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I am currently looking at an old 60's Transistor Organ (Phillips Philicorda GM752 to be exact). It has an integrated Vibrato-Unit, although the only modifyable parameter of it is the depth (amplitude) of the Vibrato. My question is: Is there a way I can integrate some sort of potentionmeter into the circuit to change the frequency of the Vibrato?

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The Service-Manual (which I've taken the schematics for the Vibrato from) is available here (It's written in German, but I can help translating if you have any questions)

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    \$\begingroup\$ Nice, 1960's style electronics :-) No microC in sight :-) \$\endgroup\$ – Bimpelrekkie Jul 7 '15 at 12:10
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The BJT is a phase shift oscillator and uses the three caps marked C411 to C413 to generate 180 degrees phase shift to create sinewave oscillation. You could experiment with these to alter the frequency.

enter image description here

This isn't the actual part of the circuit that modulates the audio but, the output should feed the modulator (the real heart of the vibrato circuit).

If you wanted more control you could use the ubiquitous 8038: -

http://www.circuitstoday.com/wp-content/uploads/2012/02/audio-oscillator-circuit.png

It has sine, triangle and square outputs and you'll get a massive range of frequencies to use. Or maybe even consider the wien bridge oscillator. This one runs from a 5V supply: -

enter image description here

There's also application note 263 by TI - it's a good read and has some interesting sinewave circuits.

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  • \$\begingroup\$ Alright, thanks. Is there any way to be able to vary them dynamically through a potentiometer? \$\endgroup\$ – jhbruhn Jul 7 '15 at 9:48
  • \$\begingroup\$ There is R422 and R423 - you could try using a dual pot to alter these values. You may get away with altering just one value. Slower vibrato = higher resistance. \$\endgroup\$ – Andy aka Jul 7 '15 at 9:50
  • \$\begingroup\$ Ad changing C411, C413: Variable capacitors do exist but not in these values. 470k is not 470000 Farad (they would be the size of a house) but 470 nF I think (470000 pF). \$\endgroup\$ – Bimpelrekkie Jul 7 '15 at 9:52
  • \$\begingroup\$ I would also try to vary R422 and R423 but this does affect the loopgain so the oscillator might stop if you lower them too much. If that is a problem you could try lowering R245 also to compensate. I would keep the ratio so if you make R422 and R423 80% of their value also lower R425 to 80% of its current value. \$\endgroup\$ – Bimpelrekkie Jul 7 '15 at 9:54
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It's a straightforward phase shift oscillator. Variable capacitors in 470,000 pF aren't widely available, so my suggestion would be to replace R422,R423 wit a 2-gang linear potentiometer, maybe 47k. And be prepared for the oscillation to stop at either end of the pot's range. Once the feasibility is established, restrict the range of operation by adding R (e.g. 10k) in series with each pot, and reducing the pot's range (perhaps by connecting another resistor in parallel with it). I can't suggest exact values.

Ideally you'd need a 3-gang pot, but the third resistance is the input impedance of the transistor stage, so that`s not so easy. Varying the phase of 2 of the stages should work over a limited range of frequencies.

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I have created an online simulation of the original Philicorda circuit and modified it with 2 additional potis. Looks like we need to move both potis at the same time or the circuit will lock up.

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