At the suggestion of @tamasgal I am opening a thread to troubleshoot stripboard builds of the Skulls & Circuits VCF-3 (a op-amp/LM13700-based state variable VCF). The circuit schematic is available here.
It seems that @tamasgal @swirv (and perhaps @Yeg0rka ?) and myself (see my post here) all used the stripboard layout by @Synthlurker presented in the verified stripboard layouts thread in this forum (link) with variable success.
Personally I checked on my build:
- The stripboard cuts with a multimeter (before populating the board),
- The LM13700 (it works)
- That the ICs received the correct supply in the correct pins (+11.7 and -12V)
My soldering also seems rather tidy.
I would appreciate if anyone can give any input on this. Something that we can go through specifically (with a multimeter or oscilloscope), before we double (triple?) check the layout against the schematic that is.
Edit: Added the voltage measurements at the IC pins. I get a very small drop at the positive rail, but appears consistent. Not sure whether this could help identify the problem.
One thing that wasn’t immediately obvious to me is that there’s a strip cut at J10, under the 1k resistor, and likewise at B8, under the 10k. Do you have those?
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Good points! I will check them later as well, but I think I have those 
Btw. @K.ostas some strip cut drills look a bit “optimistic” with the residual copper layers. I’d definitely recommend to clean them out, they could also move a bit in future and shorten out some connections. OTAs and opamps do not like leakage
Just for the sake of completeness, here is the strip board layout:
Thanks! I have them indeed.
To avoid such problems, I add in my layouts a second image indicating the cuts and jumpers, along with how many are there for quick and clear reference. So when I see a layout that does not have them, I triple check (although I’ve made mistakes after triple checking before).
Hey!
Are you sure the layout is correct? I checked the schematic and the Cutoff pot (Freq in the schematic) acts as a variable resistance between V+ and GND. On the layout is is wired between to GND and V-. You could try connecting Cutoff 3 to O26 and Cutoff 1 to D1.
(And the Resonance pot is wired differently but I don’t think that’s critical)
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Good catch! The -12 and G connections are meant for the trimpot between the TL072 pin 7 and the PNP transistor. I will connect the cutoff potentiometer elsewhere on the board and see if that works better.
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One more question: Did you use 2N3906 transistors or equivalent with the same pinout? I’m asking becauce the BC series have a different pinout.
Yes, I used 2N3906. I haven’t really substituted anything compared to the layout.
I updated the layout so that there is now easier to read cuts and links layout below. Also fixed that resonance pot.
Update: Tuning pot is back to -12V to 0V as that is how it is in the original layout
Update 2: Fixed cutoff for real, now it should work properly!
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Pay attention, now the trimpot set the voltage between GND and +12V as well, contrary to the design (-12V and GND).
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Thanks @BobbyZincone that’s a good catch! Let me also post the original schematics here. Maybe someone has a “more original” source This is at least from the Skull and Circuits website: VCF-3 – Skull & Circuits
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It seems as if part of what’s going on here is differences, or uncertainty over whether or not there are differences, between the stripboard layout and the schematic. I haven’t used DIYLC very much, but I know it has facilities for drawing schematics; does it not also provide a way to link a schematic to a layout in a way that indicates whether the connections are consistent? That certainly is a basic feature of PCB layout applications such as KiCad — the Design Rules Checker will tell you immediately whether the PCB layout matches the schematic.
I don’t know, I am also not a DIYLC user and when I do strip board stuff, I always draw by hand 
I posted it because I want to start from the source of truth and make sure that the strip board layout is correct, so yes, that’s at least my approach: make sure there are no differences betwen the schematics and the strip board layout before we dive into details
Sadly no.
The answer whether the layout was drawn from the schematic in the S&C website its only part of the story. A verified stripboard layout should work, irrespective of whether it was drawn from that schematic or not. At the moment we have two members for whom their build works, and two for whom it doesn’t. Given this, I would first try to rule out that the ones for whom it didn’t work did something wrong in the build.
It’s just a guess, but if the cutoff pot is wired from V- to GND, sweeping would not work, however, if you connect a beefy enough CV, the filter can produce the desired results, so you may consider it working. Do you know how the other forum members use their modules?
If you haven’t tried the pot wiring fix, you could try this with your current module: set the cutoff pot to the max (i.e., GND - measure it if you can), then connect a beefy LFO/Envelope/Constat voltage source, etc. That may give you some results. If yes, then thats indeed the wrongly wired cutoff pot. You can fix that easily.
I changed that layout back to the corrent one which is -12V to 0V. Having it wired from 0V to +12V should just result in the lowpass staying open etc. when the cutoff pot is turned fully counterclockwise.
In the Skull & Circuits schematic the Cutoff pot is from +12V to ground, not -12V to ground. Does this mean that the stripboard layout is not of the Skull & Circuits VCF-3?
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Now it is fixed for real. I mistook cutoff pot for the trim pot from correction from @BobbyZincone
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