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ChromaTalk Archives: June 2015

Re: OT: ARP 4075 VCF "replica"

Go to first message in thread, May 2015

Mirko Lüthge [21010245+]

[...] Anybody interested?

yes, of course. Which format ?

Luca Sasdelli [21010226]

Thank you Mirko,

format? It will be exactly the same of the original, with pin-to-pin matching, to be used as a drop-in replacement. Sales price should be around 150,00 Euro.

The pins in the original are made of thick wire, to be inserted into test unit I guess; these replicas will have zero-ohm resistors instead.


Mirko Lüthge [21010245+]

Hi Luca,

that is fine. If finished send me one. adress:

[Address removed]


Best wishes


Re: Tapper/'phantom' button presses

Go to previous message in thread, May 2015

Frank McGing [21010198]

On 30/05/2015 02:26, Paul D. DeRocco wrote:

There's no J4. Did you mean J14, the leftmost membrane switch connector?

Hi Paul, thanks for your reply! Yes, I meant J14.

If so, that suggests a problem in the switch scanning circuit, or possibly the left membrane switch panel itself. Since the STB0-6 lines on pins 1-7 of J14 also go to the right panel on J15, then they're probably okay on the board (that's assuming that all 50 of the numbered switches work when you unplug J14). That leaves the SW5-7 lines on pins 8-10, or the panel itself.

I have confirmed that all 50 of the numbered switches work fine when J14 is unplugged.

You can try things like leaving J14 unplugged and touching a wire between one of pins 1-7 and one of pins 8-10, to see if that invokes a clean switch press, or results in the same haywire behavior. If the former, then there's a problem with the membrane switch; if the latter, then the problem's on the board, and could be Z16 or Z17.

I can invoke clean switch presses by touching the pins you mentioned with J14 unplugged, and the Chroma doesn't go haywire or reset itself when I activate Param Select. I'm also not getting any flickering of the four Link LEDs with J14 unplugged. I can cleanly activate Link Unison, No Link and Edit A by touching the appropriate pin combinations with the wire. So that might indicate a membrane issue, but see below for the results of a membrane continuity test...

If it turns out to be the panel, it could be some abrasion through the insulation on the ink side of the tail, allowing it to make contact with the metal panel. I would think that would be pretty easy to fix by sliding a sliver of plastic in between the tail and the metal.

I used a piece of card to insulate the membrane tail from the metal panel but it made no difference when I powered up with J14 plugged back in. The Chroma once again went haywire.

On closer inspection, I see that the J14 membrane tail has a slight crease in it, running diagonally from pins 5 to 10. So I used a continuity tester on the tail contacts to check for intermittent connections to the panel switches, but everything seems to check out! All the problematic switches are bleeping out cleanly with no issues.


Frank McGing [21010198]

I believe I've found the answer to my Chroma's phantom button presses, and it looks like a problem with two switches in the left side membrane. :-(

After running around in circles and ending up at one dead end after another, I returned to the J14 membrane tails and tested resistance between contacts 1-7 and 8-10. All combinations measured from roughly 18 to 23 megohms, except for:

  • J14-3 to J14-8: 136 ohms (Link Unison)
  • J14-6 to J14-10: 2.5k ohms (Edit A)

Is there any way to fix this, or am I sunk? The Chroma is pretty much unusable when J14 is plugged in to the I/O board, and with J14 unplugged, I lose all the left panel switches.


John Leimseider [21030434++]

I have a couple of NOS panels left. They are not perfect cosmetically, but they work perfectly. They are $150 Cdn each plus shipping. The shipping is crazy expensive from Canada, depending on where it's going. If you need one, figure out the shipping method that you prefer.

Dave Manley [21030547]

What's the failure mechanism in the switches. Is it possible to rehabilitate them mechanically or with heat, etc.

How about "burning" them open with current and adding a switch. Undesirable, I know...

Dave Manley [21030547]

I see a few articles online regarding silver dendrite growth causing short failures in old membrane switches.

