Crumar Bit One fault

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Crumar Bit One fault

Postby Meatiness » Thu Nov 08, 2018 7:26 pm

Hello, this is my first post. I was wondering if somebody could help me out with a slight fault on one of the voices of my Crumar Bit One. There’s a noise, sounding like static, on the third voice - so it’s something to do with the third DCO, correct? Is this a familiar problem to anyone? Fixable?
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Re: Crumar Bit One fault

Postby madtheory » Thu Nov 08, 2018 11:46 pm

The most likely thing is the filter chip is failing. I had two die with these symptoms in mine. You can check by swapping the chips around, they're socketed.

Pig of a synth to work on, no service manual. Just a schematic. Doesn't even have test points. What a sound though. Nothing like it.

A nice trick, is remove at least two filter chips, put the thing in Unison mode, and send it fast arpy basslines. It will generate random syncopations as it assigns the notes to the now muted voices (actually they will click, it's cool) and random panning as well if you're using both outputs panned hard left and right.
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Re: Crumar Bit One fault

Postby Meatiness » Fri Nov 09, 2018 1:10 am

Thanks for the reply - and the tip, sounds interesting. Yes I do enjoy the quirky outputs this synth has.

So the filter chip is a CEM3328, according to a quick search. I haven’t opened it up yet, I’ll have to do so tomorrow. I’ll see if swapping them round will change which voice it affects. So you’re saying just a new one of those? I’m not really a techie - tbh I won’t have a clue what I’m doing - are they easy to see? I’ll post photos on here maybe if I get stuck.
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Re: Crumar Bit One fault

Postby Meatiness » Sat Nov 10, 2018 11:26 pm

Just as a follow up to this, the noise seems to have stopped on its own.
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Re: Crumar Bit One fault

Postby madtheory » Sun Nov 11, 2018 12:24 am

OK, I'm going to be perfectly frank with you here. I'm not trying to be rude. Just clear, and honestly as helpful as I can be. I can give you the information but it's up to you what you decide to so with it. So don't be offended by this: given that you've no experience I suggest that a reasonably valuable and clearly loved vintage synth is not where you should start learning electronics. The schematic is available, as are the data sheets and a bunch of technical tips on old usenet and email groups, if you can interpret it it's a fixable synth.

You'll see a big green circuit board behind the keyboard. 6 CEMs or SSMs in a row, depending on which rev you have. They're socketed. Follow all the usual precautions for chip removal. If you don't know those, or haven't done it before, it'll cost you about $40 per chip if you make a mistake. Your call.

The noise will return.
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