Some time it’s the time to make a decision. The rocker switches in the drawer seem to be self-made by M. Martenot and have degraded seriously over the years. Poor contact, some are sluggish, others have too much slackness. For the use of the instrument it seems much more important that the switches operate correctly without causing additional audio noise due to poor contacts than having genuine switches under the hood.

Here’s what I’m planning to use:

The Marquardt rocker switches will be mounted in an additional sheet of metal which will also keep the axles for the original transparent levers in place.

Here are some facts about the drawer – the control panel – of a Mk V Ondes Martenot.
Two swichtes (1, 2) are missing and the touche d’expression is absent as well. No fear, they are kept warm and cosy.
The switches are built from Pertinax (laminated paper) which has suffered over the decades, so that I will drop genuinity in favour of reliability and combine the old nice levers with 2013s switches.

The controls in detail, from right to left:

– switch C/R (rightmost) : selects between clavier (keyboard) and ruban (ribbon)
– three metal knobs : that’s for the circuit benders amongst the ondistes – here your finger becomes part of a series connection with a small capacitor,
directly influencing the oscillator.
– two transparent push buttons left from the C/R switch : transposes down by some amount (only in ribbon mode)
– the big rectangular hole normally hold the touche d’expession, the main volume control button of the Ondes
– switch 1 : adds a LC low pass into the D1 output (red – in, white – bypass)
– switches 2, 3 : some kind of tone control applied to the output amplifier (red – filter, white – off)
– switch 4 : determines the amount of 2&3 (red – full, white – attenuated)
– switch 5 : controls the gain of the pre-amplifier tube – probably into limitation, so a sine will become somewhat rectangular
– switch 6 : adds a filter to the metallic speaker (D2)
– switches D1, D2 & D3 : select the speaker(s) used. D1 is the main speaker, D2 the metallic (gong) speaker, and D3 the palme (with the strings)
– switch 8: adds a bridge rectifier into the signal for speaker D1, thereby doubling the frequency

The wheel numbered I..V : volume control for the main speaker (D1) – to adjust the balance between D1 and D2/D3

Upon serveral requests I’ll show you the input devices of the Ondes Martenot today.
Except from the very early models that only had the ribbon controller, the instrument has two controls selectable by a switch.
One of them is a classical 6 octave keyboard (clavier) with rather small keys, the other is the ribbon controller (ruban) which the Ondes Martenot is famous for.

Le ruban

The function principle of the ribbon is that of a variable capacitor which makes up a variable oscillator with the addition of an inductor. By moving the ribbon by means of an attached finger ring along the keyboard, a metallized segment moves between several metal plates, one pair for each octave. With this construction the inventor has built six linear variable capacitors which are tied together via weighted capacitors, which in turn are connected to the LC tank circuit of the variable oscillator. The following image shows the circuit of the ribbon controller:

The ribbon is contacted by a brass roller and connected to ground by means of a LC tank circuit with adjustable capacitor and inductor. The analysis of the meaning of the circuitry will be part of the oscillator circuit description which I will post as soon as the restoration progress has reached this stage.

To compensate for the mismatching relationship between capacitance and the frequency slope within an octave, the capacitor plates are bent in a way that the ribbon position matches the note frequency of the key at the same position. Bending the plates with a pair of pliers is actually the only method the ribbon controller can be tuned! The next image shows a close up of the bent plates and the metallized ribbon passing in between:

Le clavier

When switched to keyboard mode, a ladder of fixed inductors becomes the frequency determining part of the oscillator. Depending on the key depressed, one or another node of the inductor ladder is grounded, resulting in a varying inductance influencing the oscillator’s frequency. Here’s the circuit of the first octave’s inductor ladder which is repeated in series connection for all six octaves:

The Ondes Martenot has a low-note priority, which means that whenever more than one key is pressed, the lowest note wins. Due to the circuit principle, there’s a positive proportionality between inductance and output frequency. While the oscillator frequency increases with decreasing inductance, the resulting beat frequency decreases – and vice versa. To achive this, the variable oscillators frequency must always be below the frequency of the fixed oscillator. With this configuration, the audible frequency (the difference between fixed and variable oscillator!) gets the lower the higher  the variable oscillator …uhm… oscillates.

As you can see above, the inductor ladder is separated by some additional variable inductors. These are mounted to the keyboard, which is mounted floating with respect to the frame, and feature ferrite cores which are attached to the frame by wires. This allows for a vibrato effect by wiggling the pressed key and thereby the keyboard in its frame. The next photo shows one of those inductors:

The last photo shows the inductors for one whole octave:

In the light of a recent comment, I’d like to write something about one of the major controls of an Ondes Martenot: the touche d’expression.