There is a similar dendrite problem in Hammond organ percussion switches that some people "fix" by zapping the switch, in the open position, with a 9 volt battery. You might try a similar experiment - a short duration, low voltage, current limited "zap" and see if the measured resistance changes. Do so at your own risk!!!

Paul DeRocco [21030230]

The resistance of silver traces is generally in the tens of ohms. 2.5K therefore sounds like a really tiny bit of silver, which might well be fusible. 136 ohms, I'm not so sure about. But since it's a total loss otherwise, I'd give it a try. If it succeeds, I'd work the offending switches with my finger to see if the failure recurs, and possibly zap it again, before putting the whole instrument back together.

Frank McGing [21010198]

Good idea, Dave! I'll give it a go and see what happens. As Paul said, it's a total loss otherwise, so it looks like I have nothing to lose.

Is it common for membrane switches to 'just stop working'? I was under the impression that it's rare for a Chroma membrane to give trouble, and my Chroma has just sat on its stand in the studio for several years.

Frank McGing [21010198]

Update: I 'zapped' the problematic membrane contacts on J14 with a 9V battery and the results were interesting!

It took several rounds of 'zap, measure, exercise switches, measure' to get a relatively stable resistance measurement, and here are the results, measured again on the J14 membrane tail:

  • J14-3 to J14-8 went from 160 ohms to 8 megohm
  • J14-6 to J14-10 went from 2.5k ohms to 4 megohm

I reinstalled the I/O board, powered up, and the Chroma is definitely behaving better now. The four Link switches have settled down, and Param Select no longer causes a crash/reset. Edit A is still behaving erratically, but at least it's now semi-responsive.

Meanwhile, I did the smart thing and emailed John Leimseider to purchase one of his NOS panels!

Go to next messages in thread, July 2015

Not able to receive Program Change


Hi all!

I have a simple but yet irritating issue. Somehow I'm not able to receive program change messages to my Rhodes Chroma CC+ from my DAW. Other hardware synths don't have this problem. What am I missing?

Thank you in advance!

Best regards,

David Clarke [21030085++]

Magnus - both the Syntech MIDI interface and the CC+ allows the receipt of Program Change messages to be turned off - and so that setting will be the first item to check. xref: See The Chroma CPU Plus (CC+): User's Guide - Configuration Interface & MIDI Support.

Specifically - go into the configuration interface by using:

[Set Split][36]

You should see "EP" in the large two-digit LED display.

Press [4], to select the Program Change option. If this is set to 'off', then set it to 'on.'

If this setting is already set to 'on' then program may be changing, but it may not seem to change, depending on how testing is being done.

Specifically - a MIDI program change will change the instrument being used for a given MIDI channel. This doesn't necessarily mean (for instance) that if you send a MIDI program change that you'd expect to see the Program Number in the main panel of the Chroma change - or that the program you hear when you press the keys local to the Chroma will change.

Best regards,

David Clarke

Kybd scanner fault

Luca Sasdelli [21010226]

Hi all,

a Chroma currently under repair, has a non-working keyboard. Original CPU board installed.

No flickering at all during fast keypresses. Autotune works okay (all DVB in tune).

I've fairly checked the kybd scanning section, but it seems that all main signals are ok:

  • _KINT signal acts when pressing keys, with correct logic states
  • _E signals working on Z36 & Z37
  • note and velocity data are present at Z36 & Z37 inputs

Does anyone have some further checks to do? Or should I get a logic analyser to read data bus stream?


Paul DeRocco [21030230]

When you say "No flickering at all during fast keypresses" do you mean that you do get flickering for slow keypresses?

And when you say "all DVB in tune", do you mean that you've gotten them to play over the Chroma interface, and they sound in tune?

If it really is in the keyboard scan area, the only thing I can think of that might account for failure to play, while KINT is still happening, is if the driver for the velocity byte is just dead, and the main CPU is reading zeroes all the time. But somehow, that seems unlikely.

Luca Sasdelli [21010226]

Hi Paul,

thank you for your prompt answer; there is no flickering at all: all displays are steady, regardless of any key action. The logic seems to work normally: during boot-up, the DVBs waveform on TP1 &2 follow the usual routine, the ZCD Z20 triggers properly and the failed boards (an empty slot indeed, to check it) are shown in the little display. All front panel functions are normally working.

