Diagnosing Jaycar/EPE/Silicon Chip/Kees Enkelaar Theremins

Hello Dave,

Looked through thread for schematic and noticed Thierry gives the answer in the first post. Again your oscillators are at 400 khz and 500 khz, you do not want this much difference. Try and have the two frequencies meet in the middle like 475 khz or higher, I doubt being right at 500 khz is critical. The real problems begin to arise when you start un-soldering parts.

In Thierry's example one osc is 500 khz and the other is 501 khz, so the audio frequency is 1000 hz. The most exciting moment in theremin design is the first time you hear the audio pitch sweep by.

e) Check the AF section.

- Recheck the frequency at Pins 1 and 10 of IC1 and make sure that there is a difference of about 500Hz - 1kHz

- Check with an oscilloscope at Pin12 of IC1 if this difference frequency of about 500Hz - 1kHz appears with at least 250mVpp

- Same check at Pins 5,6 and 7 of IC2 and at the audio output.

T

Hello again Touchlass. Thanks for your comments but I think you're missing the main point that the volume antenna circuit is not working. Sure, the pitch antenna oscillators are not adjusted properly yet, but no amount of adjusting them is going to alter the fact that there is no volume. There is zero volts being applied to the bias of IC1 because the volume circuit is defective. There should be about 7 volts applied to the bias pin on IC1, not zero volts. I have found that the two antenna circuits are working and adjustable so I can work on adjusting them after I solve the problem of the volume antenna. Thierry pointed at it to another person on this thead that the correct way to approach this problem is to start with the volume oscillator circuit around Q3. Currently this circuit is not oscillating so I must focus on that before trying to adjust the beat frequency in the pitch antenna circuit. But thanks for your interest. Dave. 

There are only a handful of active people at TW today, then a few observers. I missed your main point because I am staring at the elephant in the room. I found the schematic so let us see what other exotic animals might be here.

All we can do is exchange ideas so let me mention that your volume may be defective idk but most of the time a bad volume control will not quiet the sound fully, in my experience. There are still a couple of theremin smart people around here. I am too rebellious to be in their league.

What voltage is on pin 5 of IC1, it shows at ground would be volume silence. Full volume would be several volts positive.

MC1496 Datasheet   This shows if you connect 12 volts through a 10k resistor to pin-5 you could bypass the theremin volume circuits for a test.  This data sheet is a bit too geeky for me but do remember when it comes to theremins everyone is an expert.

Hello again. I hope you manage to do something about that elephant! 

Pin 5 of IC1 is at 0.05 volts and it doesn't change. The volume circuit is dead.

i have now exchanged the JFET Q3 but it made no difference. This moved my attention to the other main component of the volume oscillator, the IF transformer T3. I have discovered that there is an open circuit on one of its windings. The other two IF transformers have a small resistance across each winding. I am fairly confident that this is the cause of my problem. I will get another one today and swap it.

Thanks again for your interest. The circuit you posted is slightly different to mine. I am using the Jaycar mk2, but I think your one is the modified one that has been mentioned in other posts. I hope to get the basic circuit working before considering modifying it.

Dave.

I replaced the faulty IF transformer and I now have a working theremin! I have ordered a frequency counter so I can follow Thierry's instructions to adjust it properly. I will do some mods on the aerials too to try to improve range and performance.

Thanks for taking an interest 

Dave.