Fixing Claravox volume antenna

I had an other look at the Aetherwellen page.
Although it is not clearly stated, it seems that the proposed modification for issues nr 9 and 14 could solve the non mute issue.
The curves show a minimum level around -87 dB.

Issue nr 9
In addition, it seems that the audio signal cannot be reduced accordingly when playing at very low volume; the cut off distance can not be adjusted properly.
...
Solution : Include volume-control improvements as already advised in the EW modification documentation according to the original designs of Leon Theremin – all is now included at the CVC-Theremin VREB-10-2021] (Volume Rockmorizer Extension-Board)...

Issue nr 14 :

Volume control circuit of the CV-C Theremin is extremely sensitive to deviations of mechanical and electrical tolerances, inserting the proposed Wave Blocker is likely to change the cut-off position of the hand near the volume-antenna. It was noticed that complete cut-off did not occur and was not adjustable at the volume-oscillator tank or the volume control-voltage amplitude defining trimmer R57, located at the volume-oscillator board.
...
To overcome all this, a dedicated piggy-back board (CV-C Theremin VREB-10-2021) was designed to minimize the additional stray capacitance, provide at least trimmers for cut-off (R59) and control-voltage amplitude (R57), hold the antenna extension-coil (L15) and the 320kHz blocking parallel-resonator (2x 470uH / 220pF).
...
The CV-C Theremin will now (probably for the first time) work correctly applying the Wave-Blocker, when volume-oscillator tank, volume control-voltage amplitude trimmer R57 and especially the cut-off trimmer R59, which has a big influence on the low-level volume response, are adjusted correctly.

I'm able to make and install this piggy back circuit but this will of course void Moog's warranty.
So It's a big and difficult decision to make !

Before I do that, let's have a look at sound levels of the Claravox.

I connect my 50 years old Hameg HM412 oscilloscope to the Main Out jack.
A cable is plugged in this output. The other end is plugged in my mixer. So the output is loaded.

Maximum output level, with my hand far from the volume loop : 2.5 V (peak to peak). This is close to standard line level.
Minimum output level, with my hand in the loop : 2.5 mV (peak to peak).

A voltage ratio of 1 000. If 2.5 V is 0 dB, 2.5 mV is -60 dB.

OK. I'm an old man, I have somme tinnitus. I do clearly hear this - 60 dB sound and it is very annoying.

I got a very long and detailed answer from Aetherwellen !
His modifications of the Claravox do fix the volume mute issue.

He also has solutions to improve the sound of the Etherwave.

Since I'm not interested in the modern mode of the Claravox and only looking for the best possible and most beautiful sound out of any theremin, I have to decide if
I start making modifications of my Claravox (voiding the warranty)
or if
I modifiy My Etherwave (not warrantied anymore) and sell my Claravox. (Anyone interested here ? )

Today I did some screwdriver and wrench work on my Claravox.
I completely removed the volume oscillator board and the plastic sockets of the antenna.
I wanted to see what was hidden behind this golden plate.
I didn't find anything unexpected.
I reassembled everything and found that inserting the antenna needed much more effort than before.
May be the spring was blocked somewhere and not free to press against the antenna rod.
I switched the Claravox on, shaked it... no more cracking noise at all !
Issue of my message nr 38 is now solved.

The issues with Claravox continue to amaze me. Many of these problems seem incredibly bush-league and cold have been avoided through some basic acceptance testing and/or very low cost changes to some of the components used. That said, I have found it to be a very enjoyable instrument to play underneath it all. I do have concerns about the long-term viability of the instrument because so much of it is dependent on software which Moog (now InMusic) may not have much incentive to maintain.

Some news.
I'm talking everyday with Aetherwellen who is of great help.
The diagnostics he suggested show that the volume oscillator board is unable to generate a control output voltage high enough to completely mute the Claravox.
The piggy-back circuit that he developed will solve the issue by changing the gain and offset of the final stage.
It's not really simple to do but I think I can do it.
Problem : there are quite a few pins to solder on the oscillator board + a track to cut. That will definitely void the warranty.
A decision to make...

My personal $0.02 is that I wouldn’t worry so much about the warranty per se because Moog’s already done all they are going to do for you. If you regret the purchase you’d be within your rights to demand a refund based on the problems. If you want to keep it (I still have mine) then I say go for it

My concern is not about getting a free fix from Moog (who does not reply anymore, by the way).
My concern is to have a free service from Moog in case of any other failure.

Ça y est !

The "no mute" issue is fixed !

Alexander (Mr Aetherwellen) probably knows the Claravox better than those who designed it.
After asking me to make several mesurements, he proposed a modification of the main board (this is not the piggy-back module).
A very simple mod, actually, but nowdays, those SMDs are so small I was really frightened to make a mistake when soldering !
I used to design and assemble a lot of electronic devices including with SMDs, but never worked on so small components.

So I exploded my Claravox on the table, made the modification, reassembled everything and, when it came back to life, the issue was gone.
Perfect muting and a smooth progressive volume curve. Beautiful !

Edit : too smooth, may be...

Cherry on the cake : absolutely no more squeak noise when touching the volume antenna.

I can only be grateful to Alexander who saved my life.
My wife will not kill me for buying an expensive and non functioning device.

And Moog is still silent...

IMPORTANT

The modification that Alex (aetherwellen-musik.de) suggested and that I implemented, must be considered as an improvement to MY Claravox, after making several specific tests and measurements.

For the moment, this improvement cannot be published here or on Aetherwellen site because it has not yet been fully tested, particularly the influence on thermal drift.

This improvement needs a complete disassembly of the instrument and manipulations of SMDs of the main-board that may (will) void the warranty.

I have performed the proposed measures and modifications by myself.
My Claravox can now reach full mute level, with the hand a few centimeters above the volume antenna.
I'm currently evaluating the musical results of the improvement and any other issues that could have been created by the modifications.

If you are concerned by the non mute issue of your Claravox, you may contact Alex at Aetherwellen but although he spent a lot of time solving the issue with me, he is not a professional of musical instruments maintenance (he has an other job) and cannot provide this improvement as a service.

If you have the skills to safely solder extremely small SMDs on circuit boards, I'm sure he will be glad to help you for your specific case with a solution that might be different from the one that worked on my Claravox.