My Theremax project

I am more and more convinced that the combination of the air coils and the RCA-style antennae was too much, but will have to wait on the extra coax (thanks, Philip) to be sure. My multitudinous attempts to get the Tmax tuned have been far more educational than just turning it on and having it work straightaway would have been. I think the antennae alone should be OK. If so, that will leave me with several satisfactory mods:

1) RCA-type antennae
2) Longer case, increasing distance between antennae
3) Latest PAiA electronic mods
4) Successful tripod mount
5) Power supply can be disconnected from case
6) Push-button mute

Now, if the air coils come out, what can I put in their place? There's enough room for a carefully packaged sandwich, but some sort of electronic accessory would seem more appropriate. Or maybe it would serve as a place to stow the wall wart and a wrench for the antennae, if nothing else.

Oh, another discovery: no need for lock washers on the bulkhead unions I used to mount the antennae. A plain washer on the inside of the case is enough.

Alan,
It seems to me that most Theremax builders that added an extra inductance in series with the pitch rod, set them vertically, near the pitch rod inside the case. Perhaps, instead of totally abandoning your worthy research into how gradual increased inductance effects pitch linearity, you should use one of the coils in a vertical position and try again to achieve tune.
Just, my thoughts.

Good Luck!

teslatheremin

Well, I'll think about that, Tesla. Scott Lee at PAiA had a number of suggestions, and Philip is sending me a length of coax adequate to reach the pitch antenna mount, which will allow me to troubleshoot more effectively. I tried attaching both the stock and RCA-type pitch antennas to the pitch lead (which is too short to reach the right end of the case), and it seemed to make a world of difference. The pitch and volume sides of the instrument do not seem to be independent of one another; change one side, and the other must be adjusted. If I must choose between the RCA-style antennae and the air coils, that's easy--the antennae (they really do look good). If I understand the pitch preview mod correctly, it requires another jack, which could go where I put the knob for the pitch coil selector and would also serve for keyboard input. That would be a tolerable use for the space. Add some way of securing the wall wart within the case for transport, and it would be OK, I think. Well, we will see. Probably next week; I work the "kamikaze" shift this weekend--somewhere between 55 and 60 hours straight.

Man, I gotta get back to work on my Tmax. I've just been overwhelmed of late, but that should pass before long. Gotta bypass the air coils, and then see what happens.

Well, the air coils experiment seems to have been a mistake, at least combined with the big RCA-style antennae, or at least the way I did it. I couldn't get the TMax tuned. I got buried in other stuff the past several weeks, and only today I got back to work and bypassed the air coils. (Thanks for the coax, Philip!) After a little tinkering, I seem to have it tuned about right, both for volume and pitch response. There is a VERY faint pitch response over a short part of the volume control range, but that's close enough for tonight. To compensate for the RCA-style volume loop I had to adjust L4 a bit more than for the standard antenna, but it could still be done without difficulty.

Now the question is, what to do with that space where I put the air coils, and the hole in the right side of the case. Pitch preview seems like an obvious substitution.

Tesla--I recall you saying that you had added pitch preview, which seems like a reasonable use of the space devoted to the air coils. Could you give me some pointers? The instructions at

http://paia.com/ProdArticles/therefaq.htm#therauxm

are about as clear as mud to me. Looks like they make some connections to the board, somehow add in a little amplifier circuit, and have a headphone jack coming out of the case. Am I close?

I think what went wrong with my air coil mod may have been the grounded plate I put underneath it. That was wired together with the one underneath the circuit board. I think the AC output from the air coil fed back through the grounding plate and into the circuit board. Perhaps it would work without the grounding plate, or with it disconnected from the one underneath the circuit board. Well, that's an experiment for some other time.

I seem to have the pitch tuned OK, and I have discovered that for volume adjustment I must tweak L3, then put the cover on and try the response. But one problem remains. Once I have the volume responding about right, as I move my hand toward the volume loop the volume drops off as it should, goes through zero, then goes through a zone of reversed response (louder near the antenna) and finally drops to zero. As I adjust L3 inwards, this reversed zone becomes smaller and closer to the antenna, but never goes away completely. Does this sound familiar to anyone? Any suggestions?

Alan,

Depending on how you tune L4, the Theremax will get softer as you approach the loop, louder as you approach the loop, or softer then louder as you approach the loop.

Just tweak L4 to get the response that you want.

[i]-- Kevin[/i]

Thanks, Kevin--I don't think that's in the manual, but it ought to be! I was beginning to catastrophize about some odd electronic mismatch. I will set to work at once!

That did it, all right! With L4 cranked in two turns, the volume response seems to be just as it should. I spent some time fiddling with the FX presets on my amp (list downloadable at
http://behringer.com/K900FX/index.cfm?lang=eng
link in right column) and could get quite a variety of sounds. Many thanks, Kevin!