PT-03 Review

Glad to hear it Thierry - congratulations!

My Theremax occasionally picks up radio signals as well - I think it is a common problem with the circuit.

One of the things I like about the Theremax is it has an unusual sound different than any other theremin I have played.

I have been too optimistic. I played around with it a few minutes (the RF suppressor coil made it really better, but there are still some dirty spikes in the audio signal...) in order to optimize its tuning when I discovered that the variable pitch oscillator occasionally stopped when there was a slight percussion on the circuit board. Unsoldered the IF can L1, fixed its wiring and resoldered it. Still a problem corrected. Now I have to find the source of these spikes...

Edit (some 30 minutes later):
Could reduce these spikes by soldering a 4.7nF capacitor across pins 12 and 3 of IC1. Sounds much cleaner and better. Since it's 01:40AM here, I'll stop for today, but in an optimistic way!

Now I think I'm finally almost at the point where this "device" seems to operate like it should.

The remaining noise disappeared systematically when I put my cell phone at least 8 meters / 24 feet away... *rolleyes*

The only thing which troubles me still is that even though I turn the timbre control fully counterclockwise in order to get a more or less pure sine wave, I have a very weak, but audible first harmonic, that means the double audio frequency or one octave higher. Is that "by design" or have I still to search for a problem? May anyone of the "real" Theremax owners confirm this?

Thank you in advance!

Thierry -

My Theremax originally had the octave doubling as well - it is faint, almost like a tone color. I made many mods to my circuit board to change the sound, which also had the effect of making the upper harmonic disappear. I wish I could tell you exactly what I did, but it was many years ago. I remember changing the cap values in the amp and mixer circuitry until I liked the sound. If I remember, it was pretty easy to figure out which caps conrolled the frequency rolloff.

Did some maths and also "real" tests today. The traditional linearizing technique with a coil in series with the antenna comes to its limits at such high frequencies as used in the Theremax (>800kHz). Out of that there are big differences in the pitch antenna design of the original Theremax and the PT-03, which has a 60cm long and 4mm thick thread rod. So the Paia method with a relatively small capacitor in series works better as has been proved in practice.

In order to be sure I lowered the frequency of both pitch oscillators to about 480kHz by soldering temporarily 220pF capacitors in parallel to the 100pF in the tank circuits. This allowed to improve the linearity by replacing the 5pF capacitor by two 4.7mH coils in series (as expected). But the amplitude of both oscillators became so week that the quality of the audio signal was unacceptable.

Conclusion:
My first task, making this PT-03 work correctly (as a Theremax) is accomplished. I had to fix one of the coils (a replacement was not available), to exchange a 390k resistor with a 4.7M (production fault) and to replace the LM339N (wave shaping did not work and there was no pitch cv). Out of that I tin-plated most of the circuit board because of very poor etching quality (lots of hairline cracks). As written above, I do not at all recommend the purchase of this bad Theremax rip-off.

My second task, doing some tests for a future project (still a secret), could be done after the device worked correctly and was successful.

My further idea, improving the linearity (which is already not bad at all by default, compared to the B3 and the Silicon Chip) could not be realized without important changes on the oscillators design. So I decided to postpone that until I'll have a real Theremax for myself (my birthday is soon...) instead of tinkering a friend's instrument.

I will meet him next Monday/Tuesday in southern Germany, give him back the repaired PT-03 and test another of my new circuits in his Etherwave Pro (but this will hopefully be the subject of a new posting next week).