Skip Navigation.
Section 0

ChromaTalk Archives: July 2009

Chroma Polaris Service Manual

Chris Ryan [21030691] · Wed, 1 Jul 2009 00:47:07 -0700

New on the site this month is the entire Polaris Service Manual in hypertext format, including large format schematics in PDF.

The Computer Tutorial may be useful as an introduction to the state of the art in the mid-1980s. Circuit Descriptions provides an overview of the main board, control panels, power supply, and output board. Disassembly covers just that; Diagnostics and Adjustments and Checkout should be useful for those who are repairing a Polaris keyboard. Options deals with memory and EPROM upgrades; and the Parts List "outlines most parts needed to service the Polaris."

Thanks to Steve Grom [21030502] for supplying a copy of the manual.

Re: Dual Footswitch on eBay

Go to previous messages in thread, June 2009

Jesper Ödemark [21010135] · Wed, 01 Jul 2009 12:54:34 +0200

Try it and/or post some pictures. The Performance Manual Specifications say "Stereo phone jack - Sustain on ring/Latch on tip. Operates on standard TTL (LS) with 10K ohms pull up to +5V."

Can't see any resistor inside, but it sure would be modifiable since the rest seems to fit the demands...

Patch sheet for Chroma

Joe Porter [21030058] · Fri, 3 Jul 2009 14:54:25 +0000

Hi,

I know this is pretty old school, but is there a Chroma sheet for filling out patch info?

I will be installing the CC+ card shortly, and there are a few patches from the current expanded memory that I'd like to have when I install the CC+. I have no tape player so can't export them that way. There are only a few I am really keen to keep, so was planning to just go through and write down all the patch settings. Since I don't want to forget something wondered if anyone had a paper sheet I could fill out for each.

Chris Ryan [21030691] · Fri, 3 Jul 2009 20:28:12 -0700

I've attached a quick job I put together in OmniGraffle (saved as PDF) based on one I did with pen and typewriter (!) in the 1980s. Let me know if this works. If anyone has suggestions, let me know. Will add this to the Parameter Chart page.

Joe Porter [21030058] · Sat, 4 Jul 2009 10:49:35 +0000

Thanks for that.

FS: Rhodes Chroma

Chris Ryan [21030691] · Tue, 7 Jul 2009 07:41:34 -0700

On craigslist Vancouver; located in Calgary (is this Cantos? John?). CAD$4500.

[craigslist posting ID 1255705461]

John Leimseider [21030434++] · Tue, 7 Jul 2009 09:02:27 -0600

They are from a friend of mine. I'll never sell my Chroma and Expander... The ones for sale are both working perfectly, in very nice condition...

Chroma and Expander For Sale

Chris Ryan [21030691] · Sun, 19 Jul 2009 13:28:27 -0700

"We are selling a Chroma and Expander. They currently belong to legendary B3 Player John Novello from Prog Rock Band Niacin who hasn't used them for years and so we're cleaning house. Expander seems fine other that after auto tune it say error on one voice card. Chroma has several voice cards showing error. So they both need a little maintenance and in good shape cosmetically. It doesn't have pressure sensitivity to my knowledge. The batteries are fine.

"Chroma serial# 21030509 [new to registry] Expander's number was quite wore off and hard to read but it looked like serial# 16330035 [probably not as this is Steve Grom's, unless he sold it]. They are not listed for sale yet anywhere. Would like 4K for the both of them."

Contact: [removed]

Keyboard Scanning Computer Woes

Randel Osborne [21030467] · Mon, 20 Jul 2009 11:18:03 -0400

Hello-

I've spent many hours on the bench with my Chroma lately, trying to correct a problem with the keyboard scanner. I'm hoping that someone may be able to share some wisdom, or point me in a new direction, as I've exhausted my patience, and am feeling defeated.

Upon power-up, my instrument tunes all voices, and plays beautifully. After a short period of time (1-5 minutes), all voices play and hold a random note at maximum volume, without me touching the keyboard. The notes sustain until another patch is selected, and the keyboard is unresponsive. All of the switches and LEDs operate, except Bank Select. The instrument will respond to MIDI via the CC+ board. The instrument will return to "normal" for a while after resetting the power.

The voltages from the switching power supply are rock solid.

I've replaced Z34, Z35, Z32, Z36 and Z37 on the I/O board, and tried to look for anomalies in the signals in the data lines of the keyboard scanning computer. If I remove Z36, I am able to use Bank Select when the problem occurs, so I suspect the problem is related to velocity.

I've cleaned the contacts on the stack switch boards, and replaced the bank select decoders. The only thing that I've found is a little noise on one of the sense amps, and I have new ones on order.

The symptoms would point to something that fails over a short period of time - like a leaky capacitor, yet I am unable to locate anything suspicious.

Does any of this ring a bell for anyone?

Help me, Obi-Wan, you're my only hope...

