Oscillation won't pass voltage buffer

Posted: 11/7/2016 7:39:11 AM

Joined: 8/25/2016

This is the full schematic of the circuit

It didn't work on the first try.

A theremin is an electric instrument which emit a constant note and you can change it's tone (frequency) and volume (amplitude) with your hands and a pair of antennas.

On this schematic we have a theremin which only varies its frequency. There's 3 transistors, on the sides two BC546 (I'm using BC547B that have more hFE) that are working as oscillators each with its own tank circuit consisting on an 100uH inductor (L1/L2) and two capacitors of 27pF (C4/C13) and 15pF (C5/C14) on parallel. Either oscillator on both sides are the same and are quite responsive on frequency if I get my hands near them but, the left one has a 4.7K potentiometer (R10) on series with the 15pF capacitor to vary the base oscillation frequency and the right one should have an antenna connected to its transistor collector through a 15pF capacitor (C16), making it more responsive.

The output of both oscillators (OSC1 & OSC2) are fed to the output stage on the middle through 1M resistors (R8-R9) using direct coupling. This should sum them up in one signal and rise the impedance of the output signal.

The main problem is, on Q1 base, where the signals meet, there's no signal.

If I disconnect each oscillator, on OSC1 and OSC2 I get a healthy output of approximately 8Vpp 1~20KHz with an offset of ~10V with a new 9V battery.

For testing purposes I disassembled everything and reassemble only the left oscillator.

My mission: be able to make its output signal usable.

Using a 10nF decoupling capacitor on OSC1 I'm trying to buffer this output to get a stable audio signal without success.

I've tried a simple voltage follower with an LM358 Op-Amp but the signal won't show on the output.

I noticed while trying to offset the signal to Vcc/2 to feed it to my Op-Amp is that a 4.7K+4.7K voltage divider will drop down the signal to 0V just on the decoupled side of the capacitor, but with a 1M+1M voltage divider the signal will be kept healthy. So I guess it's an input impedance problem on the buffer side.

So I went with a common collector voltage follower with a second BC547B (collector to Vcc and a 1M resistor between emitter and ground, feeding the signal to its base), and I did get some but minimal and unusable output at the emitter. Before the output decoupling capacitor (emitter) on this voltage follower I got the offset-ed signal near Vcc, so I guessed the transistor was getting saturated by the high voltage signal.

Then I tried to divide the input signal by an order of magnitude with a 1M+100K divider before feeding it to the base, but the signal will be lost right after the decoupling capacitor on OSC1, just like it happened with the op-amp.

What should I try next?

I guess is obvious that I need a high impedance voltage follower but get a lower input signal too. But when I try to divide it before feeding to the buffer it will drop down to zero.

Posted: 11/7/2016 11:24:43 PM

From: Northern NJ, USA

Joined: 2/17/2012

Where is your schematic?

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