Hohner International

Electronic Piano

I haven't managed to find much conclusive information about this neat 61-key unit online so far. Most of the info presented here is based on my own and others' speculations and deduction, repair logs, as well as some Hohner advertisements from the mid-1970s (see further down). I even contacted Hohner to try to find out more, but all I got was a confused fellow and service manuals for various Clavinet models, the Clavinet-Pianet-DUO, and the Electra-piano. This unit is unlike those famous Hohner models in many ways. First of all, rather than being electromechanical (i.e. with mechanical tone generation and electrical pickup), it is entirely electronic. Of course, it is of the electronic piano instrument family, members of which generate their tones using analogue electronics that attempt to imitate the sounds of an acoustic piano, and invariably harpsichord as well, since the nature of the tone generation makes it well suited for both piano and harpsichord-like sounds. The resemblance is not great, and when compared to a "real" piano or harpsichord is often described as "thin" and "cheesy". But as stated on the ECH page, so what!? Such a tone can fit well in certain situations.

This unit was likely made around 1975, and is probably the earliest of at least three models of "Hohner International Piano", with later models having a "bass" feature on the front that this one lacks, as well as having 61 oscillators (one for each key!) instead of just 12 that are divided down on this one, some with slightly different vibrato controls, and very different electronics overall. The date was derived by looking at some of Hohner's magazine advertisements. One from 1975 reads "And now there's the new Hohner International Piano" (and also list some rather amusing tonal possibilites not printed on the unit itself: "Honky-tonk piano" and "Hawaiian guitar"). A later ad from 1976 advertises a later model, the "Hohner International Piano II", and I presume when the II came out the "I" was discontinued. This would suggest an approximately 1- to 2-year production run of this first model, which seems to be supported by how rare it is; I have only found one other of this model for sale online. Another factor likely contributing to its rarity is that these units sometimes get harvested for their rare SAJ110 frequency divider chips, which are used in other more sought after keyboards, and are often worth more alone than an entire working instrument (which is ridiculous, and the people responsibe for destroying these for the chips should feel bad). The "HIP II" seems to be called by the model designation K1, and seems to have similar vibrato controls to this early one (see the links section for more info on these). Another version has different vibrato controls, with what is possibly a vibrato depth slider. My unit has no obvious model designation, serial number, or any such useful info. All models look outwardly quite similar and also sound much alike.

While the brand name on the outside of this unit is Hohner, the electronics bear the name of the obscure Italian manufacturer ELEX (ELettronica EXcelsior), which is apparently the true manufacturer (of the electronics, at least). The casing is made of particle board with black tolex, and along with the overall styling and metal stand resembles that of the Clavinet-Pianet-DUO and the Clavinet E7, leading to some speculation about whether or not ELEX also made the casing and stand. My unit unfortunately is not only missing the stand, but has had its stand mounting holes totally wrecked by a previous owner. Oh well, it's the perfect size to sit on top of the Yamaha CP-30 anyway, so for the time being, a stand is not necessary.

Features

  • 61 full-sized semi-velocity-sensitive keys (F1 to F6 / 5 octaves + semitone)
  • completely polyphonic (all notes can be played at once) analogue tone generation
  • 2 tones ("piano" and "steel guitar / harpsichord") with volume sliders, plus overall volume slider (towards the front = louder!)
  • variable speed vibrato with knobs for speed (doubles as on/off switch) & delay (on/off switch, non-adjustable)
  • 1/4" jacks for output & pedal (sustain is normally ON with no pedal attached)
  • standard IEC power plug

Trivialities

  • 9 single-sided PCBs using all through-hole components, including one approx. 7" x 33" board!
  • 181 transistors in total, of 6 types:
    • 1......2N3055FA NPN power
    • 122..2N5172 NPN
    • 2......BC173B NPN
    • 52....BC208B NPN
    • 2......BC252C PNP
    • 2......BC253C PNP
  • 7 ICs, all of the same type:
    • ITT SAJ110 seven-stage frequency divider (red DIP-14)

Repairs

I got this unit for pretty cheap under the impression that it wasn't functioning at all. I was told by the guy who sold it to me that "the power light comes on, but we couldn't get any sound out of it", which of course could be caused by any number of issues of greatly varying difficulty to diagnose and repair. As it turns out, the issue was probably the simplest possible: the volume sliders go the other way! As you pull them towards the front of the keyboard, loudness increases. I'm guessing they tried to go full blast by pushing the sliders all the way towards the back (as in a mixing board) and figured it was busted when no sound resulted. Admittedly, I spent far longer than I should have measuring voltages and looking for bad parts before I discovered this myself, but nonetheless, it was a nice surprise.

