Actual information on Claravox

Posted: 9/4/2021 9:52:45 AM

From: germany, kiel

Joined: 5/10/2007

Volume processing analog (here the "traditional" mode): the working frequencies (osc, antenna, filter) are important and have to be well-matched. A factory-set can be done with an isolated/coated antenna. The Claravox antenna with its coating and the inner electrical contact of the antenna port build a capacitor. By sanding of the antenna you just have short cut that cap, presumably resulting in a different working frequency. If the connection has not been the problem, you now may have a further one, at least in traditional mode.
Volume processing digital (here the "modern" mode): it has been my understanding, that there just has to be a variable oscillator whose frequency will serve the electronic brain. A set-up routine (volume-range calibration) should do even with the oscillators frequency being off from the factory-setting, or? But i see in the manual that the volume osc works with 510kHz, ways too low for digitizing i guess..
All in all the Claravox with these both modes seems to be a confusing instrument. One e.g. wants the "traditional" sounds, but cannot play them with the adjustable volume response. And where is the Vox of Claras instrument? I seem to love one trick ponys! For sure Moog will solve problems and i wish all CVox owners working devices in the near future!

Posted: 9/4/2021 12:34:40 PM

From: Northern NJ, USA

Joined: 2/17/2012

DOMINIK, we think alike! :-)

The working theory is as you describe, the tuning is done at the factory with a "stock" antenna so as not to scratch up the one the customer gets, but connectivity can widely vary due to the insulative coating on the brass and brass plated parts, forming capacitances, resistances, etc. that kill oscillation if a similar electrical situation isn't developed every time.  The double resonant type of parallel / series (LC tank and EQ coil) oscillator generally employed by Moog and others can stall if not properly tuned, sometimes with damaging results (depending on the topology and components).  There is a series capacitor on the volume oscillator board that is likely there to maintain oscillation when the loop is touched by the hand, and I believe this is coming into play in ContraDude's latest video.

The pitch antenna could have similar connectivity issues, so sanding or filing the end of that tube might be warranted as well.  The contact point is on the other side of the EQ coil, which might help / hinder the situation (low impedance vs high impedance).  The pitch arm brass interlock seems more likely to establish electrical contact than the volume antenna studs, which relies on spring tension to make contact.

Not to slag on the Claravox, but that plastic guide for the volume antenna doesn't look like it would survive a fall to the floor.  The pitch side interlocks with the brass plate, so it's probably sturdier.  It looks like a black zip tie is integral to the design, holding the contact spring thing in place?  I'd beef it up, and maybe try to employ a banana plug going into the end of the antenna rod for electrical connectivity (Roger did this on his P3).

Say what you will about the Etherwave plumbing fixture antennas, at least they made good contact.

Posted: 9/4/2021 1:36:37 PM

Joined: 12/27/2013

"Oh, unlike my EPro, I do get a chirp if I touch the volume loop.  Just need to break a bad habit, I guess." — johnthom

That's good to know. In that case, I will cover the volume antenna with a sleeve or heat-shrink tubing as soon as the instrument is back from RMA.

"Waiting on the Windows version of the editor to be release so I can tweak the timbre presents, the ones shipped are mostly worthless to me." — johnthom

In the meantime it's possible to run the Claravox editor inside a macOS virtual machine, with USB passed through.

Getting a macOS VM is relatively painless nowadays, without resorting to piracy. The installation files are freely available directly from Apple.  They do require some modifications before they work in a VM but there are scripts that automate the installation process. This one worked well for me (on Linux):

Posted: 9/4/2021 3:08:02 PM

From: Portland, Oregon

Joined: 2/22/2018

The volume-loop chirp is kinda cool when you use a whole lot of delay :-)

Posted: 9/4/2021 4:25:20 PM

From: Scotland

Joined: 9/27/2012

'The working theory is as you describe, the tuning is done at the factory with a "stock" antenna so as not to scratch up the one the customer gets, but connectivity can widely vary due to the insulative coating on the brass and brass plated parts, forming capacitances, resistances, etc. that kill oscillation if a similar electrical situation isn't developed every time.' - DOMINIK via dewster

This is exactly what I was thinking too.

Given the problem seems to be mechanical rather than within the guts of the electronics, the fix should presumably be relatively easy if the hardware allows. Maybe...
One would think that MOOG would be aware of this problem, if not through their own research then from looking at this site.
If not, would it be an idea to inform them?