Leonardo Ascarrunz [no serial number] · Mon, 20 Jul 2009 11:47:24 -0600

Is it always the same voice board that the note gets "stuck" on? Since other switches work, sounds to me more like a problem with the channel motherboard than the computer.

Randel Osborne [21030467] · Mon, 20 Jul 2009 14:07:41 -0400

Leonardo-

Thanks for the suggestion.

All of the voices get stuck, yet they play fine via MIDI, so I'm guessing that the problem lies within the I/O board.

Leonardo Ascarrunz [no serial number] · Mon, 20 Jul 2009 15:54:51 -0600

You are right, if the midi commands work fine the problem is probably in the scanning circuit. That is a very weird problem indeed. Maybe you have an IC that is malfunctioning with heat?. Spraying some dustoff on I/O IC's (too cool) might be a way to identify the culprit. Could also be the scanning computer is crashing. Its been a while since i troubleshooted one of these so i may be way off here.

David Clarke [21030085++] · Mon, 20 Jul 2009 20:24:32 -0400

... all voices play and hold a random note at maximum volume, without me touching the keyboard. The notes sustain until another patch is selected, and the keyboard is unresponsive. All of the switches and LEDs operate, except Bank Select. The instrument will respond to MIDI via the CC+ board...

Randel - you might try pulling the ribbon cable to the stack switch assemblies. If the problem still occurs in the same way, then you can pretty much rule out any electronics on the stack switch boards (e.g., sense amps, etc.).

If I remove Z36, I am able to use Bank Select when the problem occurs, so I suspect the problem is related to velocity.

To clarify - will the problem occur when Z36 is removed, or are you removing Z36 after failure, while the unit is still powered up?

For the issue in general, there is a handshake required in order for the keyboard scanning computer to pass over a keyboard even to the main computer.

The general idea is that the scanning computer will determine note number and velocity, and will output this to its data ports. Once ready it will force a KINT to occur - which will tell the main computer there's data available.

The off-the-cuff implication here is that KINT is firing and that the main computer is reading data. As all voices sound, it would seem that multiple KINTs fire, as if multiple keys are hit.

If the problem can be made to occur just by leaving the keyboard on (without pressing keys), then you could monitor KINT and see if it happens all by itself. If it does, then at least a confirmation of the failure mode may be possible.

Some checks could be made to ensure that signals from the CPU board do get to the I/O board where expected (e.g., 8MHz clock, KINT, etc.) A brute force approach would also be to swap out Z49 (74LS10, involved in KINT generation), Z31 (the scanning computer itself) and Z33 (the PROM, responsible to tell the keyboard scanning computer how to act).

By the way - how did this problem manifest? E.g., is it something that has recently started or perhaps something that has just gotten worse? Did anything seem to accompany the occurrence of the bad behavior (e.g., CC+ installation, PSU installation, etc.)?

Randel Osborne [21030467] · Tue, 21 Jul 2009 10:21:08 -0400

David-

Thanks for your insight and further explanation of the keyscan process.

I need to spend some time with the instrument to try some of your suggestions - removing the ribbon cable to help localize the problem, and monitoring KINT.

Last night I was monitoring 2 of the inputs of the Keyboard Scanning Computer, KD6 and KD15 on Z34. During normal operation, the waveforms looked like the following:

_'_'_'_'_'_'_

with a single-pulse square wave when a key was depressed.

When the instrument failed, the frequency of the cycle was halved, then quartered, intermittently, like the following:

__'__'__'__'__'__'__'

____'____'____'____'____'

with the 0v portion of the waveform very jittery.

The problem will not occur when Z36 is not in place when the instrument powers up.

I have a spare I/O board, and have swapped Z49 and Z31, I have not yet swapped the PROM.

I purchased the instrument with a dead power supply, and acid-damaged computer board. It has been resurrected with a switcher and CC+ board. This particular problem started a few months ago, and the only change is the length of time between I turn it on, and when the failure occurs. Sometimes it happens in seconds, and last night I played the Chroma for about 15 minutes until it happened.

As luck would have it, it often runs quite well when the cover is off, the panel is flipped up, and oscilloscope probes are hanging off of everything. The minute I put it all back together, and bring it upstairs to the studio to record, however...

Arun Majumdar [21030348+] · Tue, 21 Jul 2009 21:27:39 -0400

Sounds almost like a temperature-sensitive issue. Usually happens with bad caps or resistors or solder joints --- or (like in the case of my Synergy synths) the ic mounting (which I don't believe should be the case.

I could be dead wrong about this, and *DO NOT* attempt this if you do not have an intuitive feel for electronics!

The way I would test is as follows:

  1. Have a warm air blower (a hair dryer) at a distance of about 2 feet (else you will overheat the boards) and open the case, turn it on and measure the time to fail
  2. Use a cold-spray *freeze* (or better, a cool air blower from a mobile AC unit) --- and measure the time to fail (if it does).

If you notice a time to fail is fast with (1) and slow with (2), then it is usually a connection problem (first) or then a capacitor problem and lastly, possibly though hard to image how, a resistor problem.

