Theremax question

Good catch, Teslatheremin.

Ah, but the plot thickens...

Indeed, I put a 10K resister across the coil ends of R11 and R6 which imparts harmonics to the otherwise sine-wave tone.

Thus, my coupling scheme is using the secondary side of the coils!

As much as I would like to claim that I utilized "great knowledge and skill" to come up with this, apparently I just attached my clips to the wrong parts, tested it out, liked what I heard, and went with it!

I haven't really played my Tmax much over the last year or so. It had developed a lot of "distortion".

Turns out that the detected (audio) signal from the volume side was bleeding into the volume control circuitry. The hint was that I was NOT hearing the distortion in the pitch preview signal (which taps the pitch prior to the amplifier circuit).

After many hours, I traced it down to C25, and discovered that I had removed one of its leads! I must have been experimenting with mods to make the volume more responsive. Turns out that C25 passes most of the audio frequency signal from the volume side to ground. Restoring C25 got rid of the most severe distortion (which closely resembled ring modulation).

There is still a little bit of bleed into the volume circuit and I may experiment with different values for C25 or some of the other components in that part of the circuit.

Anyway, just wanted to post this because I have commented that my Tmax had some "interference" issues. At least I have it narrowed down to the bleed into the volume circuitry and have ruled out any RF interference.

[i]-- Kevin[/i]

Kevin--
Reading back through this thread I came to your post of 3/5/2006 9:28:07 PM about adding a new pitch antenna and having to tweak L1. I am having troubles with initial tuning of my Tmax because of unauthorized mods notably including antennae. I find that I can get things kind of tuned with both antennae disconnected. But when I connect my RCA-style volume loop, the volume control range becomes huge; there is also a reversed response close to the antenna. No L3 adjustment seems to fix it. I am beginning to think it may be necessary to adjust L4. Does that sound right to you? So far I have backed L1 and L4 full out, then one turn in, and left them there (as per Tmax manual).

Yes, you are on the right track. For the volume side I have to tweak both L3 and L4. I haven't found a "scientific" way to go about it, though.

I am still using the Tmax's stock volume antenna.

You might want to hook up the stock antennas and make sure that it work with them. The Tmax design seems to be pretty "forgiving" -- it should work with different antennas other than stock with some tweaking.

[i]-- Kevin[/i]

Thanks, Kevin. I previously disconnected the air coils and and both the custom antennae, then connected the stock pitch antenna; the Tmax tuned and the response seemed good, which indicates to me that the board and front panel are OK and my mods are the problem. With the custom volume antenna in place I have been adjusting L4 inwards a quarter turn at a time and then going through the manual's tuning procedure, and I seem to be getting somewhere. The L3 adjustment point seems close to all the way in, and I suppose it should ideally be in the middle; Moving L4 in seems to move the L3 setting out, so I will turn L4 in a bit more. I noticed that although the manual says to turn the volume knob (far right) to max, that produces a 60-cycle hum, so I backed it off from there. At one point the neighbor across the street started up his electric grinder, and the motor came through loud and clear on my Tmax! My plan, after the volume side seems to be tuned, is to first connect the pitch antenna directly to the pitch lead and go through the same sort of incremental procedure; then add in one air coil, then check the volume side again. I have hopes it will eventuate in a functional Tmax. Oh--the timbre control really does seem to work well with the latest suggested PAiA mods as noted on my project thread. Nice steady change, and *very* nice [IMO] tone part way between sine- and square-wave.

Alan,

Sounds like you are making progress with your Theremax project.

I had some spare time this weekend and I installed some additional shielding on my Theremax and I installed a 1/4" jack on my newly purchased toy theremin. Here as some pics and more info:

http://kevinkissinger.com/tmaxandtoy.shtml

The bleedthru problem was the result of grounding issues. As I've mentioned in previous posts, the best way I have found to ground the Tmax is to ground it to myself via a wrist strap.

The capacitor C25 diverts the audio frequency signal away from the amplifier section via the ground. If the ground isn't REAL GOOD then the bleed-thru occurs. So, in a sense, all the shielding in the world isn't going to be effective unless the unit is well-grounded.

Same with the toy theremin -- without the mod (that is, without an exposed ground point) it really isn't playable. However, to have an exposed ground point that one can touch immediately makes the unit playable. Or, if plugged into an amp then the sleeve acts as a ground point.

For some reason, there is a lot of electrical interference in the neighborhood here. In fact, I cannot recieve a.m. radio here due to all the interference.

[i]-- Kevin[/i]

Kevin--I'm stumped. By doing a little tweaking here and there, deleting this and that, I have reached the point where I have a seemingly acceptable response (albeit with some other whoo-oops in the background) with the pitch rod (with the jumper from D1 to R83-3 in place. Then I take the jumper off and adjust L3 inwards to find the double-humped sound, stopping just short of the volume peak. That gives me a qualitatively correct volume response, although it is very buzzy rather than a nice tone.--but now there is no response to the pitch antenna, and the sound continues no matter which knob is turned. Does this sound familiar to you?

TIA
Alan

Fired up my Tmax just now to see if I could figure out anything. Realized (as I should have last night, but I was tired) that the volume antenna is acting as a pitch antenna. It must be that I have to tweak L4 to get to the correct L3 setting.

--Alan

Alan,

If the volume and pitch oscillators are close to the same frequency, then the interference will cause the pitch to vary with the volume.

The way around this is to put in your jumper and, while listening to the signal, adjust L4 such that your hand motion around the volume antenna doesn't affect the pitch. Then remove the jumper and tune L3.

Hope this helps.

That sounds like a primo suggestion; thanks, Kevin.