More voice board swapping and a liberal dose of Split 50s brought the healthy voice board count up to six yesterday. This morning I've set about the Chroma with a multi-meter and calibrated the SPSU. It was showing 5.03 so a small tweak was needed. Next the I/O board which was showing .003 so a small adjustment there has yielded .000 - not sure that my meter is actually sensitive enough to calibrate as finely as required by the service manual, but it's certainly brought the voltage closer to where it should be. As I'm not encountering tuning problems I'm inclined to call that good enough.

Next I've dialled up the preset 0 pulse wave patch, tweaked it as suggested then run Split 31 and discovered that voice A on Board 3 (one of the 'dead' boards) is working fine in that mode.

Then I trimmed the pulse-width on all working voices (including the voice on Board 3) and discovered that one voice on Board 6 (passed by the Chroma as a 'healthy' board) doesn't respond to changes in waveshape 'Width'. To be specific - turning the trimmer on the board causes it to change tone as expected, but it responds only very slightly to even extreme changes from the Chroma's 'Width' control. The tonality of the waveform is slightly different to the other voices, like a thin buzzy harmonic overlaid on a square wave, even at the nearest I could get to a null position.

Next I switched my preset 0 patch from pulse wave to noise. In this mode Board 3 outputs noise on both voices just fine, though the cutoff is offset lower on voice B than on the healthy voices.

Next I turned 'Resonance' to full and found that it produced a self-oscillating sine wave on Board 3 voice B and Board 2 voice A. The sine wave behaves as you'd expect and can be swept with the 'Tune' parameter, manually or via LFO etc

Lastly I played with the amplitude envelope; all the voices that I could get to make any sort of sound responded as expected, so VCAs seems fine.

In summary -

Board 2 -

  • Voice A doesn't output VCO or noise but with resonance set to maximum it self-oscillates and produces a sine wave.
  • Voice B is dead as a dodo, outputting nothing at all.

Board 3 -

  • Voice A works fine but only in Split 31.
  • Voice B outputs white noise but not the VCO. Voice B's VCF is functioning but the cutoff is offset lower than on the healthy boards. At max resonance the filter self-oscillates (though not if white noise is selected as the wave type, in this situation it behaves as a healthy voice.)

Board 6 -

  • Voice A doesn't respond to changes in Waveshape 'Width'

If any of this suggests particular problems then I'd be grateful for any pointers.

Leonardo Ascarrunz [no serial number]runz [[no serial number]runz">no serial number]

Now that the DAC is calibrated you must find out if it is a problem with the individual voice boards. You can easily determine this by swapping the position of a "bad" voice board with that of a good one, and see if the bad channel moves with it. If it does this is an indication that the voice board is the source of the problem. If the bad channel number stays the same then its most likely a problem with the channel motherboard.

Matt Thomas [21010021]

Hi Leonardo,

Sorry I should have mentioned that - I've moved the position of the voice boards several times in the last few days. The faults are all specific to the voice boards regardless of their location on the motherboard.

Leonardo Ascarrunz [no serial number]runz [[no serial number]runz">no serial number]

Unfortunately there is no easy and quick fix for a bad voice board. You will need an oscilloscope to narrow down what the problem is for each board. After the faulty chip has been identified you will need to replace it. A used analog oscilloscope will run you around $100US.

Once you have the oscilloscope you must check the control voltages on each board. If you look on the sample and hold circuit (see Dual Channel Board Schematic 2) you will see that they are pin 7 & 1 of z21,z22,z34 and z24. These pins deliver the control voltages to the oscillator,filter and amplifier.

Luca Sasdelli [21010226]

Hi Matt,

in the past, I've got a dead-like board like as your Board 2 and I solved simply by replacing Z4 (expo converter) with a TL072, Mosfet (Hi-Z) equivalent to 4558, with a better linear curve and very low noise; this is simply a common dual operational amplifier.

If you decide this way, remember to install a 2x4 tulip-shaped IC socket before the IC itself. Even Z5 and Z10 are same type OAs and you can replace 'em as well, if you suspect them as faulty. Z5 is the square-wave Schmitt-trigger, and Z10 is simply the master output current amp.

Z6 is a precision voltage comparator and, if needed, you have to replace it with another LM393. Together with Z5, it is responsible for the "width" issues, providing that the "width" voltage does make sense.

