Circuit Troubleshooting Help

[rahulpsharma]

Dear Experts:
I am trying to copy and make a LVDT excitation Circuit from the following link:

https://www.mikesflightdeck.com/lvdts/lvdts.html

I need help in understanding and troubleshooting the last circuit schematic (Signal Source) in the above link...!!

The stated aim of the circuit is to generate 1kHz frequency at 2VAC at the emitter of 2N3906...

When I first soldered all components and turned ON the power supply, I got exactly 1.078kHz @ 1.95VAC at Pin 8 in the schematic.. However, at Pin 6 the freq remained 1.078 kHz but voltage dropped to 0.6VAC... And at the emitter of 2N3906, both freq and voltages were '0'...

Today, when I tried working on the circuit again, NOTHING is working... The results I observed yday are also gone... Freq and voltages at every pin described above is 0...!!! :-(

So now, I wish to gain some understanding of the ckt and logically proceed to troubleshoot it...

I've made a PCB of this ckt and soldered components so I am ruling out the possibility of any loose connections... Moreover, I've replaced the main IC TL084 but there has been no change...!!! Please help me understand this circuit:

I have the following questions now:

1) Which components of this ckt determines the frequency and AC Voltage Levels...

2) From the ckt, there appears to be no external signal applied to the Op AMP, then how does it generate freq and voltage at Pin 8...

3) What is the purpose of the final part of this ckt comprising of two transistors, three diodes and one section of TL084 (Pins 5,6 & 7)...

4) What will be the shape of the waveform when the ckt works: sine wave?


Some deviations that I have taken from the original circuit are as follows:
a) I didnt have a 7808 IC readily available so I have used 7809, instead...
b) Instead of Polar capacitors, I have used Ceramic ones
c) I could not find a 20 pF exact value so I am using 22pF

Thanks and regards
Rahul P Sharma

 

[FvM]

The oscillator seems to work at the expected frequency f0 = 1/(2pi*100k*1.5n), it should be sine without too much distortions.

The class-AB output stage ist obviously not working, either the output is shorted, or it's somehow wired incorrectly. As a first step disconnect the LVDT drive coil, then measure the bias point of each related circuit node.

The component values should be still fine.

 

[pjmelect]

The circuit is not clear, there should be a dot at the junction of pin 7 and the diode and the transistor and the 5.6K resistor. Try it with the coil disconnected first.
Now that it is not oscillating check the outputs of the three op amps, each should be the inverted output of the previous stage, the stage that is not inverted is where the problem is.

 

[FvM]

The circuit is not clear, there should be a dot at the junction of pin 7 and the diode and the transistor and the 5.6K resistor.

Yes. It should be obvious to anyone who understand the circuit.

Now that it is not oscillating

The circuit was reported to oscillate correctly. What do you mean?

 

[pjmelect]

Yes. It should be obvious to anyone who understand the circuit.

The poster is obviously inexperienced with electronics, and this was just a suggestion as to a possible reason why the output circuit was not working, he states in the first post that he did not understand the circuit.

The circuit was reported to oscillate correctly. What do you mean?

In the first post he says "Today, when I tried working on the circuit again, NOTHING is working" I just suggested an easy way to check the oscillator.

I know all of these suggestions may seem obvious to you but I think the poster is not very experienced and needs a bit of hand holding. We all needed help when we first started.