No sound has been got from the instrument yet. After auto-tune, all boards have the CHA and CHB square wave between 140-300Hz and 5.0kHz, depending on the TUNE parameter, that correctly affects all DVBs together. At low frequencies, the falling edge has a sort of modulation (the falling edge appears as if the frequency has some modulation, more evident at low end), even if all mod depths are zero, that makes difficult for my two digital scopes to define a given measure.

I'm about to read the data bus with a little logic sniffer to see.


Luca Sasdelli [21010226]

Hi Paul,

now it sounds. I did exactly nothing. Power cycled, autotune okay, and now it sounds.


David Clarke [21030085++]

Luca - a couple thoughts (before digging into the schematic):

  1. 1) Do you have an internal MIDI interface that you can temporarily use (e.g., KMX/Syntech, JL Cooper, etc.)? If so - then it would be good to confirm that MIDI IN will allow everything to sound - as well as to confirm that nothing is seen on MIDI out when the keys are pressed.
  2. 2) Is there any info on the 'history' of this unit? E.g., was it working and then stopped - or has it never been seen to work with the current combination of HW?
  3. 3) It might seem seem like an odd question - but do the Main/Link transpose buttons on the panel actually 'work'? If not - and if you're appearing to not see any response to the keys - is there any chance that instead of having Chroma firmware in the EPROMs that the CPU board is running Expander firmware? (EPROM Code) Maybe someone previously tried to get it to work by swapping in an original CPU board (not realizing that it was originally from an Expander?) The expander and chroma versions of original firmware are almost identical - they just different in a few areas where the front panel buttons are different - and of course, expander firmware is set to specfically ignore anything from the keyboard.

Luca Sasdelli [21010226]

Hi David,

thank you for your thoughts: in the next days (months? this one should have the CC+ I've ordered, so until september it will stay here) I'll investigate in that strange behavior. Right now, the instrument seems working properly.


FS: Chroma Polaris

Nicholas Vining [21030777]

For sale in Vancouver BC - one Chroma Polaris. This is the one that was retrieved from Trading Musician in February 2013, and has languished in various project bins and states until recently. It has recently been recapped by Anthony at Music.Teck services in Victoria (who is also an excellent repair person for all things Chroma and Polaris), and the only thing that was wrong with it when I got it back from him was a dud CEM3374 VCO. I have now replaced it with one purchased from eBay, and the machine fires up and all six voices pass auto-tune.

It has the original membrane, but all membrane panels and buttons work. I kind of don't trust the master slider, and you might want to replace that in the near future, but otherwise as far as I can tell it's in good shape.

I've listed it on Craigslist Vancouver for $900 CDN, and Vintage Synth for $900 USD, commensurate with the amount of nuisance involved in shipping it. That said, please feel free to make me an offer. I'm happy to ship this basically anywhere in America, but it is heavy and it is also at the buyer's own risk - I cannot guarantee it'll be fine, especially given the membrane panel situation. Local pick up in Vancouver is best, and I *know* there are Vancouverites on this list...

(I also have a Yamaha CS40m currently looking for a good home, for "some amount of money that's around $1000.00", if anybody's interested...)

-- N.

PS. Other Chroma News: my Expander is currently languishing on the floor, not being used, but I hope to do something interesting with it soon. Chroma 21030777 has developed all kinds of crazy habits, including holding a tune just fine and then suddenly trying to drop all of its voices *at once*, AND random knocking noises from the tapper, so it's back in the shop. Ugggggh.

One of two Rhodes Chromas for sale

Dylan Wahl [21030410]

Hello all,

I have two Rhodes Chromas, and I'd like to sell one of them. I guess I can sell either one, they differ in price and condition. One is nearly fully restored, woodwork redone, has the SPSU and CC+ installed and would be around $4500, the other needs restoration but is clean, and would cost $3000. Both have road cases.

If you are interested, I am also willing to swap the SPSU and CC+ to the unrestored one for around $600.

Email me if you would like photos or more info. Thanks!