Pain in the resistors

Well that was a pain in the… resistors.

Finally got the new 3320 VCF module working after tearing my hair out a little. With the lockdown, I’ve had to source the components in dribs and drabs and can’t always get the exact components stated on the circuit board.

About a week ago, I finally took delivery of the last capacitors I needed then excitedly plugged it in and heard… nothing.

A bit of headscratching later, I chanced upon the idea of sending a voltage through the CV input which initially did nothing, until I swept the filter knob, which revealed a comb filter type noise, but only when the knob was pointing straight up. Resonance did nothing to the sound, and I had silence at either end of the filter range.

Where do I start?

My first thought was that perhaps the 100k B potentiometers were faulty, or that I had bought the wrong ones and got my Linear vs Exponentials in a twist. So, I checked stock at and fortunately they had some.

Then, this morning, I had the joy of desoldering the old pots and installing the (arguably superior) new ones. Unfortunately, it didn’t fix the issue.

I checked my soldering again – all looked sound to me – so as a last resort double checked that I’d used the correct resistors. And low and behold, the 91k resistors I had been sent (and which were in a bag labelled 91k) were actually 910k! In fairness, the difference between the two is just a red stripe rather than an orange one, and even my wife wasn’t sure what colour they were when I asked her to check. So I desoldered all four of them, and replaced with 100k resistors (the closest I could find) and I’m pleased to say the module sprung into life.

I still didn’t have any resonance though, but tracked this down to some damage I’d inadvertently caused on the board around one of the resonance knob pins. I could just about work out where the pin was meant to be going though, so added a jumper wire to link to the appropriate resistor, and I have resonance too!

Whether the 100k vs 91k resistors makes a difference to the sound, I’m not really sure, but it sounds good to me and I’m currently enjoying feeding some sample and hold through the CV input.

This was the first time I’d built a module just from a PCB and Panel, rather than a full kit, and it definitely makes me appreciate the convenience a full kit gives you, even if I did save a bit of money along the way.