Etherwave component help

This isn't so much construction as resurrection....

The story began with the theremin tumbling from its stand... I thought little of it, as the chip in the case was the only visible damage.

Next time I plugged in, the tuning was off, so I whipped the case off, adjusted, (worked nicely) then re-fitted the case. With the case fitted, the pitch was off, so again; off, adjust, replace. After repeating this proceedure a few times, I noticed the setting screws in the variable inductors were feeling a little "scratchy" .... Before they refused to turn entirely, and crumbled.

Now, I assume the variance in pitch with the case off is attributable to the partial demise of the inductors, (or perhaps proximity of components after the fall?) and that having replaced them, the beast will live again.... Snag is, I've no idea of the value of variable inductor that I need. (the script on the casing of them only gives a part number)

Any ideas?

Thanks.

John.

It is absolutely normal that the tuning of the Etherwave changes when you put the cover back. When you look at the bottom side of the cover, you will find that it is kept together by 9 big screws whose capacitance naturally influences tuning. That's why you should tune your Etherwave without cover with the pitch knob in center position for zero-beat at about 4" or 10cm from the antenna. The additional capacitance of the screws will then stretch the pitch field to its normal size when you put the cover back. That is behavior by design, not a damage.

Now you have killed your variable inductors for nothing... I wonder how the ferrite cores got crumbled. Didn't you use a special RF alignment tool which is made from plastic or ceramic in order to avoid such damage? Normally there is one included with each Etherwave.

The only way to get your theremin back working is to replace the variable inductors and to retune it from A to Z. For this, you will need a freqeuncy counter. Or you send your circuit board in and get it back repaired and adjusted. There is a small theremin repair shop in Colmar, France. You may contact the theremin luthier by email: theremingenieur(at)gmail(dot)com

Sorted. Mine is one of the late type 'boards, so I can use the Coilcraft inductors. Nice that they'll sell small quantities to private customers too... I'll be ordering them today.

My Etherwave was ex-demonstration when I got it, and I have no recollection of a special tool... So presumably I used a conventional hex key to adjust (It was a few years ago that I did this- It's been sat in it's case ever since - so I honestly can't remember) ... I'll get myself a tool this time! 

I presume I'll be able to get software counter/scope easily enough....

Cheers for the reply.

If R2 and R6 are 2.2kOhm (red-red-red-gold), you'll have to put the older TOKO inductors.

If R2 and R6 are 1.5kOhm (brown-green-red-gold), you'll have to use the coilcraft inductors.

The choice of the inductors is not depending on the board revision but on the biasing of the oscillators. Changing from one coil type to another will need replacing of about 12 resistors in the whole theremin. There have been 211C and 211D boards with both coil variants on each.

A cheap software scope which goes through the computer's sound card will not work up to 300kHz. What you will need is a high precision frequency counter for the initial setting of the coils.

Sorted. The Coilcraft inductors worked fine (and I bought some spares while I was at it!) 

A few minutes of trial and error sorted the tuning. To make life easy, I drilled the top of the case (I have rubber bungs to cover the holes, so it'll still look OK!) to allow tuning with the case in place.

Replaced the chipped front part of the case with a nice bit of spalted maple while it was apart.

Got hold of a couple of broken guitar practice amps too, so I'll be dragging one of those back to life next, then making a suitably classy/crazy speaker... Then I'll just need to get my head round playing!

Thierry,

Besides R2, R6, R16 and R15, R19, are there any other resistors that need to be changed when using the Coilcraft inductors?

I'd really like to build an Etherwave and publish the updated schematics (if permitted by copyright law, that is), as well as my PCB and notes.

I would suggest not posting the whole schematic, but perhaps you could do a write-up on the modifications and post something for our "Mods" section?

Welcome Airj,

You seem to have good skills, the theremin will test them as she often develops attitude.

Christopher

"Thierry,
Besides R2, R6, R16 and R15, R19, are there any other resistors that need to be changed when using the Coilcraft inductors?
I'd really like to build an Etherwave and publish the updated schematics (if permitted by copyright law, that is), as well as my PCB and notes."

No, that's basically all.

Now it depends from which schematic you start... There have been lots of small circuit improvements since the original "EM Theremin". The schematic in the Hotrodding Manual is incomplete too.

The most important change are the linearization coils: Only 3 x 10mH on the pitch side instead of 4. Only 2 x 2.5mH and 1 x 5mH on the volume side instead of 2 + 2.

What has been added since board revision C is an additional buffer stage for the
-12V supply bus, it's now buffered and decoupled via a RC filter and an emitter follower stage, before it goes to the front panel potentiometers (pitch, volume and waveform). You'll need a 2.2kOhm resistor, a 100uF electrolytic capacitor, a 2N3906 pnp transistor and a 47n ceramic capacitor for that.

Thierry, what's the effect of the additional buffer stage?  Is it a for a cleaner power signal?

I'm still playing an original BB Etherwave and an early Signature Series EW.  I'm wondering if the later models perform even slightly better?