As the instrument produces a continuous tone of variable pitch, there’s an urgent need for some kind of volume control.
Almost like the common volume potentiometer in any audio amplifier, the touche d’expression controls the output level by limiting the cathode current of the output tube (EL84) depending on the pressure applied to the white button (see the photo below). Instead of a linear or circular carbon film in a common potentiometer, the Ondes Martenot uses a small leather bag filled with carbon powder, similar to the microphones used for telephones prior to the electronically amplified microphones that came up in the 1980s.
For some thoughts about the contens of the bag, see this discussion on radiomuseum.org (in german language)

This is a photo of the touche d’expression from a 1961 Ondes Martenot still needing to be refurbished. Using the pretension thread, some pressure is applied to the carbon powder just enough to keep the output silent. The 470k resistor tied to plate voltage lifts the cathode potential of the EL84 up some volts to allow for complete silence with the pretension adjusted properly and optimum response to the pressure applied by the player.

A second unit of this kind is shown on the right side of the schematics above. This way some low pass filtering is implemented, controllable with a knee pedal under the instrument.

As this is a techie blog, I’ll start with some impressions of the electronics as I found them when I disassembled the Ondes Martenot.
I assume some knowledge about the function of this instrument. If you’re new to the Ondes Martenot, you’d probably like to have a look at Thomas Blochs website and Wikipedia

This is the main electronics unit from below.
Almost all film capacitors (most of them of the paper type, sealed with tar in a glass tube) and resistors need to be replaced either of degradation in value, short or open circuit.

On the left side, within two separately shielded boxes, are the two LC oscillators, each featuring an EF94 (6AU6) pentode tube. As you may already now, the Ondes Martenot is based on beat-frequency oscillators, like the Thermin or the Spärophon. The fixed (more or less at least – it is tunable to achive tuning of the instrument) oscillator is told to oscillate at 200kHz – I still have to verify this, but for now, let’s assume it is correct. An additional transpose switch also works on the fixed oscillator.
The second one is set up to be variable between 180 and 200kHz by means of either a keyboard or the ribbon controller with the finger ring typical to the Ondes Martenot. While the ribbon controller works on a gigantic variable capacitor (the conductive ribbon passes an array of capacitor plates, photos will follow), the keyboard switches a string of inductors. This gives the higher note priority known from many other monophonic synthesizers. In addition to the keyboard or ribbon control, two push buttons with switchable capacitors and 3 metal knobs also connected to the LC tank circuit via low value capacitors allow for further influence on the tone. Switch No. 7 adds some lose coupling from the variable oscillator output to the grid circuit, its function will be further analyzed when the mixer circuit is operational as it also changes the mixer configuration.

The third EF94 tube is used as a mixer, bringing back the outputs of the two oscillators into the audible range between 0 and 20kHz.  The mixer is followed by the fourth EF94 as a pre-amplifier. Its screen grid is connected to some circuitry in the drawer (see below) to allow for various sound modification.

Final amplification is accomplisged by an EL84 output pentode. Volume control, which is major important for the Ondes Martenot because of its free wheeling oscillators, is achieved with the “touche d’expression”, a large button on the drawer with compresses a little bag with conductive powder. Working like a carbon powder microphone, it emulates a potentiometer in the EL84’s cathode circuit. The output transformer is somewhat special because of its three secondary windings for three different speakers which could be operated simultaneously on the Ondes Martenot. Two filter functions are realized within the output amplifier: Switch No. 0 allows to connect a choke in series with the output transfomer’s primary winding, and a capacitor in series with a potentiometer connected in parallel with the primary allows for tone control. This potentiometer is also of the carbon bag type, operated by a knee pedal.

This photo shows the inside of the drawer. This small panel contains almost all important controls of the Ondes Martenot.
On the right side you can see the switch toggling between keyboard and ribbon control (upper right corner) and the push buttons and metal knobs influencing the variable oscillator mentioned above.
The big rotary switch in the lower center allows for logarithmic control of the output level for the main speaker (D1, “diffuseur principal”) to match its volume with other speakers. Nine toggle switches allow for various sound modifications, while the switch in the upper left corner connects a bridge rectifier (the blue disky thing on the left) in series with the output.

One can assume that all those modifiers enable interesting effects on the originally generated sine wave. The next step towards the synthesizer would probably be a sawtooth oscillator like the Trautonium used.