Help with em theremin construction

Hi Roy,

First, its probably best to take what I say with a pinch of something strong - I am something of a pedantic when it comes to design and construction.. Really, the EW works with its inductors on the board, and EM's do, and Bob Moog only moved inductors close to the antenna on highe end theremins like the E-Vox and E-Pro (I know those have the EQ inductors off-board, and I think the 91 series does.. but whatever.. he thought the EW was acceptable - who am I to argue with Bob !?)

Its just that, to me, having some of the antenna inside the theremin (which is what the wire connecting to the antenna actually is) is wrong.

But I dont think it is critical, unless one makes the mistake of having a large ground plane or whatever close to the inductors.

If you are starting from scratch, I would advise you to locate the inductors off-board and close to the antenna/s, regardless of which theremin you were building.. But if you had, for example, a grounded back plate, and by moving the inductors they came close to this, then the board might be a better place.

Bottom line - keep the antenna wire and antenna inductors away from anything conductive as much as possible.

Fred.

Hi Fred,

thanks for the reply: I have no problem with pedants:-)

One more basic question: I'm looking at the illustration of the component placement on the prototyping board (figure 4 in my version of the article) and am a wee bit baffled as to which way the tracks on the board go because when I think I've figured out that they run vertically, I see a link which says they go horizontally.
Are my glasses needing replaced and am I just plain daft not to spot it or is there something else going on here? I'm assuming that prototyping board is just a fancy version of veroboard?

Apologies for the daft questions!

Roy

Hi Roy,

Not daft questions at all.. I dont know what sort of board it is - but I suspect it was some sort of copper pad board rater than stripboard - the sort of board we used in the old days for wiring up TTL IC's, they had tracks with 3 linked pads, seperated by 2 holes followed by 3 pads .. like this oooxxoooxxooo where the o's are linked pads, and the x's are holes with no copper links...

One would put yhe IC down so that the two xx holes were across the body, and each leg of the IC then had two pads (after soldering its pins to pads) onto which wiring could be connected.. So most interconnection was with point-point wiring.

looking at the layout diagram, I think this makes sense - if you take the LM13600 (replace with LM13700) and view the diagram using its pins as a reference, it should be something like 2 or 3 of the pads above and below each pin of the IC will be connected to the pin, followed by a break in the track of probably 2 holes repeating thereafter..

But there were sometimes variations, with power carrying tracks laid down the centre of the IC locations (IE, viewing the diacram, horizontal tracks, with the tri-pads being vertical)

To be honest, I have never copied any layout, even as a kid .. the schematic gave me what In needed to know, I couldnt have afforded the specified boards, so used whatever I could find (even removing components from junked boards I could buy for peanuts, scratcing off the tracks, and using these ;-)

Fred.

IT WORKS!!! The pitch that is. Moved the linearization inductors to the perfboard and low and behold it works perfectly...even with all the lab equipment around it (Will move to an open area later for final tuning). 

Now off to testing the volume section.

Hi Fred,

I had no idea there were so many variants of fancy veroboard substitutes out there! Maybe that could explain peoples' failures to get their EM working; they have the wrong type of protoboard.

Good info though-in light of that I'll do my own circuit board/veroboard/hard wiring if I ever get round to making one.

By the way, I like the idea of making something out of the bits that are redundant. I have an old valve wireless out in the hut which is really in tatters. If I ever have a notion to make a valve theremin, the chassis might be useful...

Roy

"I had no idea there were so many variants of fancy veroboard substitutes out there!" - Roy

Oh, there certainly were! - Not so many these days (sadly) but still quite a few. I really dont like the original strip board.. My favorite is square-pad board and 2nd is round-pad board - these have no tracks, but allow one to wire tracks wherever on wants.. This way you can design a "PCB" on an 0.1" matrix, fit the components on a 0.1" proto board, wire up as per your layout, and have a layout ready to produce simple boards quickly.. although, in reality, one would usually move the components closer (select say an 0.05" grid) so as to reduce PCB size.

I have put my copy of the EM article up at E-14.. It is reasonably clear, one can zoom to the board picture (on the first page) - You can see from this that the board is certainly no standard stripboard!

http://www.element14.com/community/docs/DOC-50631/l/build-the-em-theremin1pdf

Fred.

For the volume side, I'm able to get -5V to 0V across the detector(on volume antenna circuit) as i move my hand towards the volume antenna but the voltage at pin 12 of the vca processor stays constant at -11.513V or so. In a previous thread, it was mentioned that there should be a capacitor at this pin to ground like it is in the etherwave theremin schematic. Does anyone know if this will help me get the 0-12V i'm looking for or is there any other solution to this?

Hello again.

I got my EM theremin to mostly work but i'm a little unsatisfied with the volume side of things. 

The volume is muted with the hand still about 8inches away from the volume antenna. Is there a way to tune the volume antenna so that the it mutes when the hand is much more closer to the antenna?

Right now the volume oscillator is running at about 460 - 470 kHz (I haven't looked at the oscillator in a while) and the voltage at pin 12 goes from +11V to -11V.