I have found hairline fractures in printed circuit board traces (though not on my Chromas) that with temperature cause circuit failure --- you could trace it with a resistance-sensitive beeper but more often than not, you have to change the temperature to find such a fault - they're nasty.

I hope this commentary helps -- I've only done this myself on some other gear once in my life about 20 years ago!

Randel Osborne [21030467] · Mon, 27 Jul 2009 09:08:27 -0400

Keyboard Scanning Woes - Solved!

Thanks to all for your suggestions and insight regarding my Chroma with the keyscan anomaly.

I'm pleased to say that it is up and running, and for anyone that is interested, here's the rundown:

One of the data lines from the stack switch boards (KD4) had a fair amount of jitter - the other data lines looked like _'_'_'_'_'_'_'_'_'_'_, while KD4 looked like _'_'--'_'--'-'_'--'. The jitter worsened over time, eventually becoming so bad that the keyboard scanning stopped, with hung notes.

The cause of the problem was a cold solder joint on Z9 on the stack switch board - the resistor bank that pulls up the outputs of the sense amps. KD4 should have had a 10K resistor connecting it to 5V, when, in fact there was much greater resistance. Temperature change, as well as the vibrations caused by playing the instrument caused this problem to manifest itself intermittently.

Cleaning and reworking the solder on Z9 fixed the problem, and the Chroma is off the bench, and back in the studio making music.

Jesper Ödemark [21010135] · Mon, 27 Jul 2009 16:28:16 +0200

Re: Keyboard Scanning Woes - Solved!

Congratulation! :D

Andrew Dalebrook [21010180] · Tue, 28 Jul 2009 08:14:03 +1200

Re: Keyboard Scanning Woes - Solved!

Excellent news!

Chroma 21030504 lives and breathes again...

Michael Krewitsky [21030504+] · Wed, 22 Jul 2009 08:29:56 -0700

Hello,

I am pleased to announce that after 14 years of non-operation and a recent Nuclear Powered Chroma PS modification, that my Rhodes Chroma SN 21030504 is back among the living. It is an even finer sounding and playing instrument than I remembered...

Now, on to the Expander [16330032].....

We [ProSound Inc.] are ready to tackle Chroma repairs as always....

Randel Osborne [21030467] · Wed, 22 Jul 2009 11:35:20 -0400

Congratulations!

You may now reclaim the small subroutine in your brain that has been devoted to fixing the instrument for the past 14 years. At least for a while...

Rob Belcher [16330123] · Wed, 22 Jul 2009 18:17:44 +0100

Congrats on your repairs!

Actually my Chroma Expander is also now working too, just came back from an excellent tech friend that has been looking after all my repairs for last few years..

I had a real bad tuning issue which has been resolved, a faulty chip on the digital board.. Ive done a hell of a lot to my Expander (replaced all the capacitors throughout/ new switching psu/ new woodwork, had the top and back, resprayed black, fitted CC+, calibrated it, general service etc) .. cost a ton of cash too.. but im glad i stuck with it it sounds fabulous.. so good in fact i decided i could sell my Jupiter 8!

Michael Grossman [21030350] · Wed, 22 Jul 2009 12:37:31 -0700

What was the tuning issue and which digital chip fixed it?

Rob Belcher [16330123] · Wed, 22 Jul 2009 21:31:35 +0100

No idea mike im afraid, just regurgitating what my repair tech told me. Im just glad its finally working! :)

Jesper Ödemark [21010135] · Thu, 23 Jul 2009 09:01:44 +0200

I am pleased to announce that after 14 years of non-operation and a recent Nuclear Powered Chroma PS modification, that my Rhodes Chroma SN 21030504 is back among the living. It is an even finer sounding and playing instrument than I remembered...

Now, on to the Expander.....

Congrat's and good luck! :

Jesper Ödemark [21010135] · Thu, 23 Jul 2009 09:06:42 +0200

Congrats on your repairs!

Actually my Chroma Expander is also now working too, just came back from an excellent tech friend that has been looking after all my repairs for last few years..

I had a real bad tuning issue which has been resolved, a faulty chip on the digital board.. Ive done a hell of a lot to my Expander (replaced all the capacitors throughout/ new switching psu/ new woodwork, had the top and back, resprayed black, fitted CC+, calibrated it, general service etc) .. cost a ton of cash too.. but im glad i stuck with it it sounds fabulous.. so good in fact i decided i could sell my Jupiter 8!

Another "congratulations" goes out to you then and yes, I'd keep the Chroma over any Jupiter (if I'd owned one - the closest to me owning one was a ProMars) any day!

Chroma for Sale on Craigslist Minneapolis

Chris Ryan [21030691] · Thu, 23 Jul 2009 07:47:17 -0700

USD$750. "Doesn't make sound, but cosmetically near-mint. Comes with fitted Anvil case."

[Craigslist Posting ID 1284180410]