Three Items on eBay

Chris Ryan [21030691]

CPU Board for parts, item #270257834844, starting bid US$39, no bids yet, ends July 26. From the description: "This board does not power up. There is a good set of R14 Roms installed. Board looks to be complete. Some battery acid damage on battery holder. Item is sold AS IS for parts."

Dual Channel Voice Board, item #270257834854, current bid US$39, ends July 26. From the description: "I think that this board used to have tuning problems. Board looks to be complete and in good shape. However, item is sold AS IS for parts."

Dual Pedal Board, item #270257834858, current bid US$39, ends July 26. From the description: "This is the Sustain/Damper board assembly that came with the Chroma. Has two on/off type switches terminating in a 1/4" stereo plug. Unable to test so sold AS IS."

All the same seller.

Chroma at home with earth tone graphics

Mark Smith [2103-PT-002]

Hi -

Here's a picture of my home setup - Chroma, two EMT10s (sampled piano), and a D50. I have an Korg M1 in the computer room.

Richard Willoughby [21030348]

those colors do look very sweet...........

if I wasnt so broke, I'd bid on those panels in a heartbeat......

Jesper Ödemark [21010135]

Why didn't they stick to that design - it's so beautiful!

(I'm talking Chroma here, not that sorry piece of the worst synthesizer ever on top of it! (Sorry, D-50's get me saying stuff like that!)) ;)

Voice Card Incompatibilities

Ben Rossborough, Cyberwave Ems

Hi All,

I wonder if anyone has any information regarding possible incompatabilities between voice card revisions and different Chromas? It has been my experience that the earlier trace cut type do not function in some otherwise ok Chromas, I am not sure whether this is on OS issue or something else.

When a card is put in one Chroma it works fine. When put in another the autotune fails and on setsplit 31 there is no osc b keyCV from the channel motherboard. Both boards of different revision (early trace cut and later Bsuffix versions) will work fine when put in their host machine. This is not a PSU or calibration or channel motherboard issue.The B keycv seems to be cut off by the OS rather than any component failure. Initially after extensive diagnostics I assumed I must have 8 faulty voice boards! all the same revision! all the same fault ! I have never had too much problem fixing Chroma voice cards but after even more extensive working on all boards (All sh chips caps. etc. etc . running Akey cv through B osc- works perfectly) I got nowhere there was nothing wrong with them, they worked in other Chromas. I just couldn't get the B keyCV on any of them in this machine (rev 12 OS) Put in a B revision card and bingo, there it is! It is of course possible that I have made a mistake somewhere. Any comments from wiser minds than mine would be much appreciated.

I am a professional synth tech based in the UK and I am always looking to buy non working Chromas. The last 2 sold in the UK came from me and both Peter Forrest [21010096] and RL Music said very nice things about them. Thanks in advance.

David Clarke [21030085++]

I wonder if anyone has any information regarding possible incompatabilities between voice card revisions and different Chromas?.It has been my experience

Ben - there are differences between different voice cards in that some of them may have different values of certain components installed. They can affect the range of tuning (i.e., can affect the behaviour of how far they can be out of tune and still have the Chroma re-tune them.)

There are also differences in different versions of firmware in terms of tweaks to the auto-tune algorithm.

That having been said - as long as the card is in good shape, and all other adjustments are OK - I would expect a 'good' card to be able to be tuned by a 'good' Chroma.

You did specifically note the use of Rev 12 firmware. It's been a while since I wrote Comparison - Rev 12 vs Rev 14 firmware [August 2000], but the implication of one of the points seems to say that the auto-tune code in Rev 12 would 'give up' on the tuning once it got to a particular point of failure.

Taken at face value, this might imply the parameters used for the channel could be bad, even if Set Split-31 was run. (i.e., the 'setup' values for that voice could be left in a bad state, thus pushing a setting all the way to one side or another making it seem like the voice was bad).

For debug purposes I'd likely suggest installing a set of Rev 14 ROMs and re-evaluate operation. At least this might get you to the point where you can see what the Chroma thinks is bad.

On the different value side-of-things, a quick check of docs like Field Change Notice FCN2-001 may point out other differences in the cards.

I can report that I've never come across voice cards whose operation was machine-dependent - other than as it related to voltage/DAC/motherboard operation of that machine.