Rhodes Chroma Polaris

Hello all!

I've owned my Polaris forever but it's spent almost all of the last decade in storage.  I recently pulled it out with the intent of selling it, but it's developed some issues I'm hoping you folks might have some thoughts on.

The first is the ultra-common one: bad membrane switches.  I tried to fix them myself but have only succeeded in making the situation worse.  Are new ones available?  Google yielded some references to someone selling them several years ago but I didn't see anything recent.

The other problem it's having is really weird.  As soon as you connect the MIDI output to something, there's a click in the audio output every time you hit or release a key or move a slider.  I think basically it's making a click every time data is sent out the MIDI port.  Has anyone else seen this problem?  I'm thinking it could be a decoupling cap issue but I have no idea where to look.  It's not a ground loop because it does it when connected to a laptop computer that's not connected to anything else.

Thanks in advance for any thoughts!

> From: eric@...
>
> I've owned my Polaris forever but it's spent almost all of
> the last decade in storage. I recently pulled it out with
> the intent of selling it, but it's developed some issues I'm
> hoping you folks might have some thoughts on.
>
> The first is the ultra-common one: bad membrane switches. I
> tried to fix them myself but have only succeeded in making
> the situation worse. Are new ones available? Google yielded
> some references to someone selling them several years ago but
> I didn't see anything recent.

Contact me off-list. I've got them.

> The other problem it's having is really weird. As soon as
> you connect the MIDI output to something, there's a click in
> the audio output every time you hit or release a key or move
> a slider. I think basically it's making a click every time
> data is sent out the MIDI port. Has anyone else seen this
> problem? I'm thinking it could be a decoupling cap issue but
> I have no idea where to look. It's not a ground loop because
> it does it when connected to a laptop computer that's not
> connected to anything else.

It could be a grounding problem somewhere. I recommend disconnecting and reconnecting everything with a connector inside, since contacts can get dirty with age. It could also be a power supply problem, but I'd think you'd hear lots of hum on the output if that were the case.

--

Ciao, Paul D. DeRocco
Paul mailto:pderocco@...

Well I've figured out the "clicking" thing... and the Polaris isn't broken!

AFAIK the MIDI spec calls for the input on a device to be nothing but an optoisolator conected to pins 4 and 5 of the DIN socket... no other connections.  But some newer/cheaper MIDI interfaces, including the one I was using, "cheat" and steal power from the MIDI input by grabbing +5 volts off pin 4 and using the shell as negative.

Well in the Polaris, the MIDI output connector shell is connected to the chassis, and only to the analog/digital grounds through a 1-megohm resistor.  So when current flows through the shell, it energizes the chassis relative to the digital + analog grounds.  The current flow varies depending on whether data is or is not being sent out the MIDI port and I think that was causing the clicking I heard.  Just for reference, with nothing plugged into the MIDI port I measured a few hundredths of a volt between chassis and analog ground (at the PS board), with the interface plugged into the port I measured almost a volt!

It seems like what needs to happen is (other than manufacturers making MIDI interfaces that follow the spec, and good luck with that) to connect the MIDI connector shell to digital ground.  But doing that with the Polaris in the factory configuration would result in digital ground being connected to the chassis which I think would be bad for a number of reasons.  The only solution to *that* problem I can think of is to replace the MIDI output connector with an isolated one and I'm too lazy to do that.  ;^)

> From: eric@...
>
> Well I've figured out the "clicking" thing... and the Polaris
> isn't broken!
>
> AFAIK the MIDI spec calls for the input on a device to be
> nothing but an optoisolator conected to pins 4 and 5 of the
> DIN socket... no other connections. But some newer/cheaper
> MIDI interfaces, including the one I was using, "cheat" and
> steal power from the MIDI input by grabbing +5 volts off pin
> 4 and using the shell as negative.
>
> Well in the Polaris, the MIDI output connector shell is
> connected to the chassis, and only to the analog/digital
> grounds through a 1-megohm resistor. So when current flows
> through the shell, it energizes the chassis relative to the
> digital + analog grounds. The current flow varies depending
> on whether data is or is not being sent out the MIDI port and
> I think that was causing the clicking I heard. Just for
> reference, with nothing plugged into the MIDI port I measured
> a few hundredths of a volt between chassis and analog ground
> (at the PS board), with the interface plugged into the port I
> measured almost a volt!
>
> It seems like what needs to happen is (other than
> manufacturers making MIDI interfaces that follow the spec,
> and good luck with that) to connect the MIDI connector shell
> to digital ground. But doing that with the Polaris in the
> factory configuration would result in digital ground being
> connected to the chassis which I think would be bad for a
> number of reasons. The only solution to *that* problem I can
> think of is to replace the MIDI output connector with an
> isolated one and I'm too lazy to do that. ;^)

Interesting. The shell is connected to the chassis, but nothing in the cable is supposed to be connected to the shell. The shield is supposed to be connected to pin 2 (the middle pin) at each end, which is connected to digital ground in the transmitter but left disconnected in the receiver. If you measure 0 ohms between the shells of both plugs, then it's the cable that's violating the spec, and tying the two chassis grounds together, which defeats the purpose of the optoisolator.

A small completely floating MIDI device could safely ground itself to pin 2 of its input. Taking power from pin 4 doesn't necessarily work, though, because the spec doesn't actually require that pin 4 be connected to +5V and pin 5 be driven from an active low output through resistors; that's just a suggestion. Pin 4 could be connected to an active high output and pin 5 to ground through resistors.

