Sounds like it’s oscillating but at very low frequency (~0.5 Hz). In which case maybe the control voltage is much lower than it should be, or I’ve also seen similar problems when the linear FM pin network isn’t right. But that’s just two resistors to ground and 12 V in this simple version and those look right to me. So check those but if they’re OK then the question is what’s the voltages at the top of the two 100k resistors connected to pin 15, as shown here Annotated Simple 1V/Oct Oscillator Stripboard. I think they need to sum to something like 9 V or more to get an audio frequency signal. And check that one of them does change when you change the tuning pot. Also verify the 470R to 10 nF to ground from that pin.
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