My New Year gift to TW: A new theremin circuit

Hello! Do you happen to have the PCB File availible so I can order a board? Thank you.

Hello!

I´ve tryed to build the Thierrymin using J113 which, according to some posts here, should work. But I can´t get sound. COnnecting the audio out to a scope shows a falling voltage, starting at 4V, slowly dropping down to 0.
At Q11 there´s a nice waveform 0-12V.

What am I doing wrong? 

It´s build on a "real" pcb, not veroboard, and all connections are fine.

All component sizes are original, except the J113s

Most thankful for all help, this should be a xmas present for my son, still got some days left...

power on/off (audio out)

https://imgshare.io/image/cmv0j

q11 source
https://imgshare.io/image/cmZ0p

q11 drain
https://imgshare.io/image/cmMyX

jazzer73, I'm getting "that page doesn't exist" errors from your image links.

Is it oscillating?

jazzer73, I'm getting "that page doesn't exist" errors from your image links.Is it oscillating?

I´ve updated the links, should work now.
Its oscillating at Q11 D and S, bot not really anywhere else

Did some more measuring:

At Q41 nothing happens

Q21 D steady at 4.5 V

S almost steady a 1V

G oscillating at +/- 100 mV

So, one oscillator looks like it is already working well, but from your pic, I can't see if it's the correct frequency. Now, look after the other one. Both have to oscillate on distinct but close frequencies, so that the heterodyne mix product (difference of both oscillators' frequencies) falls into the audible range and passes the output low pass filter.

So, one oscillator looks like it is already working well, but from your pic, I can't see if it's the correct frequency. Now, look after the other one. Both have to oscillate on distinct but close frequencies, so that the heterodyne mix product (difference of both oscillators' frequencies) falls into the audible range and passes the output low pass filter.

OK, meaured frequency on Q11, it´s 460 kHz. Not so audible.  Sound like I´ve got a wrong cap size? The trimmer moves the freq between 460 and 470 khz

Getting the same frequncy on the antenna and Q21

Did some more measuring:
At Q41 nothing happens
Q21 D steady at 4.5 V
S almost steady a 1V
G oscillating at +/- 100 mV

Between 460 and 470kHz is perfect. With a theremin, you never hear any oscillator frequency, but only the heterodyning signal (difference) between two different oscillators' frequencies (which are both in the RF range). That's the principle of operation of every theremin in case nobody told you this until now...

Make the oscillator around Q41 work in exactly the same way as Q11. Then, and only then, you'll get an audio signal at the D of Q21. The indicated DC levels around Q21 look fine, so it will do its job as an active mixer as soon as it gets the signals from both oscillators (Q11 and Q41) at his Gate.

Now I´ve checked the caps. Seems right
3.3 pF ceramic
1.5 nF ceramic
150pF poly

10uF electrolyt

all in the right spots

Trimmer is 6.5-20 pF, which is slightly higher lowest value than in the schematic. (4-22 pF)

As I said, the first oscillator around Q11 works well which proves that caps and trimmer are ok. Now, I say it for the third time, fix your circuit around Q41, so that the second oscillator operates in exactly the same way as the first one around Q11. As you can see on the schematic, the only difference is that there is an antenna at the place of the trimmer. Both oscillators should work even without antenna/trimmer, just on a slightly higher frequency.
Before you do not achieve that, it makes no sense to look elsewhere. Your luck is that you have one of two identical oscillators already working, so you can directly compare between both, which should help you with finding the issue.

jazzer73,

good to see a fresh face. When I constructed the Thierrymin it was the 10% tolerance of the coils I chose that messed me up. Needed to tack a 4.7pf capacitor to the bottom of the board to lower the freq of one oscillator a little more than the other.

I recommend not to using a scope which can load down sensitive circuits, rather use an AM radio tuned around 900 khz (double freq) to hear where each oscillator is tuning in at. Then adjust accordingly.

This is a good teaching design but I found thermal drift to be a bit of an issue.

I added Pot-1, a 100 ohm for tuning, works good.

Christopher