CEM 3328 - Crumar Bit 99

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klickflip
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CEM 3328 - Crumar Bit 99

Post by klickflip » Fri Apr 17, 2015 4:33 pm

Hi, Ive a crumar Bit 99 thats always been a bit temperamental.
I've been digging a little deeper into it and trying to adjust internal pots to get it in tune again and sound right.

First Question is if I have dead cem 3328s is there any way to tell , I was thinking of pulling them out and putting one in at a time , would that be bad for the circuitry or anything? ( as there isn't any info I can find on a test mode)

I think I may only have one 3328 working and with that it's not giving any resonance but does filter.

There is a strange revolving note problem, it won't play ( or will only play a marginal volume or noise version of that note) unless three keys are pressed then the forth will sound ok, and only a new one will sound when the current sounding (forth) note is released.
Or if in double mode it works monophonically with first note pressed but second or subsequent notes have to be pressed after the current note has been release- physically and the amp asdr sustain finished.

So im not sure if its voice chip related or filter chip related, I can expand on this if anyone has knowledge to help.

Initially I just want to assertion if the 3328s are working fine and if they could be the problem.

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madtheory
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Re: CEM 3328 - Crumar Bit 99

Post by madtheory » Fri Apr 17, 2015 6:59 pm

Dead 3328s will cause exactly the symptoms you're describing. Pig to tune oscillators too. There's no "voice chip". Oscillator board is big, and IIRC VCAs are on same board as filters. Great sounding synth though. I have the schematic somewhere if you want them, there was never a service manual unfortunately.

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Re: CEM 3328 - Crumar Bit 99

Post by klickflip » Fri Apr 17, 2015 9:02 pm

madtheory wrote:Dead 3328s will cause exactly the symptoms you're describing. Pig to tune oscillators too. There's no "voice chip". Oscillator board is big, and IIRC VCAs are on same board as filters. Great sounding synth though. I have the schematic somewhere if you want them, there was never a service manual unfortunately.

Cheers Madtheory! If you do that would be great.

Cool, first person I've had reply on a forum that 3328 will give that problem, normally everyone with Bit experience says that it could be a multitude of things apart from just dying!
I've found some on ebay so thinking just buy a set and hopefully get a matched batch.
Yes tuning/ calibrating on it is a bit strange , another blog posted some advice on how to calibrate them with oscilloscope which I think I might invest in next.

Like you say it is an amazing big sounding synth.. its was great when I first had it 1996! and about a year after it started to do drop voices and tuning. Sent in for repair and they were stumped I think they didn't look to hard tho, got it back with a new battery and they said that was the problem but it wasn't!.

Soundwise , let me tell you that I keep on trying to program sounds I made back then on VA synth and well they never quite cut the rawness and depth. I actually gave a juno 6 away back then because I was so dissapointed by its sound compared to the Bit !

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Re: CEM 3328 - Crumar Bit 99

Post by klickflip » Fri Apr 17, 2015 9:07 pm

Does anyone know if all cem 3328s are the same in spec, i.e. some advertised for a Mirage should be fine on a Bit or other synths, or are there specific types or sub spec for different synths?

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Re: CEM 3328 - Crumar Bit 99

Post by madtheory » Sat Apr 18, 2015 11:21 am

All 3328 are the same. The trimmers on the Bit will need a little tweaking- but I imagine that's due more to the Bit's construction than the CEMs!

You should be able to figure out how many you need- see how many notes you can actually play at once. From your description you have either 3 or 4 voices working.

I'll have a look for the schematic. It's old fashioned paper, an original. Not making any promises...

Can you post the tuning tip you found please? Never heard of a method involving a scope. The more service info that's around for this synth, the better!

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Re: CEM 3328 - Crumar Bit 99

Post by madtheory » Mon Apr 20, 2015 9:28 am

Just talking to Dan at Hideawaystudios over the weekend, if you're having trouble with the oscillator tuning you might want to check for faulty capacitors on that board.

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Re: CEM 3328 - Crumar Bit 99

Post by klickflip » Tue Apr 21, 2015 8:56 pm

Cool, I've just bought a couple more 3328s as one isn't working. And got a some new dual op amp chips. There is definitely some strange tuning issues.
It will be in tune when it first starts up, but strangely there is no resonance signal getting to the filters. Once warm - an hour or 5 mins with a hairdryer! Resonance kicks in but this also leads to the oscillators detuning and it seems impossible to get them in tune. The way it cycles the voices seems strange to me too.

