My gradfather owned a Hohner Pianet N. Sadly it stopped working several years ago and spent the last years merely as decoration for his flat. After he died I had the choice between scrapping or saving it. I didn’t like the thought of a musical instrument beeing scrapped, so I kept it and hoped to repair it one day. This is what I did and here are some things about the process that I want to share.
First of all: I like the manuals and advertising, that came with the Pianet. It is so different from everything you see today.
Let’s start with details about the Hohner Pianet N itself.
What astonished most about this device is the fact, that the whole thing is designed for easy access to all its parts and easy replacement of parts. All you need is a screw driver, a soldering iron and the part you want to replace. Of couse this is the toughest part, because there are no original replacement parts anymore. However it feels great to know that you could repair it easily. This definitely isn’t true for any other device I dismantled so far. It is even better, that the device manual contains detailed schematics.
Let’s continue with a short description of the inner workings of the Pianet.
The way the Pianet generates sound is fundamentally different from other keyboard instruments. Harpsicords and pianos have strings as the basic sound generator. On pressing a key on a harpsichord a string is plucked, while the piano hits the string with a special kind of hammer.
So what happens when you press a key on the Pianet? Check this image first:
The key is connected to a metal rod. This rod has a sticky hammer attached to its end. This hammer is glued to the metal reed for this key with some very weak adhesive. When the key is pressed the hammer rises and pulls the metal reed with it. At some point the force of the lifting key exceeds the force of the adhesive sticking to the metal reed. The adhesive rips of and the reed starts vibrating, which creates the sound. It fades out until you can’t hear it anymore or until you release the key. After releasing the key the glue sticks to the reed again, stops its vibration and the key is ready to be pressed again. This process sounds a bit like playing the chimes.
But wait – isn’t the Pianet an electric device? Yes it is and the story continues. The metal reeds are attached to a metal bracket with one end while the other end can vibrate freely. Next to this vibrating end there is a straight metal bracket leaving a gab of approximately 0.5 mm to the reed. One of these brackets is connected to ground, the other one to 300V. Together the form a giant capacitor separated by air as the dielectric. Pressing a key lets a metal reed vibrate, which causes the capacitance to oscillate slightly. This causes a small charching/discharching of the capacitor, therefore causing a small alternating current alternating with the same frequency as the metal reed. This alternating current is first amplified in the Pianet, then fed to the Footswell, back into the Pianet and then finally to the audio output connectors on the back side of the pianet.
The Pianet also has a vibrato effect built inside. When I studied the schematics and the circuit board I was really amused how this effect was achieved: There is an oscillator circuit built into the circuit, that flashes a small light bulb a few times per second. Next to the light bulb is a photo resitor which catches the light. This resistor is part of the amplifier stage of the Pianet, so the flashing of the light bulb toggles the amplification factor between “high gain” to “low gain”. A simple, yet surprising way to solve this task.
Condition of the Pianet, when I got it.
The device is pretty old and so are all its parts. I first had a look inside the device, the amplifier and the schematics in the manual. Things I noted were:
- There are a lot of leaking capacitors in the amplifier, so it was completely useless.
- The Pianet came without a Footswell. At that time I did not know, that this was essential for the device to work.
- The capacitors on the circuit board of the Pianet seemed to be intact. However they looked very old.
- The light bulb on the circuit board was broken, so I removed it. It is used for the vibrato effect and I can go without it.
- The schematics of the Pianet showed, that it has segments working with 300V. I really had to be careful when poking around with it. This meant especially not touching anything while connected to mains.
- Almost all sticky hammers degraded over time and were useless. I expected producing my own replacements for it to be very tricky.
Then I began my work.
I googled for the sticky hammers and found a webpage dedicated to some Hohner devices: http://clavinet.com.
They sell sticky hammers made of silicone, which work better and more durable than the original ones. Additionally they offer upgrade kits containing all capacitors and two resitors neccessary for my Pianet circuit board. I was lazy that day and ordered the upgrade kit instead of getting the parts myself. Some weeks later I recieved the parts. (The delay was not caused by clavinet.com. They sent out the stuff one day after they recieved my order).
Reworking the circuit was pretty straight forward. Clavinet.com also offered some annotated photos which made the replacement very convenient. Here is a copy of the tutorial from clavinet.com (Upgrade Pianet N). When I was done soldering I connected the Pianet to mains and waited for it to explode, which luckily didn’t happen.
Now it was likely that I had a working circuit board. I tested it with the Pianet amplifier, but as I expected, it was not working due to the leaking capacitors. I needed another way to test it. Finally I decided to connect it to my stereo. The output of the Pianet is rated with 300mV max, Google told me how to connect the audio signal and ground to a standard audio jack, the schematics contained information where I had to hook into the circuit and I was ready to go. But nothing happend. I turned to the schematics again and finally I understood why it wasn’t working. I had no Footswell. Its purpose is to dampen the audio signal to add some way of volume control. By-passing it was easy and I finally got something (Recording of first sounds I got test1).
The sound was dim, there was a lot of humming, but it worked. So things left to do:
- Replace the sticky hammers by the new silicone ones. This also requires some cleaning of the metal reeds.
- Find another way to by-pass the Footswell. This was achieved by plugging a short piece of wire into the pedal connector of the Pianet.
- Find a more permanent way to connect the Pianet to a “modern” amplifier like my stereo. I hacked the original connection cable between the Pianet and its amplifier and added a standard chinch connector. (Of course I could repair the original amplifier, but I want to leave this for later.)
This is a recording of the final result: test2
Future work: Learn how to play.