Newbie trying PAiA Theremax

Hi all-

Can anyone help with suggestions for a PAiA Theremax I’ve (sort of) built?

Back in 1998, a friend and I got kits, and started building them. We’d solder four components, then trade circuit boards and inspect each other’s work. Just as we neared completion, I met my wife, and suddenly had a much harder time finding time for our project. My friend hooked his up and it worked, while mine was mothballed. As time went on I got married, moved twice, and had kids. Then, last Saturday, as my 6 year old son watched, I checked my email, and a kitty+theremin video popped up. He loved it, and I told him about the incomplete theremin downstairs from a decade ago!

So we got it out of storage downstairs, and followed the instructions. The power LED turned on when the theremin was turned on – so far so good.

I turned L1 – L4 all the way out as instructed, and set the knobs as instructed (vol trim & pitch trim in middle, CV, timbre & vel all CCW, Vol at 45%), and shorted lug # of R83 to the band end of D1. Then, I turned L3 slowly in while listening for the null point. I was using just a little set of computer speakers for an amp. It was pretty low volume (turned up all the way), but it worked.

I found the rising and lowering tone, and noticed that it was vol trim, not pitch trim, that then changed it (unlike what the instructions said). At that point I could get a (weak?) response from having a hand near either antennae!

But then I noticed that I had null pointed L3, while the instructions said L2. I turned L3 back to it’s all the way out position, and tried to null L2. No response. I tried taking off the short – no response. I tried putting everything back the way it was, and couldn’t replicate what I had done with getting it to sort of work previously.

I thought I might have had the short on the wrong end of R83, so I turned off the power and switched it (to lug #1). I turned on the power, and the power LED came on, then quickly faded. I immediately unplugged it, worried that I had damaged it, and then restored the short on R83 to lug #3. Plugging it back in, the LED came on fine, but as before, I still couldn’t replicate the response from the antennae, no matter what I did.

So can anyone help? Is it clear whether or not I damaged it by having the short incorrect – or was that a harmless mistake? Or is this just because my amp is so small? Should I try wiring it into the line/ext of a boom box? Is it clear what is wrong, or what I can try next? I've found a nice case and the kids are all excited.

Thanks for any help anyone can provide.

Equinox

Hello,
Welcome to Theremin World!
The T-Max needs a GOOD Earth Ground. A pair of computer speakers that are powered by a wall-wart does not provide a GOOD Earth Ground.
I would try setting the inductors back to the manual start settings, borrow a guitar amp and then try tuning the T-Max again.

Keep us posted! I am sure some other PAiA guys will deposit their couple of 'sense'.

Good Luck!

teslatheremin

I'll give that a try, and report back!

Thanks!

PAiA has provided a youtube video demonstrating a "quick tune method" for the theremax.

http://youtube.com/watch?v=ay6XCYs4_3c

OK, no amp I can borrow, so I'll buy one (I'll need it for the Theremin anyway when it's fixed anyway).

This one (http://www.radioshack.com/product/index.jsp?
productId=2891927) is just $30, but the closest store that has one is 20 miles away. I'll get it sunday (travelling for father's day).

I checked out the you tube video. It's the same procedure in the book, which I've tried. It's nice to have that as another reference though - thanks!

Equinox

I bought the amp, and tried it. No sound at all. I've got the short between R83 and D1 in place, the treble and bass on the amp set midrange, the amp volume up to nearly half way, and the theremin volume I tried at around 1/3 and at 45%. Other settings are as recommended. I tried turning in L2 and L3 their full range in turn. No sound.

Equinox

Did you verify the oscillators with a frequency counter and an oscilloscope if they really oscillate at the correct frequencies? Which input signals can be found at the mixer?

I have'nt done so. I'd love to check that, but I don't have a frequency counter nor an o'scope. I'd have to also be instructed as to where to hook up the leads to get those signals.

Equinox

If those oscillators aren't oscillating, then you're at square zero. Go over every bit of work with a magnifying glass. If you can, have someone check your work....there may be something you have missed.

Check that transistors are in the right spots, as are their leads, correct resistors, cold solder joints, etc.

You can miss something time and time again. If you have a shortwave radio you might be able to check the oscillators that way. That is how I started. I eventually got an old oscilloscope that did WONDERS in helping me troubleshoot this stuff.

I would not worry about any mods until I got the basic theremax working first. THEN, when that is achieved, do the mods.

Good luck and keep all of us posted.

Philip

Hey, with no o'scope and little experience, I wonder if someone isn't too far whom could test it with their o'scope. I'm about to go on a vacation, driving from mid-michigan, through Indiana, to Atlanta, to Orlando, then back up I-75 to Detroit. Is anyone along that route who would like to take a look at it?

I have carefully looked over it for any shorts/open circuits, etc, and couldn't find anything.

Equinox