While looking on the Moog website, I saw a schematic for the Melodia theremin sold in the 60's. It is virtually identical to my Kustom, right down to the parts values.
Moog put the trim caps for their instrument on the front panel and, unfortunately, Kustom chose to bury them inside where a screwdriver is required to adjust them. A royal pain in the extreme lower back.
I've managed to add some reversible mods to it to get better low frequency response, and a larger playing field, but it looks like there's no way to transform it into a respectable performer while retaining any collector value.
I've decided to just build a new one....Sigh.
Here is the construction article. I agree it would be a good place to start. Linearity is not to bad and construction is reasonably simple.
The mods I made were to install a bit of resistance in series with the oscillator outputs and add a one turn pickup loop loosely coupled to the variable oscillator coil so I could use a scope to get the tuning perfect. At antenna resonance there is a sharp dip there.
My new theremin will probably not bear much resemblance to a Melodia. Hartley oscillators, buffers, harmonic filters and a real VCA.
I'll document everything.
Photos of the beast and a schematic.
Any common transistors should work. The construction article gives winding instructions for the inductors and transformers, but the parts values are a bit hard to read.
"While looking on the Moog website, I saw a schematic for the Melodia theremin sold in the 60's. It is virtually identical to my Kustom, right down to the parts values." - W0ttm
I wonder if Kustom bought the rights to Bob's design, or whether they just ripped Moog off ?
After a closer look, there might be enough differences to avoid a lawsuit, but just barely.
The Kustom pitch oscillators run quite a bit higher than the Melodia, and there are minor parts variations.
My guess is rip off.
If they did pay uncle Bob anything, it was a bad deal considering they only sold a couple of them.
I think I'll build a Melodia just for fun. I probably have all of the parts in my junk box :-)
These links may be helpful - The entire article:
http://moogarchives.com/therem61.htm Front of magazine - no data just picture
http://moogarchives.com/there61a.htm 1st page, no data, description of theremin/s
http://moogarchives.com/there61b.htm Schematic + "how it works"
http://moogarchives.com/there61c.htm "how it works" continues, and start of construction
http://moogarchives.com/there61d.htm Construction continues, coil details
http://moogarchives.com/there61e.htm Coil adjustment etc, how to play..
It should be noted that the transistors are Germanium and probably not cheap or easy to get - there is an 0.3V bias on the bases of the oscillator transistors, and I suspect that at least this will need to be changed (to say 0.65V) (change R14 and R15) to have a chance of getting Silicon transistors to work.
I do not actually think this would be a good starter theremin, just from quickly looking over the schematic, I can see many potential pitfalls for any "newbee" - For example, the "VCA" looks like it works by driving a transistor into saturation - interesting! - But without getting data on the (germanium) transistor used (I suspect these have a much lower Hfe than most modern silicon transistors) and carefully selecting a replacement (and probably making some modifications, at least to component values if not the circuit) I really doubt that getting it to work well will be much fun.
That's one of the differences with the Kustom.
It uses silicon, I think. I made note of the transistor types, bu I don't remember where I put that note.
The melodia bias resistors are 2.2k and 330, where the Kustom uses 2.2k and 820.