Upgrading the Rhodes Chroma Power Supply

I just replaced the power supply on my Rhodes Chroma with a modern switching supply.  Why would I do such a thing?

• The original Chroma power supply is not a good design.  It was an attempt to use a single-voltage power transformer to supply all of the various required supply voltages, trading off circuit complexity for a simpler transformer .  However, since the largest current is drawn by the lowest voltage (the +5.0 volt digital line) a significant amount of voltage drop is required.  A linear regulator alone would dissipate far too much power to be practical, so the Chroma power supply included a 120 Hz switching preregulator circuit for the +5.0 volt digital line.
• The Chroma is overly sensitive to line transients.  Turning off a fan in the room often plays a random chord!
• The supply is heavy, and its mounting location makes the Chroma lopsided and difficult to carry.
• The power supply circuit generates too much heat which has a noticeable detuning effect on the oscillators.
• The transformer generates inappropriately large magnetic fields that affect nearby guitar pickups.
• My particular unit happens to be acoustically noisy; it makes an annoying buzzing sound.

But I really, really like the Chroma as a musical instrument and it has been my main keyboard instrument for a while.

Switching power supplies are generally not the preferred choice for synthesizers or other pieces of low-level audio equipment.  It is best to keep the switching noise out of the picture.  Switching supplies should really only be considered for situations where there is a high power requirement.

In the case of the Chroma we have a sizable current draw from the microprocessor circuitry (which itself creates a certain amount of switching noise).  An alternative approach would be to replace the microprocessor circuitry with low-power CMOS equivalent chips.  This would minimize the draw on the +5.0 volt digital power supply and you could use a small commercial linear power supply.  This is a major task though; every chip swap would have to be checked over every way, new PROMs would probably have to be blown, and that is more work than I am up to right now.

In the years since the Chroma was manufactured, switching power supplies have become smaller, lighter, more reliable, far less expensive and increasingly practical.  This is mostly due to the economies of scale in supplying the successful personal computer industry.

Disclaimer:  It should be clear that this article is a report on a project of mine and not a recommended procedure.  Such a project is only for folks who seriously grok electronics and are nuts about the Chroma.  It is vitally important to check the wiring many, many times since one bad connection can literally fry hundreds of chips.

Power Supply Requirements

According to the service manual, the Chroma circuitry has the following power requirements:

(The tapper is a solenoid that makes a mechanical thump when the player presses the membrane switches to provide tactile feedback.  The manual is not specific about the tapper's current requirements.)

The replacement supply I choose is a Power-One MAP 80-4000.  Here is Power-One's web site, and here is a copy of the data sheet in Adobe Acrobat format. It is an 80 watt supply that puts out +5.0 volts at 13.0 amps, +12.0 volts at 4.0 amps, -12.0 volts at 1.0 amp and -5.0 volts at 1.0 amp.  (The -5.0 volt output is not used here.)

Power-One MAP 80-4000 switching power supply.

As an added bonus, this power supply also handles all international line voltages automatically.

Adapting the MAP 80 Supply

Very importantly, the MAP 80 features a TTL power-fail output signal. The Chroma uses static RAM chips for patch memory, and it is likely that the select and write lines of the chips will go through inappropriate chaotic changes as the power is turned on or off.  So the power-fail signal is necessary to disable all the RAM chips when the power supply is less than proper, especially during power up and power down operations.  Additionally the power-fail line resets the microprocessor at power up.  The power-fail signal is generated in the power supply itself since the power supply will be the first to know when the power is applied or removed.  (If your particular Chroma suffers from "bit rot" in the patch memory, the power-fail signal might be the cause.)

Unfortunately the MAP 80's power-fail output does not have enough positive output current capability to drive the Chroma's power-fail line, so it is also necssary to build an an active buffer circuit for that.

The MAP 80 does not provide for a separate analog +5.0v output, so I had to make that myself.  It is a basic regulator circuit based on the National LM317T regulator chip running off the +12.0 volt line.

The analog +5.0 volt regulator circuit and the power-fail buffer are built on a small vectorboard card bolted on to the side of the power supply.

This is a schematic of that board:

The extra add-on circuitry for the Chroma power supply.

[Note: On my version, that 720 ohm resistor is really 705 ohms; I used a 1.0k and a 2.4k in parallel as I wanted to hit exactly +5.0 volts.  Later I read in the Service Manual that the analog +5.0 volt supply is best adjusted a little high, to about +5.05 volts.  So 720 ohms is a better value to use.]

What about the tapper supply? It is possible to tap off (no pun intended) 15 volts from the MAP 80 circuit board right before the 12V regulator circuit.  Here you can see the blue wire soldered directly to that point on the copper side of the power supply circuit board and running to the connector.

Also, a fuse needs to be added between the AC line input and the MAP 80.

Connecting the Power Supply Outputs

The original Chroma power supply had four male Molex connectors soldered into the power supply pc board, and cables plugging into those ran to the I/O board, the Channel Motherboard and the EQ board.
I built up some male Molex connectors with small roughly 4-inch long wires with spade lugs on the ends, and these wires connected directly to the power supply terminal strip.

Here is the cable wiring.  It is a good idea to match the colors of the original wires as this provides an additional wiring check as you plug in the connectors.

The various grounds (Analog ground, Digital ground, Tap return ground) all connect to the two ground terminals on the power supply.

Results

It works very well. The Chroma has now slimmed down from 72 lbs to a svelte 65 lbs. At this weight, this difference is important and since the weight is more centered now, the Chroma is easier to carry.  All of the problems mentioned at the top of this article are gone.  The tapper voltage is a little lower, which I think makes for a better feel.

Chroma with original power supply. I did not think to take a "before" photo; this one was graciously donated by Ulrich Behrenbeck. You can see the large power transformer, a large filter capacitor, and the switching transistor mounted on a little board on top of the cap.
Chroma with switching power supply installed.  The new supply takes up a tiny fraction of the space of the original unit. Also barely visible here, below and to the left of the AC line filter, is the added AC line fuse.
A closer view of the power supply. Here the Molex connectors are plainly visible as well as the bolt-on board for the analog +5.0 volt supply and power-fail signal buffering.

More notes

I should point out that this was a lot of work. Mechanically I had to disassemble the beast down to every bolt.

Make sure you get a copy of the Service Manual too.  You can still purchase copies from Fender for a reasonable price, just give them a call.  Know that you will probably have to convince the folks you talk to that they used to be in the synthesizer business.  "No really, this was a product of yours.  And it was one of the best damn keyboard instruments ever.  No, not the Chroma Polaris.  Look, do you have any folks there who've been around for a while?"

Also, you will need to perform the complete calibration procedure in the Service Manual after the new supply is installed.

And lastly, since I had the Chroma disassembled so completely, I refinished the woodwork too.  This instrument had been stored for many years in its flight case and over time a chemical reaction occurred between the finish and the foam inside the case leaving the finish looking pretty bad.  I removed the old finish, sanded the pieces down, and applied a few coats of semi gloss Varathane Diamond Finish.   Here is a photo of the result:

The refinished Chroma.

Postscript

April 2001, five years later: I'm cleaning up this article a little bit, tidying up the HTML, making the tables prettier, fixing a couple pieces of confusing language and so forth.  I'm happy to report that my Chroma has performed flawlessly since the power supply upgrade.