Equivalencies for Pop. Electronics Theremin

Hello, I'm working on building the Popular Electronics theremin from 1967 (http://www.swtpc.com/mholley/
PopularElectronics/Nov1967/PE_Nov1967.htm) and I have a question about finding the equivalent parts. The man at Radio Shack helped me find some of the transistors, so to replace MPS3638 and MPS3708 I now have PN2907 and MPS2222A, respectively. Looking for more information about the TIS-59 I found Art's 144 Theremin plans, which suggest that the correct replacement for the TIS-59 would be the 2N5484. (http://home.att.net/~theremin1/144/142transistors.htm) Does anyone have experience with these components and can verify that these are the correct choices for equivalent transistors?

Secondly, I have a question about the induction coils. Will the equivalencies for these (50-300 ìll adjustables) be fairly simple to find?

Any advice regarding this project would be appreciated, it's my first major electronics undertaking.

Why this Theremin? as far as I can see, it is limited in having no tone control of any kind, and the circuitry is extremely basic.
As for the inductors - There are some coil formers with ferrite slugs which would probably work - Jaycar sells them - but you will probably need to wind your own coils, and need a inductance measuring device such as the Peak inductance / capacitance analyser (£95?).

If this is your first electronics project, I do not think it wise to take one from 1967! Better to go for a more modern design for which parts and support is available.

JFETs can be fussy! You may need to buy a bunch and sort through to find ones that will work in that circuit. The last remaining one Radio Shack sells (MPF102) is particularly useless for this sort of application.

I agree with Fred that if you haven't gotten too far into this project, you might want to try a different one.

Don

I thought this one looked fairly simple, but perhaps you're right. I've had a hard time finding a plan. I want to build one that has both pitch and volume antennas, but so far I've had difficulty finding plans for instruments which have both but are also "beginner" level. Maybe you could suggest a plan that meets these specifications?

Christopher's RS Illusion is a good choice. It is pitch-only, but he also has a photoelectric volume control (VAPP) you can build.

You will learn a lot building this and the end result has very decent sound.

Don

Hello everyone,

sadly, I am also interested in the Popular Electronics theremin.
I am a first-course physics student, therefore I am eager to learn about electronics, yet have little knowledge so far. This looked like a suitable schematic for me, so I decided to give it a try.

And, I actually built it some time ago.

However, I can't get it to work. Is there anoyone out here who could provide some suggestions for diagnosing the problem?

The funny thing is, it worked to some extent with my poor quality hand-wound coils. However, as I received some factory-made coils, I transferred the whole thing onto a new PCB.
The previous version was terrible, and I blame it on the poor coils. It did make some theremin-like sounds and reacted to my hands. However, after the transfer, it stopped producing any sound at all.

Any ideas?

--

Althoug I am a student of physics, it is not a "student project", I am actually interested in making a more-or-less playable theremin.

1.) Check VFO at Q1:E , be sure this is running. If you have a frequency counter, check its frequency and how much it varies as the hand is moved towards / away from the antenna.

2.) Check Ref Oscillator at Q2:E , be sure this is running. If you have a frequency counter, check its frequency and adjust L2 so that its frequency is at the same as the VFO when hand is away from antenna.

3.) Check mixer: Q3:C should have a audio frequency equal to the difference frequency.

If the above all work (or you can get them to work) .. You need to check the (awfully drawn) volume CV generation circuit.. Note - +V comes in via R18 and R19 which is drawn below the 0V line! .. Check the oscillator is running (Q4:E).

C20 taps the volume oscillator , L4 is tuned to filter the frequency and D1 rectifies the resulting amplitude changes from variation in the volume oscillator frequency, the result should be a changing DC level on Q5:C, which changes as hand is moved closer / further from volume antenna.

Now, a slightly weird bit.. Q6 acts as a voltage controlled resistance between the audio signal (derived via C13/R13 from Q3:C) and the +Ve supply! Q6:D is entirely capacitively coupled.. And, to be honest, I cannot see how this could work reliably – it is one of the most horrible “VCA” designs I have ever seen! – I imagine the characteristics of Q6 will be critical.

Q7 is simply a audio buffer – Q7 can be almost any FET .. 2N5484 is common and will work.

Why this Theremin??? Build Dr Moogs EM (Etherwave) (http://www.element-14.com/community/docs/DOC-16870/l/build-the-em-thereminpdf), and have something designed by a genius – not by a hobbyist! The EW is probably simpler and cheaper!

Fred.

Even if you carry on with this Theremin.. Read the article by Bob Moog above - it has useful guidance on debugging the Theremin with minimum test equipment - and much of this advice applies.

Vilnius!

The birthplace of Clara Rockmore, the greatest thereminist who ever lived.

Hints concerning special parts for this design:

The tunable inductors "SLOT TEN-5-14" from Coilcraft inc. are perfectly suited. The 5-15 may also be used, it is only important to use the same type in all stages of the circuit. These Colpitts oscillators are not critical and will work fine with every small signal transistor which has a sufficient current amplification factor.

So the PNP transistors (MPS 3638) may be replaced by the 2N3906 and the NPN (MPS 3708) by the 2N3904.

The FETs are also not critical, one is used as a variable resistor, the other one as a source follower/impedance converter. The relevant parameters are a drain current of 10mA and a pinch-off voltage of -3V. The 2N5484 (as cited above) may just work, but I would recommend using the 2N5485.

Fred M ->
Thanks a lot for the detailed input!
I just wanted to ask: is there a way to check if the oscillators are running without an oscilloscope?

// on the other hand, it just occured to me that probably the article will have some info on that

coalport -> wow, so it turns out that I have a "solid reason" to carry on my theremin effort :D

Will have to try these replacement parts.