[LectronFanModerator]

Hi everybody,

After building project 42, I noticed that the sound coming out of the speaker didn’t resemble a soft sounding sine wave.  It was rather a harsh sounding tone.

When connecting the oscilloscope at the collector of the 2nd transistor, I saw a spiky waveform !

Time to dive into the circuit and see what’s causing the malfunction.

When putting my scope at the base of the 2nd transistor, I saw a sine wave. So far so good. I suspected thus a faulty biased amplifier.  Time to look up the data sheets of the 2N3704 transistor. To my surprise, Lectron decided to use this type of transistor with rather low Hfe (amplification factor) of about 180 😯

The Ge transistors used in the earlier sets had higher Hfe.

After doing some math (calculations for the collector current and the base current), the bias resistor of 100K was  replaced by 10K and the 120 Ohm resistor in the emitter circuit was changed into 220 Ohm. I also change the 100 µF capacitor into 10 µF. Now a current of about 18mA is flowing through the emitter-collector, setting the speaker transformer at a comfortable level to amplify signals without distortion.

Also note the use of a volume potentiometer at the base connected in an odd way. I will come back to this later …

And now the speaker outputs a soft sounding sine wave resembling a bit of a Theremin.

Time to fiddle with the oscillator circuit. I observed that turning the tuning potentiometer was not comfortable.

To make a story short, I changed few components to improve this.

I will publish in a later stadium the details and some calculations and also oscilloscope signals.

But let’s start with the schematic,

To be continued …

• This topic was modified 5 years ago by Michael.
• This topic was modified 5 years ago by Michael. Reason: Changed font and size styling for easier reading

[LectronFanModerator]

Hi everybody,

As promised, here are some measurements on the circuit :

The first image is the original output at the collector of T2 (Point E) :

This doesn’t quite resemble a sine wave, isn’t it ?

So, we need to bias T2 properly, so these are some calculations :

To act like a “perfect” amplifier, T2 should divide the battery voltage. Let’s say about 5V. This voltage should be across the E and the C. To be safe, we assume that the DC current (without signal) flowing through E and C is about 20mA max.

So, 5V / 20mA = 250 Ohm. The resistance of the transformer is 30Ohm, thus 250Ohm – 30Ohm = 220Ohm

Therefore we take a 220 Ohm resistor and place it into the Emitter circuit (if we would put it in series with the transformer, we could get a loss of amplified signal across the resistor)

The Base current Ib = Ic / Hfe.  So Ib = 20mA / 180 = 0,1mA.

To achieve this, we use a resistor divider. Now comes the cool part. We use the volume potentiometer of 10K for volume but also for bias adjustment.

So, when turned at max. the volume is the loudest and the current through the E and C of T2 = 20mA.

If we turn it down, less signal is fed to the base, but also less bias. This means that when the volume is at zero, our transistor T2 is completely cut off, which means that no current flows through the transformer, saving the battery.

The resistor divider gives this 0,1mA. Now it’s up to you to calculate Ib !

Please note down your calculations in a comment.

Now, for some oscilloscope pictures :

The picture at the left shows a 60° phase shift in the oscillator between A and B.

The picture at the right shows a 180° phase shift in the oscillator between A and C. This phase shift causes the circuit to oscillate. We call this type of oscillator a phase shift oscillator.

 

Here we see the input (D) of amplifier T2 and it’s output (E).

Note how the sine wave is now correct.

Greetings

[MichaelKeymaster]

Thanks so much Frank for this very interesting analysis and circuit optimization!  I wonder how many other circuits need this kind of remediation?  🙄

[LectronFanModerator]

And now some time for some more experiments 🙂

Lectron is such a great tool to do quick changes in your circuit, as following examples show off !

What do you think will happen when you try these changes ?

Try these out to see (and hear !)

• swap the capacitor of 0,47uF with the capacitor of 0,1uF mounted between points A & B.
• exchange the straight line block next to this 0,47uF capacitor with a capacitor of 10uF (+ to the right)
• swap this capacitor of 10uF with the choke coil
• replace the 5,6k resistor with a 100k resistor
• add a resistor of 120 Ohm between the volume potmeter and ground
• add the light bulb between the Emitter of T1 and ground

There are of course more experiments possible, maybe you can write these down in this forum.

Happy experimenting !

• This reply was modified 5 years ago by Michael.
• This reply was modified 5 years ago by Michael.
• This reply was modified 5 years ago by Michael. Reason: Changed font and size styling for easier reading

[MichaelKeymaster]

Thank you Frank!  Love these challenges and variations 🙂

• This reply was modified 5 years ago by Michael.

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