Etherwave tuning issues...

I've been working on a custom cabinet build (photos soon) using an old Big Briar Etherwave, and wow... is it giving me tuning troubles!  Note, this theremin does not yet have an ESPE01 module. 

I have 2 main problems so far:

First, it's almost like I can't tune L6 quite far enough counterclockwise.  I can get a pitch range of about 30cm, but as I move my hand back further, the pitch begins to rise again.  I adjust L5 counterclockwise, and start again, per the instructions, but eventually I run out of room to turn L6 further.

Second, the pitch knob doesn't seem to have any effect on the circuit at all... except if I turn it to the far right, the pitch either jumps up to hear hyper-sonic frequencies or it just disappears altogether.

I suspect some bad solder joints and perhaps a crossed wire... but I thought I'd toss out these issues to the crowd to see if you've had a similar problem and know where I should start looking.

Jason, please look first at your circuit board: Is it a newer version where the transistor Q9 exists or an older version without Q9?

Then check the voltages at the pins of the pitch pot against ground (green wire):
- black wire -11.3V(with Q9) or -12V(without Q9)
- center pin of the pot must vary between the voltage of the black wire and 0V as you turn it from one end to another

Jason, you mentioned in a previous post that you were planning to make an aluminum cabinet - or wood with aluminum facing. Is this by any chance the project you were referring to? If so, could the custom cabinet be causing the problem?

Jason said: I've been working on a custom cabinet build (photos soon) using an old Big Briar EtherWave

My question is which carries the most weight, the Big Briar needed a new cabinet or you have a nice cabinet design you want to add the pro workings of a theremin?

Christopher

Photo album uploaded here.  I'll check on the board revision this evening when I get home.  Cheers!

http://www.thereminworld.com/photoalbums/view/14793#14794

Peter - I was originally going to cut out some holes on the front of the case to fit in an aluminum panel.  But then I decided to go with smoked black plexiglass (which turned out to be much easier). 

The case itself is mostly wood, though its density definitely seems to have an effect.  I've been trying to tune it with the case fully open, but the tuning changes pretty dramatically when I close it.  In one of the photos you can see it partially open, which I think would be my preferred playing position; so I could adjust the knobs while performing.

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I hate to be the bearer of bad news but that metal ring around the rim on the lid of the case is not your friend!

The case and dimensions are very nice but the upper metal ring should be removed. Oh....IMHO

Christopher

... and the carrying handle? Is that where the pitch antenna is at? I know it's made of black plastic but the anchors are metal...

If this is a parallel tank with series EQ (?) I'd say ignore the fixed oscillator initially and adjust the VFO section for maximum voltage at the antenna (oscilloscope probe held nearby) - that should get you in the ballpark.

Got it, I think. Jason, your beautiful case has already more than enough static capacitance thanks to the metal parts. Remove the aluminum foil on the inner right side and things will be much better.

Oooh La!

Jason, does the lid remove, or is it permanently there.. ?

I agree with Thierry that you have loads of extra capacitance and reduction of this would be moving in the right direction..

But what also worries me is the effective coupling between the volume and pitch hands via the lid loop.. Any movement of your volume hand towards the lid, will effectively be coupled to the pitch antenna as if your hand was the combined distance of the hand to the lid and the lid to the pitch antenna.. So if these distances were say 3" each, you have effectively put your pitch antenna 6" from the volume antenna.. And I actually think its probably worse than that.

My strong advice would be that, if possible, you should remove the metal trim and replace this with plastic or rubber. I think with a bit of fiddling, you could manage to tune your theremin - But I also think that you wont get good performance from it while the metal loops are there - no matter what you do at a tuning / electronics level.

Sorry.. I hope I am wrong, but I dont think I am! .. It may be a lovely traveling theremin - but unless the loop / coupling issue is sorted out, I dont believe it will be a theremin worth taking anywhere!

Fred.

I just noticed " hate to be the bearer of bad news but that metal ring around the rim on the lid of the case is not your friend! The case and dimensions are very nice but the upper metal ring should be removed. Oh....IMHO " - Christopher

I think Christopher is spot on here.. And think that until this is done, well, you have almost no hope of it behaving as it should - for many reasons.