I'm getting 2 voices that are much louder than the others which coincidentally run from the same dual op amp chip ( the filters and amps seems to be arranged in pairs which makes sense, and one voice that will only produce the same tone on every key.
I've tried swapping the 3328s around and its not the filter on that voice it seems to be the voice signal itself. I think there probably is a few capacitors or resistors not running properlly , or even cracked solder that are doing some strange things to several signal paths.
Testing everything is bait beyond me , so I;m going to drop it into a synth repair centre.

Madtheory, do you have the circuit diagrams for the 99 that you could take hi-res photos or scans? , I'f found one for the 01 but they are slightly different.

One good thing is most of the chips used are still available for not very much, i've checked most on Osc board and there's still some about.

I'm planning to make a resource on it , and document what chips do what and get data sheets for them all for future.
Just wish I knew a lot more about circuit designs.

I also found a AU/ VST to enable editing on the BIT 01 and 99 which I was blown away by last night! Cant find much info on who made it but the links are here, makes editing so easy, I thought it was pretty easy already but worried by the masses of button clicks , which will fail one day. and if you send new sound changes between notes you can make it appear as you are tweaking it in real time, or even simply change patches on the fly which is awesome.

http://sourceforge.net/projects/bitcontrol.berlios/

in action, not me, I'll make some videos soon!




Blog on tuning it by Bob and Tauntek, I mentioned before.

http://www.tauntek.com/BIT99.htm

And blog that made me dig it out from under the bed again!

https://xbs111.wordpress.com/2013/12/26 ... -part-one/


Hopefully will start one myself and get it all documented for others.. tho I don't think there are that many left working thesedays.

S

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Re: CEM 3328 - Crumar Bit 99

Post by madtheory » Wed Apr 22, 2015 11:15 am

That would be cool to have a resource. I might do a blog on mine one day, it was a Bit One though sorry, that's what my schematics are for. The Bit One doesn't support sysex.

LOL I left mine under the spare bed for a few years too, because it developed so many problems. Then I sold it because I simply didn't have time to fix it.

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Re: CEM 3328 - Crumar Bit 99

Post by Bunk » Thu Jun 18, 2015 2:18 pm

As mentioned earlier check the caps in each voice as they do go bad. Another good tip is to check the Freq and Res CV in's on the 3328. get hold of a pinout for the 3328 and test each voice with the chip in circuit. Write down the voltage values for each voice in turn with the filter wide open then closed. You could even set the cut off to a midpoint and test again. The 12 (or 18) voltage readings should be pretty close to each other. If you've got one or more thats a mile out then it's possible that the CV is holding the filter closed. If so check the S/H caps and associated OpAmp for that voice(s). If that is the case at least you know your 3328's are actually good but 'look' bad!

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Re: CEM 3328 - Crumar Bit 99

Post by sam » Thu Jun 18, 2015 6:04 pm

I bought a bit one a few months back....seller said it was working but not when I tried it out, anyway I look forward one day to fix it....just a pile of other peoples synth to get through..
Anyway the best tool for quickly checking cem or any other voice/ filter chip is to get a signal checker.

You can check audio in and out...simple.
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Re: CEM 3328 - Crumar Bit 99

Post by DavyBoySmith » Thu Dec 24, 2015 5:20 am

madtheory wrote:Dead 3328s will cause exactly the symptoms you're describing. Pig to tune oscillators too. There's no "voice chip". Oscillator board is big, and IIRC VCAs are on same board as filters. Great sounding synth though. I have the schematic somewhere if you want them, there was never a service manual unfortunately.
Sir, do you have an electronic copy of the Schematics for the Bit99? I am trying to help someone debug one and there appears to be no source for .pdfs. Any help you may provide would be greatly appreciated.
Dave

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Re: CEM 3328 - Crumar Bit 99

Post by madtheory » Thu Dec 24, 2015 12:36 pm

No only hard copy of Bit One.

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Re: CEM 3328 - Crumar Bit 99

Post by Mooger5 » Sat Dec 26, 2015 1:43 pm

The Bit 01 is the rack mount version of the 99. Schematics at synfo.nl
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