It still wasn't completely working, though; the highest key (F) did not play, and the key switches were unevenly and in some areas quite severely oxidized, especially around where the switches make contact, causing intermittent key functionality. The latter was an easy fix using cotton swabs and some 99% isopropyl alcohol (avoid rubbing alcohol; some varieties can leave possibly disruptive residue). Each switch is composed of a long spring (moved by a plastic part of the key) that rests against one busbar when idle, and is pressed against another when a key is pressed. The picture below shows the switch contacts pre-cleaning, and notice the ELEX name on the PCB. While each board has a model number that might seem useful for finding info, I have been able to find nearly nothing on the company itself, let alone info based on the model numbers.


The bad high key was a little trickier. At first, I thought it was a highly oxidized key switch, but it still didn't work after a thorough cleaning. I decided to have a look on the underside of the very large main PCB for evidence of bad soldering, and sure enough...

There was an incomplete solder joint right at where the key contact signal would connect to the key's envelope generation circuitry! A quick bit of resoldering, and it worked!

Something interesting to note about the main board that you can see in the picture above is that the circuitry for each note does not line up with the correspondingly placed key (i.e. the rightmost circuitry does not match up to the rightmost key). Instead, the circuitry for a given named note is grouped together into its octaves, since each frequency divider gives out the divided octaves from its pins, and arranging the signals in chromatic order would be difficult, especially with these single-sided PCBs. Since the boards do not have any silkscreening, the information indicating how the notes are arranged is in the copper traces on the bottom of the board. Interestingly, the notes are labelled with their syllables of solfege (do, re, mi, etc.), and octaves are labelled inversely to how they usually are (i.e. the highest octave for "fa" is labelled 0, lowest is 5).

Another issue mentioned earlier was that the places where the original stand (now missing) would mount to the underside of the unit had been seriously damaged. It looks like it maybe it got knocked over at some point, causing this severe damage of the mounting points:

On the left hole, the metal threaded mounting piece was completely missing. Basically, I decided right away that it would not be worth repairing to the point of being able to mount to the original stand, so I simply used carpenter's glue to secure the broken wood and the stretched tolex to stop it from dangling below the bottom of the side panels.

As well as this, the keyboard was also subject to the usual cleaning/vacuuming treatment that all JCS instruments receive. It was quite dusty and had a yellowish sticky coating on the keys that I assume was likely cigarette residue (especially considering some little melted spots on the keys)... terrible!

The Hardware

One of the first things that I noticed on this board (while cleaning the keys) is that the keys do not seem to be extremely well moulded. There are minor little flaws on some of the keys that are clearly the result of an imperfect mould. For example, every B key has a little plastic bit protruding from the right side, and every E key has a little bump on the front. The tops of the white keys are not perfectly smooth; there are little linear marks along the axis of the keys that almost look like those made by a paintbrush (though this is not severe enough to get a good photo), and the tops are not quite flat, but rather a little "warbly".

Of course, these flaws are very minor and don't affect the playing at all, but they're interesting to note since I haven't seen such key moulding flaws before. The white key fronts are piano-style (fully extending downwards and with a lip at the top) and also have particularly sharp front edges, while the tops of the black keys are curved like those of other Hohner keyboards (i.e. Clavinet, Pianet, etc.) and accordions. They are not weighted like those of the Yamaha CP-30 electronic piano, instead being typical organ-style spring-loaded. It should be noted that the keys are also sort-of velocity sensitive (or, as Hohner stated in their 1975 ad, it has a "dynamic keyboard action"), but with nowhere near as much sensitivity as on the CP-30 or pretty much any other velocity-sensitive keyboard. More on that later...

To the left of the keys is the control panel, which is black plastic with a glued-on brushed metal plate indicating the functions of the controls. It's fairly simplistic, having only three sliders and two knobs. There is a master volume control slider, and a slider each for the "piano" and "steel guitar / harpsichord" tone levels. Of course, the loudness increases as the sliders are moved towards the front of the keyboard. There is a vibrato knob, which is used both for activating and adjusting the speed of the vibrato. The depth is only alterable with the "delay" knob, which is really an on/off switch that introduces a delay before the vibrato is smoothly brought in. Simplistic, but quite nice, I think. On the right, there's another, less wide plastic side panel that simply has the power switch mounted to it (though it's a nice one with an internal lamp, basically like on many power strips).

There is a metal cover over a large portion of the unit, which has two reinforced holes in the top for mounting the music stand (now missing, sadly) that appears to have been made of black plastic or wood (see the 1975 ad). On the back, it has the "Hohner International" logo, a rectangular hole with a friction-fitted plastic piece (which you should never take out, since it's an absolute pain to get back in) for access to the power/sustain/output jacks and fuse, and 12 holes for accessing the tuning trim potentiometers of the oscillators. Tuning can be accomplished with a small flat-head screwdriver. Interestingly, instead of being arranged in semitones, the oscillators are arranged in perfect fourths (5 semitones) apart.

