I built the top circuit in the schematic below. First I breadboarded it and it worked great. I swapped the 10k resonance pot for a 5k as I didn't have any 10k trimmers handy and used 1000pF cap instead of a 500pF for ground.
The breadboarded circuit worked great. It's a voltage controlled low pass filter, like in a synthesizer. Plugged a guitar in and it sounded great, good sweep from the 100k frequency pot, resonance range with the 5k pot was good. The 1M input trim was plenty to take the higher output of a synthesizer down to a level which wouldn't overload the filter.
Then, I created the circuit on veroboard. The only thing I changed here was switching the little ground capacitor from my 1000pF to a 680pF. As the schem recommended 500pF and I'd used 1000pF with decent results, I figured a value between that wouldn't be too bad.
I built the circuit. Connected a battery, and heard sound. It sounded good. Then when I moved the 100k frequency pot (at this point a small trimpot), it started smouldering and smoke came out. I immediately unhooked the 9v battery. I thought maybe I had a bad pot so I switched it for another - this time a proper 16mm - Same problem. I saw red light inside the pot and lots more smoke this time.
This wasn't long before I had to go to bed, so I unplugged everything and left it. Can anybody tell me what I might have done wrong in order to make this happen? It worked fine on the breadboard. I've built a lot of circuits and I'm used to debugging circuits that don't work at all or sound crackly, but I've never had a fire before!