The Deaf Were Not Able To Obtain The Dr. Flanagan Nerophone To Begin Hearing

[kolbacict]

https://www.profmt.ru/catalog/view/amplipulsy/amplipuls-5-br/
This apparat doesn't same ? 
I was doing repair it once.
I was surprised by the price of it, not at all adequate to the internal content(components) .

[rakarskiy]

I know this master oscillator circuit. Feedback with a phase shifter.
The question is, why three variable resistors, three adjustments ?
What do they give? What do they change ? As usual enough one,
or nothing at all. 

The circuit lacks a switch for three modes with different frequencies. In any case, I made similar ones on a 555 timer.

[sm0ky2]

I know this master oscillator circuit. Feedback with a phase shifter.
The question is, why three variable resistors, three adjustments ?
What do they give? What do they change ? As usual enough one,
or nothing at all. 

1,2,3 are variable inductors
similar to those used in an old tunable radio
used for a change in impedance, yes, however they also change the circuit in other ways
where a simple potentiometer would not


For example: these can tune the frequencies of each of the 3 capacitors
thats the problem when circuits get passed around over the decades
people apply Fourier or Kirchoff loops and think their rendition is equivalent
well,.... That depends on what they were doing with the original circuit

[sm0ky2]

Theres many applications for what you see in that part of the circuit
But as it pertains to the topic,
I made a short video that i hope helps


https://youtu.be/-JopICLn6N4

[kolbacict]

It's amazing what's wrong with you all today ?
The second answer is incorrect.
This is a classic oscillator with a 180 degree phase shifter. Resistors and capacitors and nothing else. I do not deny that it will also work with inductances. But ...

p.s.  If all three resistances were mechanically connected by a common shaft, then the frequency could be changed within small limits. But this is not shown in the diagram.