Restoring the Hohner Pianet T - Part 1

How It Started

Some friends of mine introduced me to this Hohner Pianet T, which had been stored in the corner of a barn for many years. For those unfamiliar with the instrument, it’s one of the first portable electromechanical keyboards, first produced in the 1970s, and was revolutionary at the time.

Unlike the later electronic synthesisers which employed oscillators to produce notes, this instrument uses tuned metal reeds. Each key is connected to a small rubber pad, which ‘lifts’ the reed to set it vibrating, and each reed has its own individual pickup coil, which provides the signal. When not connected to an amplifier the keys therefore still make ‘plink-plonk’ noises.

Only the fact that it is fitted with a handle qualifies it as being described as ‘portable’. It’s a solid lump of wood and metal which, by today’s standards, would be designated as requiring two-man-handling.

On the outside it seemed well preserved. The chrome-plated latches and fixing for the handle had the usual surface rust which can often be removed, and all the keys worked and made a noise. They were all a bit clunky, probably as a result of the felts being worn, but the sounds they made were all of varying loudness and it was clearly out of tune. I had never seen one of these before, so was very interested in taking it on as a project.

The History

The Pianet was designed by Ernst Zacharias, and the first models were produced in the 1960s by Hohner, in West Germany. They were available in several different versions, some with legs and amplifiers and the design of the electromechanical system changed over time.

This ‘T’ version was produced between 1977 and 1983, and was the black vinyl-covered variant intended for stage use, as opposed to earlier ‘N’ models intended for home use which had a Natural wood finish. Here is a good article on the complete range of Pianet instruments if anyone is interested.

Part 1

Evaluation

Marc Wolf has a sound sample on his blog if you want to hear how it’s supposed to sound. At the moment, this one sounds like someone pushing a xylophone off the edge of a cliff.

Let’s have a look at the instrument, see how it works, and get an idea of what the restoration job looks like.

Each key, which is a rocker, lifts a soft rubber pad in contact with a reed. The reed sticks to the pad due to an effect known as polymer attraction, and eventually the bond breaks leaving the reed to vibrate until the key is released.

Each reed has its own electromagnetic pickup – the coil wound around the plastic former. The post on which the coil sits can be bent back and forward to adjust the amplitude of each key. In this way the entire instrument can be levelled.

The pickup coils are all in series, it is in fact one unbroken length of wire, and terminate at an impedance matching transformer. They’re grounded at one end, and the metal chassis forms the return path. Presumably this is needed to reduce hum, otherwise it would have made sense to simply return both ends to the transformer.

The rubber pads are a simple push fit on the arms. Their surface was originally shiny, but has become matted due to long term contact with the corroding reeds. A film of high-viscosity silicone oil on the pad and the mating surface of the reed enables the polymer attraction mechanism to work. This is covered by a 1960s patent, and is discussed in more detail in the video.

With the pad removed it is easier to see the extent of the corrosion. This is rust, so the reeds are steel, and the corrosion on the key arms is typical of that found on aluminium, which would make sense to reduce the mass of the keys.

Once removed, the rust on the reeds as apparent. This is the underside, where you can see how the reed is coarse-tuned by filing. The reeds have a film of what appears to be blue lacquer, which we will need to replace in some form after cleaning.

Each reed has a brass block at the end to create the correct mass. This also has marks where it has been filed for tuning. The lacquer on the underside of the reed is mostly intact.

Another view of the underside of the reed reveals the very fine filing used to fine-tune it. This, and the other coarsely-filed section have not rusted at all, whereas many areas ‘protected’ by the lacquer have.

On the top surface of the reed there is an obvious mark where the pad rests, and the remainder of the surface has lost its lacquer and corroded. This is somewhat puzzling. As we have seen, the filed bare metal sections have not rusted.

There are several major tasks involved in the restoration:

Cleaning up and refurbishing the reeds and pads
Replacing all the keyboard felt pads – which involves disassembling it completely
Tuning the entire instrument to standard pitch
Adjusting the amplitude of the signal from each pickup to level the volume across the whole range

The next step will be to develop a way of cleaning and restoring one reed and pad assembly, which can then be rolled-out across the entire 61 keys.
As a first step we’re going to find a way to remove the surface rust, hopefully dealing with any pitting at the same time, and chemically recreate a new coating that will preserve the future integrity of the reed while ensuring the ‘plucking’ action of the pad still operates properly.