But if you've got something that's taking MIDI in and converting it to USB, then it ought to be powering itself off USB, in which case it wouldn't need power from MIDI.

If you have some device that must be grounded through its input connector shell, you could wire up your own cable that ties the shell at that end to pin 2 on the other end, and plug that into the Polaris.

--

Ciao, Paul D. DeRocco
Paul mailto:pderocco@...

The device I'm connecting to the Polaris is one of those $10 made-in-China USB MIDI interfaces, so I'm sure strict adherence to the spec wasn't their top priority. ;^)

Also I made an assumption that I probably should not have made... they may well be using pin 2 as the return and not the shell.  I think I just read somewhere that some interfaces use the shell as a ground/return so I assumed that's what this device was doing.

According to the Polaris schematic, pin 2 and the shell of the MIDI output connector are both connected to chassis ground.  I'm not too keen on connecting the shell to digital ground, but connecting pin 2 might not be so bad?  If it turns out the interface is using pin 2 as a return maybe I'll try that.

> From: eric@...
>
> The device I'm connecting to the Polaris is one of those $10
> made-in-China USB MIDI interfaces, so I'm sure strict
> adherence to the spec wasn't their top priority. ;^)

I have a cheap Chinese interface too. It's black, oval, and has a stylized G-clef on it. If that's what you've got, be careful: it has a bug which causes it to swap the order of two incoming bytes every once in a while, so I don't use it any more, except for crude testing.

> Also I made an assumption that I probably should not have
> made... they may well be using pin 2 as the return and not
> the shell. I think I just read somewhere that some
> interfaces use the shell as a ground/return so I assumed
> that's what this device was doing.

USB MIDI interfaces are powered off the USB. They shouldn't be taking power from pin 4 of their input (the synth output), or need the ground on pin 2.

> According to the Polaris schematic, pin 2 and the shell of
> the MIDI output connector are both connected to chassis
> ground. I'm not too keen on connecting the shell to digital
> ground, but connecting pin 2 might not be so bad? If it
> turns out the interface is using pin 2 as a return maybe I'll
> try that.

The first thing to try is just to cut it. If the interface stops working, then try connecting it to digital ground. I'll throw a few Molex KK-100 contacts in when I send your panels today, so you can connect pin 2 on the MIDI OUT DIN to pin 5 on J11. I think those contacts will fit into that body.

--

Ciao, Paul D. DeRocco
Paul mailto:pderocco@...

Paul,

First things first: thank you for the membrane switches and connector contacts!  You've helped give this old beauty a new life.

I do have that same MIDI interface.  Thanks for the tip on its issues... luckily I don't use it very much as a have a Midisport 2x2 that's my "main" interface.

I have no idea why that interface would draw power off the MIDI output but it definitely does.  With the interface not connected to a computer, if you plug its input into the synth's MIDI output, the red LED nearest the MIDI cables on the interface lights up.

Anyhow, disconnecting pin 2 on the Polaris MIDI output from chassis ground and connecting it to digital ground via pin 5 on J11 on the main board completely fixed the "clicking" issue.  Yay!

Anyhow if anyone's interested and hopefully this doesn't violate group policy, my Polaris is now for sale.  Price is $950 but I can come down a bit for members of this group.  Local pickup only in Wilmington, MA.  You can read more details here: favorite this post Fender Chroma Polaris synthesizer - $950 hide this posting unhide
Oh one other thing... I've seen some people on the internet comment on something that bugs me a bit with the Polaris - that the pitch-bend lever works "backwards" compared to most other synths.  It looks to me like that could easily be changed by swapping pins 4 and 6 (orange and brown wires) on the plug that goes from the levers to the left control board.  Has anyone ever tried that?

> From: eric@...
>
> I do have that same MIDI interface. Thanks for the tip on
> its issues... luckily I don't use it very much as a have a
> Midisport 2x2 that's my "main" interface.
>
> I have no idea why that interface would draw power off the
> MIDI output but it definitely does. With the interface not
> connected to a computer, if you plug its input into the
> synth's MIDI output, the red LED nearest the MIDI cables on
> the interface lights up.

That doesn't happen for me. I think there's something broken in that MIDI interface. Although it's in the same package as mine, it's also possible that it's a different board inside--they may have "revised" the design.

> Anyhow, disconnecting pin 2 on the Polaris MIDI output from
> chassis ground and connecting it to digital ground via pin 5
> on J11 on the main board completely fixed the "clicking" issue. Yay!

That's probably an acceptable solution, even when using it with a good MIDI interface.

> Oh one other thing... I've seen some people on the internet
> comment on something that bugs me a bit with the Polaris -
> that the pitch-bend lever works "backwards" compared to most
> other synths. It looks to me like that could easily be
> changed by swapping pins 4 and 6 (orange and brown wires) on
> the plug that goes from the levers to the left control board.
> Has anyone ever tried that?

Yes you could do that, but you'd also have to loosen the pot and rotate it some. It only uses a fraction of its range near one end, and it would have to be rotated to use the part of the range near the other end. However it's wired, you want it to be putting out around 2.5V at rest, so that it won't go all the way to 0V or 5V at either extreme.

I should have included a polarity switch in the software.

--

Ciao, Paul D. DeRocco
Paul mailto:pderocco@...