My Theremax project

I got the bottom of the case modified to fit the camera tripod mounting bracket (a bit of chiseling, finishing and a couple of invisible cardboard shims), mounted it (it's very sturdy) and posted a photo (scroll to the bottom of the page):

http://www.abarbour.net/tmax.html

I would love to fire it up and test it tonight, but must do some homework. All things in due course.

This is really quite a nice job you've done.

Love to hear it. Been busy here. When I get time I'll post finished pix of Gabby.

Philip

Well, my Tmax seems to be alive--the pitch antenna tuned easily but I am a bit at sea on the volume antenna tuning. After some twiddling, I now have L3 adjusted so that (with the "volume" knob fully clockwise and the "volume trim" knob at midrange) there is the proper sort of response (decreasing volume as hand approaches volume antenna), but it takes place over a very wide range starting from beyond my shoulder--then it reverses and suddenly drops to zero several inches from the antenna. It seems that somehow I have to compress the control zone. Suggestions?

Question: Are those two large coils supposed to be that close together? At that range they would be coupling to each other one would think?

Just curious?

Philip

Another question....I noted you have a Behringer amp. Which one is it?? Is it one of their new Acoustiscnic ones? I considered getting one but was hesitant.

How do you like it?

P

The coils are in series; I haven't gotten far enough with tuning things to evaluate anything like that (the coils are currently set at bypass). But the pitch side seems to be working fine. Scott Lee at Theremax sent me some (a lot, really) of troubleshooting suggestions; he though the choke on the volume antenna might be bad, but it doesn't seem to be. Not sure if the connection to the board is good, haven't had time to work through his suggestions. He also suggested a different tuning procedure than the one in the manual. We'll see.

The Behringer keyboard amp/PA is an Ultratone K900FX. I have never had any similar product before, so can't compare. The first and so far only sounds I have heard from it were the initial tuning sounds of the Tmax. There were pretty good owner reports at Musician's Friend (that's where I bought it), and from lurking here I had learned that I needed a keyboard amp and effects might be interesting in time. It has a huge number of preset effects, lines out for sound system and subwoofer, lines in for keyboard, CD/DVD player, footswitch (I ordered one to turn effects on and off), and microphone; graphic equalizer, feedback control, and Lord knows what all. Seemed like a lot for $199. And it's heavy (which I take as a good sign); 15 kg/32 lbs.

I couldn't set up/tune my theremin without an amp, and didn't feel I could go to a store and ask to try their amps with an instrument that might not work, certainly wouldn't at first, and which I didn't know how to play. So I rolled the dice and bought this one. If others have experience with and opinions about it, by all means speak up.

Alan,
I have noticed with my wooden Theremax case,(which also has a MIDI-to-Control Voltage PAiA board tucked inside), that the wooden lid makes a giant difference as to the volume tuning settings one chooses. When tuning the circuit board inductors with the lid off, one must take into account that an organic material will be placed over 'them' when the lid it put in place.
My experience has been that tuning the volume inductors is a hit or miss endeavor, but can be correctly made with a bit of patience.

Strangely, my pitch rod does not seem to be as effected as the volume loop during the tuning process.
However, my volume loop is fashioned after the PAIA shape, not the RCA shape like yours. The PAiA shape takes the metallic loop, and metal mass, farther away from the circuit board and case. I don't really know if that makes much of a difference with your Theremx case design.
I hope this information can help you.

Good Luck!

teslatheremin

Thanks, Tesla. Yes, I can well believe the initial tuning is hit or miss. Not too hard to believe the lid makes a difference too. We have determined that the choke connected to the volume antenna is OK, not fried as Scott at PAiA had at first suggested. (Have 7 volts on the volume antenna and 20 ohms across the choke, which is within range.) Next assumption is that I may have missed the first peak and settled on the second when tuning the volume loop, which some other things seem to indicate as well. I need to get some sleep tonight, will try retuning when I am well rested. Scott suggested an alternate tuning method, doing the volume antenna first and not using a jumper. Think I'll give that a try.

I finally discovered the cost of the Swagelok bulkhead unions I used for mounting the antennae: $24 for a pack of four. (I used three, but one could just as well use two, directing one end of the volume loop into a hole in the case with no nut-and-ferrule connection.)

Having trouble tuning my Tmax; suspect the board is OK, but the modifications are not. Beginning to suspect those air coils were a big mistake; hope the bigger antennae wern't. Next step is to bypass the coils m completely, but I need some more RG174 coax to do that. I stretched what I had as far as it would go, went the rest of the way with an unshielded jumper, got a seemingly decent pitch response, but had a radio station playing in the background. Don't really think it will do much good, but maybe tomorrow I will try disconnecting one of the air coils and leaving the other connected.