CHROMA POLARIS TROUBLES, CEM 3374.

justinvm · Post by **justinvm** » Sat Jul 05, 2008 9:12 am

Hello,

Well I took the risk of importing a chroma polaris to Australia from the U.S last week. It has been a nothing but touble experience. Including unexpected hefty import taxes at the airport.

I spent 8 hours last night reparing the membrane ribbons that broke during transit. (They were working perfectly before shipping)

Now the synth is up and running again I am having difficulties with tuning and calibration.

Specifically one Voice is not tuning correctly!!! I fear one of the CEM 3374 chips might be bust:

It turns out it is voice 1 which exhibiting the tuning behaviour. I have iscolated this voice for testing purposes. Now the voice appears to be in tune with the other voices when oscillator sync is on. It responds to pitch bend and all other types of tuning modification.

However with sync off, one of the dual VCOs becomes perminently detuned!

All other voices are fine.

Having a listen to it now, it almost seems that there is a perminent pulse width modulation occuring. And one of the oscillators seems to glide into tune.

What are your opinions? The synth has calibration procedures like ensuring the DAC are reading correct voltages etc. I havent bothered to perform that yet, and it seems unlinkely this would be the cause as it is only one voice that is behaving this way.

All help/opinons will be greatly appreciated!

Thanks,

- Justin

Mooger5 · Post by **Mooger5** » Sat Jul 05, 2008 8:46 pm

My Matrix 1000 had a failing 3396 chip once. One of the oscillators pure and simply died while the other functions kept working. Had to buy a new chip.

The sync is normal as the failing osc is being tracked by the healthy one.

I would not dismiss the calibration procedure... Are the chips mounted on sockets? You could swap them to find if it´s always the same chip that´s failing or if the fault is elsewhere.

Good luck.

Post by **nadafarms** » Sun Jul 06, 2008 6:40 pm

Hey I recently bought a chroma polaris from a seller here in the usa who said it works perfect.... yeah right. I just got it back from the tech and after a $100 it still has some issue with the adr amp envelope. The release cuts off instantly instead of fading out like a normal synth would.

Anyways, here are my thoughts for you.

The chroma polaris is wicked! Totally worth all the trouble and I personally think it destroys the overpriced and overrated jupiter 6 which has similar chips? Also destroys the prophet 600, and how much did you pay with import taxes? Screw it, its all worth it my friend because could you please tell me another analog poly that has velocity and ring mod? The chroma polaris is raw as h**l and I like the better than the alesis andromeda honestly its so much more raw an analogy.

So don't feel bummed its totally worth it, lift that chin up! It was worth it even you need to order a chip for $60?

Its also the only poly synth I have ever heard a decent bass tone from!

Cheers

justinvm · Post by **justinvm** » Mon Jul 07, 2008 3:59 am

Hey thanks for the replies guys. Ill start working on it today see what I can find.

I've paid way way way too much for this synth already, I don't feel comfortable calculating the total price. I'm lucky I can do the repairs myself.

I managed to get 5 voices in playable condition the other night. It did sound and feel great and I'm under no illusion that this is probably one of the best Poly's ever made.

Nadafarms, Your ADR problem seems to be a lack of calibration. The service manual outlines a method for calibrating those opamps so they dont just 'drop out'.

- J

justinvm · Post by **justinvm** » Mon Jul 07, 2008 7:50 am

Just did a test. It's not the CEM Chip.

adamstan · Post by **adamstan** » Mon Jul 07, 2008 9:20 am

So maybe it's problem with S/H responsible for pitch voltage for this osc?

justinvm · Post by **justinvm** » Mon Jul 07, 2008 9:25 am

I just checked all the ICs on the voice circuit and all are working fine.

There's only one DAC and sample n hold circuit for all voices. But i'm obviously going to have to check it out.

It's oscillator B thats has the problem.. still working my way through the schamatics.

adamstan · Post by **adamstan** » Mon Jul 07, 2008 10:22 am

justinvm wrote: There's only one DAC and sample n hold circuit for all voices. But i'm obviously going to have to check it out.

That's true for patch parameters I suppose. But for pitch voltage (controlled by keyboard) you need separate S/H per voice - and if a synth has autotune, then it has separate CV S/H cell for each of its VCOs. So maybe the opamp or capacitor at this particular cell is bad?

I've just looked at schematics. For Pitch B it's Zx04 chip and Cx04 capacitor, where "x" is the bad voice number - in your case Z104 and C104

justinvm · Post by **justinvm** » Mon Jul 07, 2008 10:28 am

That would make sense but the schematics don't seem to be suggesting that.

If you want to take a look Page 5 is voicing circuit and previous page present S&H

http://www.rhodeschroma.com/content/pol ... matics.pdf

I'll read the service manual and see what it says.

adamstan · Post by **adamstan** » Mon Jul 07, 2008 10:29 am

Updated my previous post

The actual S/H circuits for pitch, PWm and filter cutoff are drawn on Page 5 - within the voicing circuit, as these have to be handled polyphonically.

justinvm · Post by **justinvm** » Mon Jul 07, 2008 10:39 am

I've already checked out those chips. But ill try again. I'll check the caps next.

The help is much appreciated

adamstan · Post by **adamstan** » Mon Jul 07, 2008 10:48 am

For me, the next suspects (after the capacitors) would be Z101 and Z102 4051 chips.

justinvm · Post by **justinvm** » Mon Jul 07, 2008 10:51 am

ive checked all the ic's. they're socket mounted so i just swapped them with a ic's from a good voice.

they're all good.

adamstan · Post by **adamstan** » Mon Jul 07, 2008 10:53 am

Does the bad VCO track the keyboard, pitch bend etc or does it just run at constant frequency?

justinvm · Post by **justinvm** » Mon Jul 07, 2008 10:57 am

I remember it tracking to pitch bend.. But my pitch bend is not functioning correctly at the moment.

It certaintly tracks to different notes being played. It's almost like it's detuned by an offset amount. But there appears to be some modulation occuring as well. I can't quite pinpoint whether it is PWM or VCO modulation.