Readers--this is a work in progress, you can see the schem and pcb on the one page website here.
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Hidee Ho! This time I tried to turn a working OTA LFO design (post here) into a dual LFO/small PCB, based on a SOIC SMD LM13700 OTA chip.
Guess what? Damn thing didn't work, the LM13700 kept heating up, and I had no idea why. Update 9-14-25 ModWiggler to the rescue! got it working; update at the bottom of this post.
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"Seems not working"
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USELESS (?) BUILD PHOTOS
Back to the blown up dual LFO....
Maybe if I look at these pictures long enough I'll find a build fault. but really, it all looks pretty good to me.
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Glooped paste and picked/placed parts pre-bake--looking good I think |
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As usual, I heated up the plate to 215C, let it sit a minute, then back down to 150C. I am always surprised how well this works and how forgiving the process is. |
Next I soldered in THT parts like trimmers and a few caps:
Finally soldered the edge connector:
Test time...nope!
My power supply immediately went haywire--the voltages maxed out at about .25 to 2V while the current went to the 30mA limit on each rail. Not good.
NASA we have a short--the 13700 got really hot, quickly.
Removing the 13700 the short went away but (obviously) the LFO's didn't work.
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So why doesn't this work (besides having removed the LM13700) ?? |
Was the 13700 a cheap fake? I replaced the LM13700 with chips from two different vendors (Digikey, Tayda), same problem; I removed the PNP transistors thinking maybe I had a I-abc issue, nope.
I spent a couple more evenings troubleshooting, wishing the design was
through-hole (I usually am glad it's not) so I could lift up IC pins. No love.
There was a time in my DiWHY journey where I would have kept slamming away on this until fixed, but I have built so many LFO's at this point that I realized: the hell with it, I can redesign it, pitch it, and/or move on.
Above all, I had to make sure I was having fun, and more evenings troubleshooting this didn't feel like it'd be that.
I posted to the
modwiggler DIY forum to see if those folks had ideas--maybe they'd save my bacon, they had before.
If not, this one might be one and done for the dumpster.
update 9-13-25: Modwiggler to the rescue! In short order synthiq saw the problem:
If you look at the schematic (here), the power rails to the quad op amps are flipped!!!
That's at least part of the problem, right?
Not sure why this foobar heated up the OTA and didn't smoke the TL074's, but, things definitely wouldn't work as-is.
I wanted to test the rest of the design. I removed the TL074's and put on new ones flipped 180 degrees (quad op amps are fully symmetrical--so this counter-intuitive fix worked! Flipping quad op amps to solve audioDiWhy stupidity is another awesome tip from MW's resident genius "guest").
Finally, I used trimmers TR1 and 2 to set the frequencies to about 200hz, then used 4.7K for R21 and R14, and 39K for R7 and R20, to get the output waveforms closer to 10V P/P. Finally tested CVs at input --which modulated the LFO's frequencies. HA!! WORKS!!!
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Post TL074 flip: no more short circuits--made me very happy indeed. |
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Battle damage post-fix. Lots of flux spoojim visible. As I fix repair SMD boards I find myself using no-clean flux more and more. For SMD work it's crucial, the flux inside fine gauge solder doesn't get it done. No-clean leaves a mess; I honor it by not cleaning. |
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Testing post-fixes. I will lay this board out again.Flipping the TL074's is a clever fix but energizes my OCD buzzer. |