building a theremin trouble shooting

I've sorted out the problems I was having with the pitch reference oscillator so it is producing the frequency it is supposed to. I've also built the pitch variable oscillator and have it producing close to the frequency I want but I don't seem to be getting any change in the frequency when I put my hand next to the antenna. I thought the change might be too small to be noticeable on my scope so I built the mixer circuit to combine the two signals to see what I got out of that.

I seem to be getting an amplitude modulated signal on a rather poorly filtered carrier frequency but I don't know if that's what I'm supposed to get or not. Unfortunately my instructions don't give any indication of what waveforms I should see coming out of each of the modules. I also don't seem to be getting much frequency shift out of the mixer either unless I actually grab the antenna. I don't have any specs on the antenna so I just bought a replacement car antenna to try so I don't know if that's part of the problem or not.

The car antenna may be part of the problem since it has a shielded cable which adds unwanted capacitance.

Out of that, it would be easier for the techs here to help you if we knew of which theremin circuit you are working on. Are the schematics somewhere on the web?

It is normal that there is not much reaction on the scope's screen. Imagine an oscillator running at 500kHz which will be de-tuned by around 1.5 kHz when approaching the hand to the antenna. This can hardly be seen. That's why I use a precision frequency counter when working on theremin oscillators.

Hi
Thanks very much for responding and offering some suggestions.


The instructions for the theremin I'm trying to build are at http://www.strangeapparatus.com/Theremin.html . I picked this one because of the modular design that should allow me to test each section independently.

I hadn't thought of the shielded cable on the antenna adding extra capacitance but it makes sense. I suppose I could just chop the cable off and attach it with a plain old chunk of wire. I can't find any information anywhere on the dimensions or construction of the antenna so I'm just going by the pictures of theremins that I have seen posted on line.

I kind of figured that I might have difficulty seeing the frequency shift on the oscillator output which is why I thought that I would connect the two oscillators together via the mixer. I had expected to get a lower frequency wave form out of the mixer with a frequency shift that was visible at the lower sweep rate on the scope. Unfortunately my electronics education is so far in my rear view mirror that I don't know what to expect to see coming out of the mixer. I'm also not sure why the instructions say that the oscillator frequency can be tuned by adjusting the transistor base bias current. I thought the frequency was determined by the RL tank circuit on the collector. Adjusting the trim pot doesn't seem to do anything although maybe that is another effect that's too small for me to observe.

I was unable to find the specified BC547 transistor but everything I could find on line said it was just a standard small signal transistor that could be substituted with an equivalent NPN transistor such as the 2N3904 which is what I used. Since the oscillator appears to work I don't think that is causing any difficulties.

Hi there, I've just finished applying Max's modifications to the Jaycar kit. What's the best way to go about systematically checking that it works? I started by following the Jaycar instructions but I'm not sure whether the mods will change things like expected voltages. For example, the Jaycar instructions say to expect 5.3v between the case of any of the transformers and pin 8 of IC2 but I only get 3.5v. Is this to be expected or have I maybe fried the regulator or something?
I also used shielded cable to connect the antennas, line out jack and external pots, could that cause problems?
Last question - I'd like to keep the internal speaker but saw on a few posts that it can cause problems. Should i just ditch it? If so, can I just plug in a set of headphones or does the signal need to be amplified first?
Thanks :)

@patricks: The search function of this forum should have pointed you to this posting (http://www.thereminworld.com/forum.asp?cmd=p&T=4430&F=3) for a systematic diagnose and troubleshooting of the Jacar/EPE theremins.

Generally I want to add that the first prerequisite for successfully building and debugging theremins is that you fully understand the circuit you are working on and that you are able either to calculate or (with some more experience) to estimate the voltages and waveforms you will find at all points of the circuit which makes questions like "They write there should be 5.3V there but I have only 3.5V, can this be correct?" obsolete.

You will then need a digital multimeter, an oscilloscope with at least 2MHz bandwidth and a precision frequency counter for a correct diagnose. And you should be enough experienced to understand that a digital multimeter will show false values when you try to measure the sum of a DC voltage and a superposed RF signal...

Great,thanks for the redirect. I understand what you're saying about needing to thoroughly understand how the circuit works before taking on a project like this, but I'm treating this as part of my learning process:)