LM3900 VCO Advice?

TryHard · Joined: Jan 09, 2026 Posts: 2 Location: UK

Hi, I have decided to try to build a a small synth and amd starting with the VCO. I am using a single 12v supply so decided the LM3900 would be a good chip to base most of the parts on.

I have built the VCO from application note 72 with my best guess at resistor and capacitor values, but can't seem to get it to oscillate, both outputs just sit at around 0.5v regardless of the control voltage. This isn't my first time breadboarding a circuit and I have gone over it a few times to check I haven't connected anything wrong, and I've used the same chip to build a simple fixed-frequency square wave oscillator so I no it's not a dud.

Can anybody spot what's wrong with this circuit? I'm hoping somebody can take a look and tell me I've done something stupid with the resistor values.

Any assistance would be greatly appreciated!!

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elektrouwe · Joined: May 27, 2012 Posts: 149 Location: Germany

I would rise R4 to 10k and lower C1 to 1nF (unless you want an LFO).
If it still does not work and you have double checked all connections and values, your LM3900 seems to be defect or fake part.
But why did you choose a linear VCO and not an exponentially controlled ?
And if you need a linear one, why using a vintage chip ?

TryHard · Joined: Jan 09, 2026 Posts: 2 Location: UK

Both very good questions! I'm going to control it digitally with a microcontroller through a 16 bit DAC, so it doesn't really matter whether it's linear or expo as I can just create a lookup table.

As to why the LM3900.... good call! I just had some lying around from when I wanted to do something similar like 15 years ago and it seemed like a good idea at the time. Maybe I'll have a rethink and just use something more modern. Thanks for the help!