Removing four screws allows the metal cover to come off; two at the front and two at the back. The cover is silver-painted, and carefully peeling off the old price tag (which was on the left side) revealed that the 40-year-old paint is a little flaky, and my slightly damp paper towels were turning silver during cleaning of the metal, so that is something to watch out for. Once the cover is off, the boards at the back are revealed...

Looking from the back, from left to right, there is the oscillator board, vibrato board, and filter (presumably? possibly amplifier also) board. The oscillator board is for some unknown reason mounted in a nice black-painted wooden frame, and is the only board to feature some (albeit basic) silk-screening indicating connections for power & vibrato, as well as which circuitry applies to which notes. Unlike the metal cover which only has note letters, there are also the fixed-do solmization names printed above the letters (do, re, mi, etc.) on the board. The tuning pots have some brown wax which is easily broken for tuning, and had not been altered at all before I received the unit, which is a testament to how well this thing stays in tune. To the right of the oscillator board is the vibrato board with its nice green transistors with golden leads. Note also the large variation of resistor coloring/size/shape; looks like they just put in whatever they could find in the parts box! Actually, the variation of resistor types is something that can be seen on all of the boards. On the right is what I presume is the filtering board, but it could have amplification or other features that I haven't figured out yet. From what I can tell, the output from the frequency divider/envelope generator board is filtered with basic fixed-parameter lowpass and highpass filters to produce the "piano" and "steel guitar / harpsichord" tones respectively.

Also on the back is the metal panel with the IEC power connector, fuse socket, and jacks for output and a sustain pedal. It takes a 0.5A time-delay fuse, and a sustain pedal with a switch that is normally "closed". Interestingly, if no sustain pedal is attached, the sustain function is activated since across the jack terminals is an open circuit. This can of course be modified so that it's normally a closed circuit, and I thought about doing so, but decided to leave it in its factory state.

Looking from the front, there are the two plastic side panels on either side of the keys, and then the 61-key assembly itself, which is mounted on a long continuous hinge. Removal of two screws on the underside allows the keys to be moved into a more vertical position, and exposes the key contact board, "10-transistor board" (the purpose of which I am not sure of yet - could someone enlighten me?), and glorious 7" x 33" frequency divider/envelope generator board! I do not believe I have seen a board of with such great area before. Towards the front side of the board, there are 7 of the seven-stage frequency divider chips, the only ICs in it, all ITT SAJ110s in nice red DIP14 packages (these chips alone are potentially worth around 3 times what I paid for the whole unit, which is despicable). Since this is only a 5-octave + 1 keyboard (where the generator signal gets divided only 4 times except for the F which is 5), each chip is used for dividing frequencies for more than one note, hence only 7 chips instead of 12.

I have a feeling that some of the boards in this thing were perhaps designed by different people and for different inital applications. I say this especially because of the types of transistors used. The oscillator, amplifier, "10-transistor", and power supply boards all use solely BC208B NPN transistors (aside from the 2N3055 power transistor on the PS board), while the vibrato board uses a mix of BC173B, BC252C, and BC253C, and the main freq div/env gen board uses entirely 2N5172s. I assume if the boards were designed as a set, by the same people, they would be able to utilize only maybe 3 types instead of 6; an NPN power type, NPN small-signal, and PNP small-signal. There are some notable capacitor variations too; for example, the filter board uses a bunch of orange rectangular units with radial leads, while the oscillator board uses clear cylindrical units with axial leads. Of course, ELEX also manufactured organs and synths, and the generator, vibrato, amp & PSU boards are more general-purpose; they could be used in different models of organs and synths. In fact, ignoring the envelope generator stuff, the tone generator architecture is much like many combo organs; 12 main "top octave" oscillators which are continuously running, and are divided down by frequency dividers to produce the lower octaves.

Removing two screws allows the slider/knob control panel to come out on the left side (it is slotted into the case at the front), revealing the power transformer, a small connector board, and power supply board. The PS board is noticeably more "crude" than the other boards; the large electrolytic capacitor on the left is too large for its lead mounting, there are "piggybacked" resistors, a crooked trimpot up on end, and bent wiggly jumper wires. It also has a few cracks that at first I thought were the cause of the unit's non-functionality, however these are harmless and have since been repaired with very thin "Hot Stuff" CA glue (picture below is from before gluing).

The only other hardware feature that I think is worth noting as of yet is the shielding. If you take a look at some of the images above, you might notice some silver material underneath some of the boards, particularly at the back and under the envelope generator/frequency divider board. This is, of course, shielding tape which is kept at ground potential for the purpose of reducing the effect of external electric/electromagnetic fields. There is also some tape that makes contact with the metal top cover, keeping it at ground potential as well for the same purpose. All in all, the shielding seems to do its job, since the output is fairly quiet.

