Author Topic: Tractor Beam--LFO oscillation is not regular  (Read 158 times)


  • Diode Destroyer
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Tractor Beam--LFO oscillation is not regular
« on: April 01, 2021, 01:10:25 PM »
I'm building a 2020-vintage Tractor Beam from one of Peter's "barebones" kits and I have a peculiar problem with the LFO. The audio signal is passing through ok, and sweeping the "tune" trimmer gives a good simulation of the phasing effect, so I'm pretty sure the gates of the JFETs are reacting correctly to what they're hearing from the LFO. But no phasing effect when I'm not adjusting the trimmer.

Here's the strange part: if I apply my audio probe to pin 1 of the LM358 (with the Speed pot turned pretty far down) I get a soft clicking like I'd expect, but it's not regular, more of a heartbeat rhythm (like bump-BUMP, bump-BUMP, with a short pause where the commas are). Does anybody have an idea what would cause that?
I'm inclined to try swapping out the 10n capacitor that bridges pins 1 and 2 of the IC; as I understand it the oscillation is caused by that cap ramping up and down.  But before I start desoldering things I'm curious whether anybody else has ever seen this problem. Thanks.


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Re: Tractor Beam--LFO oscillation is not regular
« Reply #1 on: April 01, 2021, 04:13:34 PM »
What build docs did you use? Got pictures and voltages? Reflowed everything and double checked your compnents?
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