Hohner Clavinet D6

(formerly property of German entertainer Heinz Schenk)

The Hohner Clavinet D6 I own was the original property of the famous German entertainer Heinz Schenk (decreased in 2014). The pictures show it at Heinz Schenk's home in Wiesbaden-Naurod and were taken in August 2016 after the auction of his inventory, which was donated to the ‘Heinz Schenk Stiftung’, took place. Note the picture of him in the background!

Sadly, the top-cover was not findable. Later I noticed that a fastener for the cover at the front had been removed, too. Heinz Schenk probably did this because the Clavinet had been played in this fixed location and the fastener troubled him while sitting in front of the instrument.

The Hohner Clavinet D6 is essentially sort of electric-guitar, played by key’s like a piano. Unlike the piano, plungers press strings to the instrument’s metal chassis when hitting a key. The string now resonates between the pressed plunger (on the left) and the fixing on the right as long as the key remains depressed. This means, the length of the string for a given pitch is only in effect as long as the key is depressed which marks the uniqueness of the instrument. After some time, which can be shortened by a damping lever, the resonance does decay. There is no special decay control. When the key is released as long as the string resonates, the rubber of the plunger immediately dampens the string. When the key is fully released,  the string is fully dampened by the yarn. Therefore rubber and yarn have a big influence on the correct function of the instrument.

Of course, the instrument was not in playable condition because of ‘sticky hammer syndrome’. All rubber-plungers had become chewing-gum consistence and had to be replaced. After cleaning, a gymnastics rubber band with appropriate firmness was cut to suitable pieces and press fitted to the stubs.

The consistency of the rubber plungers is very important; if they’re to hard, the sound intensity is strong, but a residual knocking sound at release of the keys appears. If they’re too soft, the sound intensity is much too low – but the residual knocking sound is barely noticeable.  

Three strings had cracked directly at the right bracing element. A new lug was formed using an electric screwdriver. The lug was secured by soldering.

The green wool-yarn had lost its tension. The yarn was tightened again to dampen the strings when a key is released. This is very important, because if there’s not enough damping, ghost tones, which are off key, appear when the keys are released. Therefore, the strings had to be immediately dampened at key release by the rubber-plungers and the yarn to prevent ghost-tones and knocking effects.

Another issue of the instrument is noise and hum.  The back sides of the damping-slider panel and the volume/timbre panel were coated using EMI copper varnish. The area around the audio-jack was coated, too.

To reduce noise, the old carbon-resistors, which are the main source of noise (especially the big 3,9Meg Ohm resistor) have been replaced by (low-noise) metal-film resistors. In addition, the drop-shaped tantalum capacitor (which is prone to short) as well as the electrolytic capacitor were replaced. The ceramic capacitors have been replaced by Styroflex types.

The transistors (BC550C/BC413C and BC548B/148B) were replaced by new BC550C and BC550B.

Due to the circuit as used by Hohner, some residual noise can not be removed entirely. Of course, a new circuit, using low-noise op-amps or low-noise transistors in parallel configuration, as applied in moving-coil per-amplifiers, could be added. This of course would degrade the originality of this historic instrument far too much.

All replaced parts were put into a little plastic bag which is stored inside the instruments housing.

While talking about the circuit -  I discovered a failure, which is inherent in all schematics of the Hohner Clavinet D6 manuals: The 100k Ohm resistor, connected from base to emitter (ground) at the left input transistor is originally not assembled at the printed circuit board (PCB) and has to be removed from the schematic. Probably, it was intended to be directly connected to the input transformer’s secondary (at the left side of the 10nF capacitor, not affecting the DC working point of the transistor), serving as damping resistor to flatten the transformers frequency response.

It is no good idea to add the 100k resistor to the circuit as shown in the manuals, because the DC working point of the first transistor would be changed in a way which shuts him off; no sound would be output. This was proven by simulation and by temporarily adding the resistor to the PCB while measuring the output signal (which disappeared with added resistor).

The hum was reduced by adding shielding tubing to the wires at the pick-ups. Due to the leak of mu-metal screening at the pick-ups, all magnetic field producing devices, like amplifiers, transformers or even cell-phones have to be kept away from the Clavinet as far as possible. For fixed installations, hum compensation coils may be added (not tested).

So, luckily after many years, the Clavinet of Heinz Schenk came back to life again. Now I just have to learn playing the piano!