Posted: 9/4/2021 5:30:28 PM

From: Northern NJ, USA

Joined: 2/17/2012

Claravox Repair - Take 2

Jeff (ContraDude) came back by again today with his ailing serial #00218 Claravox.  As seen in his video posted up-thread, he was able to get the volume side to work by slightly inserting the volume loop while holding it with his hand, and this was repeatable both at his place and mine.  In this configuration, I measured 541.5kHz at the volume loop.  A DMM showed good conductivity now at the end face, as well at the mounting studs, as Jeff had previously worked to remove the insulative clear coat at these locations (red arrows below).  Full insertion of the now well connected volume loop resulted in no sound, and 538.7kHz:

With the loop still installed correctly, I inserted the small white screwdriver type end of the plastic hex tuning tool (provided with the Claravox by Moog) and tried small adjustments in both directions.  Turning the coil slug clockwise increased the frequency, CCW decreased it, but these small adjustments didn't lead to the unit producing any sound.

I got out my old scope and hooked my DIY high voltage probe to it.  Turning the coil slug CCW increased the antenna voltage, and I just kept turning it, maybe 2 full turns or more (!) until the voltage peaked at 60Vpp around 500kHz, and lo and behold we got sound!  I switched the scope probe to a simple wire in the general vicinity of the volume loop and played with the "Volume Antenna" knob on the front panel.  Fully CCW gave 510kHz, straight up 501.3kHz, and fully CW 492.5kHz, so this is a fairly linear oscillator frequency control.  Adjusting the slug for maximum voltage with this knob set straight up seemed to give the best volume response knob adjustment range.  Note that the volume out of the unit is directly proportional to the antenna voltage swing so: 1) if it's too low then you won't hear anything, and 2) the volume itself can be used as a tuning aid (see below).

We played around with it for another half an hour or so, trying the various modes, and exercising the antenna mount several times, and the Claravox seemed solid, but I guess time will tell.

It was actually Jeff's idea yesterday that Moog was probably using a different "stock" antenna at the factory to do the tuning.  If the fix holds then insulating varnish was almost certainly the root problem here, with accompanying miles off factory tune job.

While the Claravox was here I took another look at the mounting bracket thing:

It looks like metal from this side, but it's actually all plastic (except for the embedded mounting nut and screw threads).

I also took a DMM to the pitch arm:

There was good conductivity around the end (green arrow) and the shoulder screws (pink arrows).  The shoulder screws were loose by the way, probably to fit, but they could use some Locktite.  If your pitch arm is droopy (making you unpopular with the ladies) you might check these for tightness.

[EDIT] So a proposed volume loop "tuning" procedure sans equipment:
0. Ensure volume loop connectivity (sand the end face and locking studs if necessary).
1. Put the Claravox in "Traditional" mode.
2. Set the "Volume Antenna" knob to 12 noon (straight up).
3. Adjust the volume coil slug for maximum volume with your hand away from the volume loop (kinda hard to do while adjusting the slug!).
4. Wiggle the "Volume Antenna" knob to ensure maximum volume is around 12 noon, if not repeat step 3 & 4 until it is.

Posted: 9/4/2021 5:49:30 PM

From: Portland, Oregon

Joined: 2/22/2018

I see a future for Dewster in repairing Claravox theremins...

Posted: 9/4/2021 6:41:22 PM

From: germany, kiel

Joined: 5/10/2007

Congrats! Why did it play in the beginning?

Posted: 9/4/2021 7:04:34 PM

From: Northern NJ, USA

Joined: 2/17/2012

"Why did it play in the beginning?"  - DOMINIK

There was sufficient capacitance between the antenna and the connector to sustain good voltage swing?

Posted: 9/4/2021 7:40:19 PM

From: Basking Ridge, New Jersey, USA

Joined: 12/12/2020

A BIG THANKS to dewster (Eric) for his expertise! Here’s a video I just made that explains things in a significantly less detailed manner than Eric’s. (No, I’m not nice to Moog but that’s their problem 🤪)

I’m just holding my breath that this is the final solution! Eric has been FABULOUS! Without his assistance, this Claravox would have been sent back to Moog. He was a master at running every conceivable test to find a solution. I wonder how many others are (or have had) the same problem? So far, there appear to be at least a dozen of us. Ugh!

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