To do this will affect the tuning, however the entire instrument is completely out of tune anyway.

More details will be found in the video (below).

Part 1 – Video

Restoring the Hohner Pianet ‘T’ – Part 1 – Evaluation

Transcript

Here we have a Hohner “Pianet T”

The chrome furniture is a bit rusty, but it looks in reasonable condition.

It’s been stored in a barn for years, but the keyboard looks fine.

These are electromechanical, with a pickup, so it should make sound when played.

It’s clunky and out of tune, but we’d expect that.

The cover is held on with three screws which have cup-washers we don’t want to lose.

The other two came off with the screws.

Let’s lift off the top and see what it’s like.

The mechanism consists of 61 keys which are arranged on rockers.

Each key has its own pickup, the coils of wire, which are all connected in series and end at an impedance-matching transformer.

Each key ‘plucks’ a reed or tine whose vibration is picked up by the coil.

The pickups look fine. Nothing is broken or missing.

The reeds – at the back – have surface corrosion, and the key mechanisms have some too, but that will clean up fairly easily.

how does it work?

The rubber pad ‘lifts’ the reed until it unsticks.

This mechanism is described in a 1961 patent.

The patent refers to the original design which used semi-absorbent leather pads. The 1973 ‘T’ is a next-generation design, however the principle is the same.

If we remove the pad, you’ll see that the surface is shiny, except where it was resting on surface of the reed.

The reed is secured to a brass post by a screw.

It has some type of blue lacquer coating which has succumbed to surface rust.

The reed has been coarse-tuned by filing the edge, and then fine-tuned by removing material on the flat. A brass weight has been added, and that has been filed too.

The underside of the reed is better than the top.

You can tell by the angles of the marks that these have been tuned before being installed, and this has been done with the reed ‘upside-down’ so that the filing marks are not visible when the reed is fitted to the instrument.

The surface cleans up well with a fingernail, but the pitting will be an issue.

It will likely affect the tonal quality of the reed.

And we must create a smooth area for the rubber pad to rest on, otherwise the polymer-attraction mechanism won’t work.

Now I’ve put it back together, that key does not work because what little polymer was on it has been wiped off.

So, Where Are We?

Well, we’re back in my office making a plan.

Contrary to popular belief, I’m not as stupid as I look, and I never would have taken that key to pieces if I wasn’t already sure I could get replacement oil.

The poly-dimethyl-siloxane with a viscosity of fifty-thousand to one-million centiStokes in the documentation was quoted for the original 1960s instrument which employed semi-absorbent pads, but the principle remain the same. We need enough viscosity to create a ‘pluck’, so I’m going to start with a thick PDMS oil and see what happens.

Viscous PDMS oil appears to be hard to get. It’s used in some automotive oils, and also as a food product, where it serves as a release agent for baking tins, however these applications are all relatively low viscosity – about 200 to 300 centiStokes – a couple of orders of magnitude away from where we want to be.

After a bit of digging around, I discovered that radio-control enthusiasts use high viscosity 100% pure silicone oil, which I believe to be PDMS, in the differentials of model racing cars.

I’ve ordered some half-million and one-million centistoke oil, which will be more like a thin paste than an oil, and we’ll see how it performs.

Various people have said that oil worked better than paste for them, mainly because the paste tended to transfer itself to the reed over time and detune it.

I suspect that we need only the tiniest amount, after all, there was no visible trace of anything on that pad or reed when I took it out, and it worked.

The surface, well that’s another issue. Some say the surface rust is unimportant so long as the landing patch for the pad is smooth. I’m not convinced. I suspect it may well affect the resonance of the reed, and I’m just not happy leaving it that way.

I suspect the patchy blue lacquer won’t help either.

I’m going to try removing the surface rust and lacquer, treat the reed with Phosphoric Acid to convert the remaining Iron Oxide in any tiny pits to Iron Phosphate, and then use a chemical bluing method to create a resistant layer, which should help prevent future corrosion.

Next time at the workshop we’ll get the instrument connected to a frequency counter and oscilloscope and see what the characteristics of the reed are like before and after.

In the meantime… I await the postman.