Really weak resonance after new build? Wtf
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Really weak resonance after new build? Wtf

by saw wave analog on Thu Dec 20, 2012 4:28 pm

Hi all, this is my third x0xb0x and I have built many other kits as well, this is the first time I've ever ended up with a problem though... Hopefully someone can point me in the right direction. The synth basically works fine, and if I wasn't familiar with how its supposed to sound I'd never even know anything is wrong, but the resonance just won't scream like its supposed to. When I crank it it *almost* gets there, but the best it will do is just a little blip with the env mod up and decay as short as possible. When I increase the decay, instead of the little blip of screamyness stretching out and doing the bubbly watery thing its supposed to, it just kind of goes away. There is *some* resonance, it thins out the sound and stuff like normal, it just can't get the rest of the way home. I pulled it apart and have spent the last 15 mins or so looking for backwards transistors or caps or shorts but can't see anything. Next step is reflowing the whole filter section, but I figured I'd post here first in case any one else has run into this one before. Thanks in advance for any insight you might have to offer!
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Re: Really weak resonance after new build? Wtf

by saw wave analog on Thu Dec 20, 2012 9:55 pm

Spent a couple hours with it and no luck, resonance still sucks. I can't find any shorts and I've reflowed everything in the filter section. At a loss! Halp!
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Re: Really weak resonance after new build? Wtf

by antto on Fri Dec 21, 2012 8:00 am

hi, it would be good if you can record a audio sample demonstrating the thing (yes, i know what you mean perfectly)
preferably .wav

otherwise, you can do 2 things.. socket C29 and R97
C29 was found to be "backwards" in many (all? ) TB-303s so you might want to flip it around
R97 is the easiest way to modify the resonance (aka reso-boost mod) you can put in a 10K pot and adjust it till the resonance level is "enough" then measure the resistance and that's what you want there
We are here too: irc.freenode.net >>> #x0xb0x
..:: c0nb0x v1.00 ::.. http://forums.adafruit.com/viewtopic.php?f=7&t=29282 (new control app - win/osx)
VCO Tuning: http://forums.adafruit.com/viewtopic.php?f=12&t=24759 (do it the EASY way)
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Re: Really weak resonance after new build? Wtf

by saw wave analog on Fri Dec 21, 2012 6:43 pm

Thanks for the post antto. I shorted out R97 and it made the filter self oscillate, so that is an option, but I have never needed to modify the standard layout to get a pretty damn good 303 sound out of an x0xb0x before so I am convinced my problem is somewhere else. I was just checking the voltages of my transistors and I discovered that Q22 is at 11 volts on all 5 pins... so, a short there somewhere I guess? I can't see any short, but that must be it. I am waiting on my solder sucker to warm up to pull Q22 and hopefully find what's wrong.
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Re: Really weak resonance after new build? Wtf

by saw wave analog on Fri Dec 21, 2012 6:55 pm

Oh joyous day! It is fixed! I didn't even have time to pull Q22 either, it turned out to be a misprint on the pcb apparently. It looked normal, but the far right (when viewed from the knob/led side of the board) two pins of Q22 were shorted. I dug out the area between the traces through all of the narrow sections of that run with an xacto knife and it sounds like it should now. And here I was feeling like an asshole for building it wrong.

Thanks again for the assist antto
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