Frequency Central CEMosc - Pitch Tracking Issues

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Mr Creamy
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Frequency Central CEMosc - Pitch Tracking Issues

Post by Mr Creamy » Sat Oct 03, 2020 1:05 pm

Hi everyone!
I apologize for this newbie question (and lengthy post) as I’m fairly new to module building and vintage synths in general. I have built about a dozen DIY kits - thus still learning. I have a question about the CEMosc which is based upon the CEM3340 chipset. I currently have the AS3340 installed and wondering about it’s performance in place of the CEM3340. Rick (Frequency Central) advertises CEMosc works with AS3340. I also have two CEM3340’s chips (one vintage, one re-issue “G”) on the way, so will be sure to try them to verify this issue and also want to see if there are any qualitative differences.

Build went very well except for one thing, calibration of pitch. First, I allowed the unit to warm up for about 20 minutes before calibration. If I calibrate to the octave switch, my KeyStep notes do not track well, every note beyond 440A is off. But if I calibrate to the KeyStep, then the octave switch is off. There is no happy medium. Is there anyway to get both to track perfectly? Or is that an inherent issue with vintage-based gear?

If it can be fixed or improved, then where does the problem lie? My thoughts are two possibilities:
  1. A tracking issue with the AS3340 chipset. Do I need an actual CEM3340?
  2. The problem lies deeper within the resistor ladder for the octave switch? Build doc states “Pick all 10k resistor values similarly with 1% precision”. So I picked the closest I had within the kit and on hand. In my case, values ended up being 9k8x (where x is any number, thus 1% precision).
My guess is the resistor ladder is the problem and resistors within the ladder need to match better - maybe closer to 10k and/or 0.1% precision. I wanted to ask here as it might just be my misunderstanding of how vintage synths perform.

Thanks for any help you can provide!
Best regards,
Mr C.

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Mr Creamy
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Re: Frequency Central CEMosc - Pitch Tracking Issues

Post by Mr Creamy » Sat Oct 17, 2020 4:37 pm

Sigh, the culprit is my Multimeter. Just got a a new Fluke which indicated my resistor values were not what I thought they were.
Just goes to show; a person is only as good as their tools. ;)
Mr C.

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hawkfuzz
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Re: Frequency Central CEMosc - Pitch Tracking Issues

Post by hawkfuzz » Sat Oct 17, 2020 4:42 pm

Glad you were able to find the issue.

In future I'd recommend emailing the FC and posting on the DIY section. Might have more informed eyes reading.
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stylesforfree
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Re: Frequency Central CEMosc - Pitch Tracking Issues

Post by stylesforfree » Sat Oct 17, 2020 4:48 pm

Mr Creamy wrote:
Sat Oct 17, 2020 4:37 pm
Sigh, the culprit is my Multimeter. Just got a a new Fluke which indicated my resistor values were not what I thought they were.
Just goes to show; a person is only as good as their tools. ;)
Mr C.
How is the CEMosc? how does it sound?

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Mr Creamy
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Re: Frequency Central CEMosc - Pitch Tracking Issues

Post by Mr Creamy » Sat Oct 17, 2020 5:19 pm

hawkfuzz wrote:
Sat Oct 17, 2020 4:42 pm
Glad you were able to find the issue.

In future I'd recommend emailing the FC and posting on the DIY section. Might have more informed eyes reading.
With Rick@FC I have about a 30% success rate thus far.
Good call about where to post, I'm new so still finding my bearings. :doh:
Thx! :)

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Mr Creamy
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Re: Frequency Central CEMosc - Pitch Tracking Issues

Post by Mr Creamy » Sun Oct 18, 2020 12:08 pm

stylesforfree wrote:
Sat Oct 17, 2020 4:48 pm
Mr Creamy wrote:
Sat Oct 17, 2020 4:37 pm
Sigh, the culprit is my Multimeter. Just got a a new Fluke which indicated my resistor values were not what I thought they were.
Just goes to show; a person is only as good as their tools. ;)
Mr C.
How is the CEMosc? how does it sound?
So far, despite the tracking issues. I really like it and it seems to be a versatile. Although I admit, I haven't had a chance to do a detailed listen/scope yet (especially with the different CEM3340 chips/clones). But I noted the CEM3340G (i.e. reissue) was at lower in pitch than the others. Not sure why. Again, I have to fix the capacitor ladder first before I make any claims. Closer matching resistors at 0.1% tolerance are on order and will see if that does the trick. Will post again when installed and tested.

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