EPE Mini Theremin construction (and problems)

I have just finished building the EPE Mini Theremin which is the same as the silicon chip Mk 2. and thought it might be helpful to other potential builders to mention some of the problems I am having so that we can perhaps compare notes. First my variations from the EPE plans.
a) I have made a 1920/30’s style case (didn’t like the plastic one)
b) The antennae are 19 mm dia and not 16mm (presumably towel rails in Oz are thinner )
c) The equalising coil is fixed to the pcb with cable ties as (believe it or not) I cannot find anywhere that sells nylon nuts and bolts !

I have carried out the set up procedure with some problems (not all the voltages are as specified in the notes).With a bit of extra fiddling I have got it working, (both volume and pitch). Attempts to get the equalizing coil working have so far failed and looking at the notes I suspect that I have to add some turns to the coil (damn)requiring some careful dismantling.
The volume is very low and the effects controls make little difference at present. I suspect one trim pot may be faulty as when trying to adjust to required voltage it makes no difference at all.
I have only given a rough outline of my experience to date but if anyone wants more detail I will be happy to provide it (including where to get some of the tricky components).If anyone knows enough about this circuit to give me any useful advice that would of course be appreciated.

I had a look on the EPE website and wanted to buy the May and June issues online hoping that they were sent immediately in pdf format, but it seems that they ship only the paper version (Anno Domini 2008!?!). So I cancelled that because I do not want to get some scraps of paper several weeks later in my letterbox. :-(

Hi
I am not familiar with this circuit enough to do a fault diagnosis blind but I do have the June issue of the magazine in front of me, colour photos!

David, there hasn't been a Theremin yet that I haven't returned to its happy owner in a working state, drop me a mail at displayer2008 at googlemail dot com.

Thierry, can you do the same?

MarkT

Hi
Thierry.. you can purchase and download epe in pdf format, I have done that and have both issues in pdf. You need to go to the epeonline website then go to subscribe. You can get a years subscription or buy individual copies once you hav paid you can download the copies you have paid for as many times as you like or simply save the pdf file.
Mark.. Thanks for the comment. I have got the thing working now although attempts to get the equalising coil working have so far been unsuccesful. I appear to be getting a nul indicating resonance but when I connect it in circuit it drasticall reduces the range without doing much for the linearity. The whole epe project is a virtually straight reprint of a project the Australian magazine, Silicon Chip in 2006. I believe Jaycar produced a kit, in fact I had to send to Jaycar for the IF transformers. I will contact you by email. Cheers
David Marks

Nylon Nuts + Bolts:
These can be bought from RS Components in sizes from M2 to M6, bags of 100 minimum! (about £4 for 291-379 M3*16mm M3 nuts 525-701 £2 for 50) - Unless one is doing a lot of construction, cable ties are the cheaper option! Nylon Nuts Bolts (http://uk.rs-online.com/web/search/searchBrowseAction.html?
method=retrieveTfg&binCount=12&
Ne=4294957561&Ntt=nylon+m3&
Ntk=I18NAll&Nr=AND%28avl%3auk%2csearchDiscon_uk%3aN%29&
Ntx=mode%2bmatchallpartial&N=4294955522&Nty=1)

Im not sure how you have the cable ties setup, but there is an issue with the coil, you need to have some space between the shells. With the nut and bolt, you need to add washers on top of the core, to lift the top shell up. the more space between halfs, the lower the Xl of the coil.

I plan to build this theremin too - bought the magazines when they came out.

The one part that seems completely unavailable is the pot core (lf1060 from JayCar - but out of stock). Does anyone have any suggestions for alternatives that will work?

Also slightly alarmed to learn that those little toko coils aren't made any more and the only ones I could find were NOS.

I would be glad of any hints for trouble-free construction. I know two other people who want one if mine turns out OK!

Kit

"The one part that seems completely unavailable is the pot core (lf1060 from JayCar - but out of stock). Does anyone have any suggestions for alternatives that will work?"

If anyone has data on this coil (inductance and resistance are main parameters required) it should be quite simple to find an alternative.

Thanks for that reply.

In the end I ordered some pots from farnell (FERROXCUBE RM8/I-3C90) and it seems to work quite happily. Don't forget the spring clamps for the sides of them, as I did.

I don't know what the target standards for linearity are, but I think I'm getting about 4 pretty linear octaves and a five-octave range overall. I think the high octaves are about 1/3 more compressed than the low octaves.

The instructions suggest 10 milliHenries as a 'target' inductance and I'm somewhere in that ballpark. I don't think a resistance value was given, though it said 300 turns of 0.25mm Cu wire; I've only used about 30 turns of 0.28mm Cu wire so my resistance value is way lower than the one in the magazine.

I suspect there may be some advantages to this - the instructions are specific that the coil should be jumble wound to reduce self-capacitance. I figure that if I'm getting the same inductance with fewer turns, then I must also be getting far less self-capacitance.

I haven't yet finished putting the theremin in a case etc. so I haven't finalised the pot, but that 10mH figure seems about right. I figure my pot has about 30 turns of wire - I'll count them properly later and post a figure when the whole thing is finalised.

Initial impressions are that...

Theremin playing is hard!

It seems to work OK.

The circuit went together very easily. I get good vibes from the layout and design... Seems it was made by someone who knows what they're doing. Only problem was that my 1/4" jack plug sockets didn't fit the board. But I'm sure that some other brand of 1/4" jack socket would have fit just fine.

Skew changes the tone significantly. I'm not so convinced about the other 2 tone controls. They may improve when I have them set up properly.

There's an awful lot of radio-frequency noise in the speaker/audio output despite the rc filter in the schematic. I suspect I may have muddled a resistor or capacitor during assembly so I will do some extra checks before stating definitively that this is a design flaw. But if other people have the same issue I would be interested to hear.

Kit

If anyone is REALLY stuck and cannot get IF transformers, I have a few spares (enough for 4 Theremins..) send me an email, and I will post a set to you FOC.

[i] "I figure that if I'm getting the same inductance with fewer turns, then I must also be getting far less self-capacitance."[/i]

First - I am not an expert on inductors! I find these components exhibit unexpected behaviour (try messing with Bifilar windings, then try to get answers for the questions which arise from your observations - you will discover that the whole subject of field interactions in inductors has huge gaps, and it is difficult to tell science from pseudo-science).. So what follows may be baloney!

I do not think the thicker wire increases the inductance.. I suspect that the increase in inductance is due to different core charactaristics... Inter-winding capacitance is a funny thing - and fewer turns does not always = lower capacitance, as the capacitance is a function of overlapping area, and thicker wire has more of this! Likewise, winding style does not always (ever?) decrease inter-winding capacitance, it does, however, randomise the capacitance / inductance relationship.. In my expierience, for applications in the Theremin frequency range, how the coil is wound makes no difference at all.

I just obtained the EPE article, and noted that the equalising coil is in the order of 10mH.. I cannot see anything special about this coil, except perhaps that the enclosed ferrite construction is needed to reduce radiation and perhaps increase Q.. I would probably go for a 'high current radial' such as RS Components 228-573 as these tend to have lower resistance10mH 228-573 (http://uk.rs-online.com/web/search/searchBrowseAction.html?method=searchProducts&searchTerm=228573)