CHROMA POLARIS TROUBLES, CEM 3374.

justinvm · Post by **justinvm** » Thu Jul 10, 2008 7:27 am

Oh nice. Feel free to come over to Australia and have a jam when it's fixed

I just removed c205. It's measuring 7.29nf. So it appears to be within tolerance.

adamstan · Post by **adamstan** » Thu Jul 10, 2008 7:40 am

Yeah, that value seems correct, but if you have ordered new one, I would replace it anyway. I'm a little surprised that only PW cap was wrong, because it appears strange to me that "dirty" voltage on PWM input affects overall pitch (as PWM has nothing to do with sawtooth). However, it is possible, that as C207 was ~20 times too small, PWM voltage was rather "spiky", and these spikes could crosstalk to pitch input. So replacing C207 with correct value should fix the problem.

Oscilloscope is really handy in such cases.

justinvm · Post by **justinvm** » Thu Jul 10, 2008 7:56 am

Well i've still got 2 more caps to check 204 and 206.
So i'll see what they come up as.

justinvm · Post by **justinvm** » Thu Jul 10, 2008 8:33 am

Ok both of those caps were also within tolerence.

I'll replace them regardless. Hopefully it was just 207 that was causing the problem

It's really strange though.. If the pulse width cap was causing the problems then the seller would have to had noticed it. He has stated that all voices worked and were in tune.

The only thing that i can conlude is that he used the presets from banks A and B which mainly used oscillator sync thus covering up the problem. But still...

There was no sign of solder burn or circuit manipulation so I really think that this capacitor was installed by mistake on the factory floor. There are identical caps on the other side of the voicing circuit. Further more some of the other voicing circuits have a 'lack of solder' which makes me think the dude who put this board together was in a hurry to eat his lunch.

adamstan · Post by **adamstan** » Fri Jul 11, 2008 6:02 am

As some reviewers at harmonycentral stated, in Polaris PWM affects sawtooth wave. It may happen - waveform selectors are really strange to me - it seems that you get pulse wave in one setting, and mix of sawtooth an pulse in the other, but you cannot mute pulse wave with waveform switch. So now I'm pretty sure, that your problem was caused by C207 being too small.

BTW Pulse comparator is quite unusual (but simple - and that really counts in polysynth). Instead of feeding sawtooth into one input, and PW voltage into another, as most VCOs do, it uses constant reference voltage, and shitfs sawtooth up and down with PW voltage.

justinvm · Post by **justinvm** » Fri Jul 11, 2008 6:41 am

Hey,

I just installed new Polyester caps for width and put back in the original Polypropylene for pitch (because my replacements havent arrived yet)

It appears the detune problem is gone. Pitch tracking is still poor. So I think I'll need to replace C205 as well.

Still in the process of testing though.

I haven't had a proper play on the synth to comment about the waveform select, but pwm on the saw sounds right to me.

- J

justinvm · Post by **justinvm** » Fri Jul 11, 2008 6:56 am

Actually i'm not sure if detune is still occuring.

It could just be my pitch bend which has been having some 'responding' issues.

adamstan · Post by **adamstan** » Fri Jul 11, 2008 6:56 am

justinvm wrote:Hey,

I just installed new Polyester caps for width and put back in the original Polypropylene for pitch (because my replacements havent arrived yet)

It appears the detune problem is gone. Pitch tracking is still poor. So I think I'll need to replace C205 as well.

What do you mean be poor pitch tracking - doesn't it stay in tune across the whole keyboard? Or does it "glide" into correct pitch?

Try to disconnect pitch wheel.

I haven't had a proper play on the synth to comment about the waveform select, but pwm on the saw sounds right to me.

- J

Service manual confirms that. The pulse wave is always on. You just turn the sawtooth on and off with waveform selector.

justinvm · Post by **justinvm** » Fri Jul 11, 2008 7:07 am

It was gliding into pitch. Which would suggest faulty capacitor.
But it has stopped doing it after i callibrated the pitch bend.

I still would like to know why nobody noticed the problem with the pwm capacitor before!? And why the seller said all voices were working.

Edit:
More testing -

Sounds like there is still some modulation occuring on that voice which isn't occuring on any of the others.

Ok there's still some detune occuring. I think that pitch capacitor has to go.

adamstan · Post by **adamstan** » Fri Jul 11, 2008 7:21 am

can you post sound file?

I suppose, that changing rest of S/H caps in that voice should help.

justinvm · Post by **justinvm** » Fri Jul 11, 2008 7:24 am

Still some detune occuring. It's not much. But it sounds fat

The other caps should arrive monday. If it's still making problems i'll post up some sound files for you. Right now I dont have it in a location where i can record it easily.

thanks again for helping with the investigation

justinvm · Post by **justinvm** » Mon Jul 14, 2008 12:04 pm

The replacement caps arrived today. They were almost double the size and I had to use 2 capacitors to get the values needed on each replacement.

But the good news is. IT WORKS!

Voice 1 is having no problems at all now. It was those pitch capacitors after all.

Many many many thanks Adamstan. I very much appreciate the help. It probably would have taken me 4 times as long to fix it if you didn't give me the good advice.

Now I just have to put all the screws back and prey that I don't damage anything (the ribbon cables) in the process.

Thanks again!

- J

adamstan · Post by **adamstan** » Mon Jul 14, 2008 12:32 pm

justinvm wrote:The replacement caps arrived today. They were almost double the size and I had to use 2 capacitors to get the values needed on each replacement.

But the good news is. IT WORKS!

Voice 1 is having no problems at all now. It was those pitch capacitors after all.

Many many many thanks Adamstan. I very much appreciate the help. It probably would have taken me 4 times as long to fix it if you didn't give me the good advice.

Now I just have to put all the screws back and prey that I don't damage anything (the ribbon cables) in the process.

Thanks again!

- J

I'm glad to hear that the synth works and my advices were useful

Post by **nadafarms** » Mon Jul 14, 2008 6:20 pm

thumbs up man!

i am amazed you all are so smart!

Cellomangler · Post by **Cellomangler** » Wed Oct 13, 2021 1:25 am

Been a few years I know... but just in case anybody is still around or monitoring.
I just replaced the membrane panels on my Chroma Polaris as well as moved the battery backup to an external metal box at the rear where the serial plate was. Everything lit up. I had to reprogram a patch to test and it works and passes various diagnosis tests. But... I've got that drifting but consistent out of tune problem that's present up and down the keyboard but mostly in the higher octaves. It can't be corrected with any of the tuning functions and I've checked the pitch bend and the detune parameters. I'm pretty sure it's the capacitors from reading this conversation but of course I can't be sure - I'm not an electronics tech, but I'm decent with a soldering iron. I'm hoping someone can describe the exact capacitor I need and possibly a source. I'm including a grab from the schematics as I'd like to be sure that these are the caps discussed above/previously. Thanks so much to anyone who can lend a hand.
I'm including the link as well as attaching the image file. Polaris pitch capacitors

CHROMA POLARIS TROUBLES, CEM 3374.

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