The Sounds

Generally speaking, what defines the sound of an electronic piano is this: simple pulse-waveforms (or other simple harmonically-rich waves) have their amplitude controlled by basic envelope generation circuitry, and have basic subtractive filters applied to the resulting output without any modulation of filter parameters, but instead with adjustable amplitude of each filter's output to create a variation of possible sounds. The two types of filters generally used are very easy to implement electonically; lowpass filtering (removing high frequencies) produces a sort of piano-like sound, while highpass filtering (removing low frequencies) produces a bright harpsichord-like sound.

In the case of this unit, the basic waveforms produced are indeed simple pulse-waves with a duty cycle of approximately 25%. Each key/note has its own volume envelope generation circuitry, which has a fast and percussive attack and a long (but not that long) release time. In fact, the notes really do not sustain for as long as they would on an acoustic piano even with the sustain pedal depressed. The sustain pedal simply makes the piano act as if the keys are held for the entire length of the envelope, which of course can be done just in regular playing. Unlike the Yamaha CP-30 or an acoustic or electric/electromechanical piano, the notes do not "swell" in volume with repeated hitting with the sustain pedal on, but of course that is a somewhat advanced feature that is even lacking on many brand new digital units. The notes have a short time after a key is let go of (without sustain on) where the amplitude drops to zero, so the notes do not end too abruptly after a key is released. Though the time is longer than on the CP-30, it is somewhat comparable to the time it takes for the dampers of an acoustic piano to dampen the strings entirely. As usual, for the "piano" tone, the output is subjected to lowpass filtering, and for the "steel guitar / harpsichord" tone, highpass filtering is used.

Now, a little ways up the page, I mentioned that this thing has some very basic velocity sensitivity to it. What I mean by this, really, is that any keypress of say... medium to high velocity produces about the same result: more or less full-blast. Playing softly is where things get interesting; the resulting loudness is somewhat unpredictably between nothing at all up to just under the max. Basically, most of the dynamic range of the keyboard is only accessible if you play softly and with immense precision, it seems. Now, it would help to know exactly how the keypress velocity is determined, and while some of the details are still a bit fuzzy, I think I've figured out the gist of it. The key moves a spring contact from touching one busbar to touching another, and, like the CP-30 and many other keyboards, the time it takes is what determines the velocity. When the spring leaves the first busbar, the voltage on it starts decreasing, and this voltage would correspond to the the amplitude that the resulting note starts at. So, the point on this voltage envelope at which the spring touches the other busbar determines how loud the note is, and so an early touch produces a loud note, and a late touch a quiet (or nonexistent) one. The reason it doesn't work so well on this keyboard is that it seems the voltage goes down somewhat slowly and perhaps without the right sort of envelope, and it also goes all the way down to zero; there is no audible "minimum level" like there is on the CP-30 if you press the keys extremely slowly. So basically, most regular keypresses will give around the same amplitude, and if you want more dynamic range you have to do carefully timed slow keypresses. I don't consider it much of a problem, though; it's not always necessary to have great velocity sensitivity, I think.

A nice feature that this unit has that is lacking in other electronic pianos such as the CP-30 is the variable-speed vibrato (periodic/sinusoidal modulation of the piano's pitch, not the amplitude of the output). The activation and speed are both controlled by the same knob, so you can't just start it at full speed, but it is a nice continuous adjustment. While there is no direct control of the "depth" of the modulation, there is a "delay" function that can be turned on or off that slowly increases the modulation from none to full over an interval of about a second when a key is first pressed. If multiple keys are pressed without a gap in the playing, the delay effect won't trigger again.

Nowadays, the pulsewave-y sounds that this thing produces are commonly associated with early video game consoles. If you played this thing in front of most young people, they would probably say "Hey! That sounds like Mario!" which of course it does, since the Nintendo Entertainment System's sound chip includes pulse-wave channels which are capable of the same 25% duty cycle that this unit employs. These early video-gaem sound chips, though, lack the complete polyphony of electronic pianos (the NES only had 2 pulsewave channels, for example), and so this unit and other electronic pianos offer a way to play with such sounds in complete polyphony, and with real-time control of the tone using intuitive and convenient sliders, knobs, and switches.

Of course, while playing this thing clean is fine, throwing on some effects can really add a lot. Especially adding a little overdrive and possibly even more serious distortion can turn this thing from arguably "thin" to absolutely thick as hell. Lots of people swear by using a phaser to add some "depth" to the sounds of electronic pianos, and of course you can almost never go wrong by adding some wah-wah as well.

It has been used on Jesse Acorn's release Demos, or perhaps not... and the S.F.A.T.B.H.S. release Novoful Vyffm while tuned in an arbitrary form of just intonation.

In the future, there will be a demonstration video showing off the sounds here, so stay tuned!

If you notice any errors or have additional information that you would like to add, please contact me!


Last Updated: 7/21/2016