Hey I just got some custom PCBs that I made based on LMNC’s low/high pass filters and upon finishing them, the output of the filter is super low, the potentiometers didn’t change anyting much, and the LM13700 chip was getting warm. I was wondering if someone can check my schematic to see if anything is wrong or might be the culprit, and I will also include supplementary schematics to help as well. Any and all help will be appreciated! The first image is my version of the schematic (the one titled Sheet_1) and the other EasyEDA schematic I found online is similar to the one I made based on the Low/High pass filter and can be used as a sort of reference
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Except for the 2 leds which are not connected together on your schem, I can’t find anything for the moment
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If the chips are getting hot it could be because they use too much current. I would check your resistors values (based on the colours as you can’t really measure their values once they are on the pcb) just to make sure you didn’t use a 1K instead of a 100K somewhere (I know someone who did this 
).
Also checking the -12/+12V is applied and applied to the correct pins is always a good start.
But agree with @Dud I can’t spot anything wrong on the schematic…
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First rule of troubleshooting: “Thou shalt measure voltages”. Measure the voltages on each of the opamp inputs and outputs. Better still, use an oscilloscope so you can see what’s going on. Then check around the transistors. If anything jumps to positive or negative rail it may be a good indication of where the problem is at.
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You forgot rule zero: do not be on fire. If anything’s getting hot, turn it off!
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I checked everything and even built another one double checking the solder points and still the same problem
Here’s the René Schmitz schematic that Sam’s filter is based on, if you want to compare it to yours.
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Wait, actually looking at my PCB routing, I noticed that on pin 13, there is -12v there, is there anyway that -12v should be there or is that a mistake on my part? Like I know pin 13 is connected to pin 8 on the TL074, but that shouldn’t give -12v right?
Edit: Turns out on further inspection that pin 13 on the LM13700 was connected to -12 when it wasn’t supposed to
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Right. That junction (the red dot) connecting -12 V to the line going to pin 13 and to R2 should not be there. You might be able to salvage your PCBs by cutting the traces from OTA pin 13 to -12 V, and from R2 to -12 V (and you might need a bodge wire connecting R2 to pin 13).
Junctions where they are not supposed to be are an easy mistake to make when you have lines in the schematic that cross each other but aren’t supposed to connect. Sometimes it’s not really feasible to avoid such crossings but you should try to avoid them. There are a lot of such crossings in your schematic. Take a look at how Schmitz laid his out, offhand I see just four non-connecting crossings (one on each control input to the OTAs, one on one of the resistors on the right OTA, and one on the collector of one of the transistors).
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Are you referring to Rene’s schematic’s or mine when it comes to the non-crossing connections?
René’s has four non-connecting crossings that I can see. Yours has more.
One nice new feature in KiCad is that you can assign different line colors in the schematic to different net classes. So all my recent schematics have the power rails in orange. That’s got to at least slightly reduce my chances of connecting a power rail to a pin where it doesn’t belong.
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Now THAT could make me upgrade !
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Huh, didn’t know that. Thanks.
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