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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
MFOS Ultimate problems
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njosnari



Joined: Mar 15, 2021
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PostPosted: Mon Mar 15, 2021 3:19 pm    Post subject: MFOS Ultimate problems
Subject description: Sample & Hold, VCF, and modulation cross-talk problems
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I just finished putting together the MFOS Ultimate, and calibration went more or less fine, however, I'm having a few problems that I can't seem to troubleshoot:

S&H glide time is very short. Documentation says up to 2 seconds, but at longest, I would get a fraction of a second. I reheated all the solder joints in the section, tried a different 1M pot, made sure everything is grounded well, swapped out the LF444 for a TL074 (behaved pretty similar, but with a lot of drift on the hold), double checked resistor R207 and cap C63. The only way I could see improvement was increasing C63 to .2uf,
http://musicfromouterspace.com/analogsynth_new/SOUNDLAB_ULTIMATE/images/schematic_pg8.gif

VCF distorts at input. Input at C56 from mixer looks totally fine, but on the other side of the cap, my square wave and sawtooth get funky:
http://musicfromouterspace.com/analogsynth_new/SOUNDLAB_ULTIMATE/pdf/schematic_pg7_schem.pdf
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I've checked C48, R166, R168, and R169, Swapped the LM13700 with the one in the VCA. I'm not sure where in the circuit this problem could be.

And in general, I am able to hear the repeat gate and LFOs in the signal even when they aren't dialed or patched in. Would this just be from poor shielding in my wiring, or is there something else I can do about that?
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blue hell
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PostPosted: Mon Mar 15, 2021 4:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

Re. S&H - I dont' know .. cleaning up the PCB may help .. if it's soldered a bit messy ... after the component swaps you tried it is hard to believe it to be a component issue ...

Re. distorion in VCF .. the wave shapes you show are to be expected after the capacitor, the signal is AC coupled there. A sine wave though, should look undistorted there.

Re. The audible LFOs, that could be a shielding issue, yes, but it could also be coupling over the power lines ... maybe the easiest thing to try is to separate the cables a bit and see if that helps .. if it does you may want to shield some cables ... and otherwise it will be harder to fix .. some extra 100nF caps here and there close to ICs could work ...

Re. your first post here, welcome

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also .. could someone please turn down the thermostat a bit.
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Grumble



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PostPosted: Tue Mar 16, 2021 7:11 am    Post subject: Reply with quote  Mark this post and the followings unread

This is the step response of a circuit composed of 1Meg and 0.022uF


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PHOBoS



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PostPosted: Tue Mar 16, 2021 8:46 am    Post subject: Reply with quote  Mark this post and the followings unread

If you get the correct notes even with a short glide time then the S&H should be functoning correct. However keep in
mind that the glide time also depends on the notes you play. Notes that are close together will have a much shorter
glide time than notes spread apart. Time can be calculated with RxC so that would be 0.022uF x 1M = 0.022 Sec = 22mS
Of course the time depends on the voltage difference and to fully charge it from 0 could take 5x as long but that would still
ony be around 100mS. (more info about RC imes here)

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