#0033: Repair and modification of a Stylophone

Preamble

As a bid to get myself into creating music – or at the very least something music adjacent: I decided to purchase a Stylophone. A simple and cheap electronic synthesiser. Something budget friendly and fun looking with which to test out the waters.

What is a Stylophone and how does it function?

If you are unfamiliar with what a Stylophone is, I will briefly explain. A Stylophone is a handheld electronic musical instrument. A synthesiser that creates audible waveforms from electricity.

The most notable feature of this instrument is it’s set of oversized PCB (Printed Circuit Board) pads, which operate as musical keys. These keys are accompanied by an electrically wired stylus, which functions as their activator. To play a note, one just has to touch the stylus to a keypad. This then closes an electrical circuit within the device, in essence mimicking a keyboard (button) press.

Broadly speaking, component-wise: at the heart of a typical Stylophone lies a voltage controlled oscillator. This component creates a waveform when fed a DC voltage. This waveform is then fed to the speaker to create an audible tone. Since this is a voltage controlled oscillator, it means that it’s output waveform is dependant on the input voltage supplied. Thus voltage is used to control the specific sounds produced.

To control the oscillator input voltage, each musical note key on a Stylophone has it’s own circuit with it’s own unique resistor values that are different from all the other note keys. These resistors are used with the intention of stepping down the 9 volts input into whatever each note-key’s desired oscillator supply voltage is.

Although there is undoubtedly more to discuss on Stylophones; such as how the pitch change, or vibrato functions operate: they are largely out-of-scope for this simple introduction. Although I will go into further detail if/when I do a full device analysis on this Stylophone. (Link below when/if that article is written.) However in general Stylophones like this one are really not too complicated devices.

Tools and equipment

Tools:

• soldering iron
• hot glue gun
• toothbrush
• tweezers
• desoldering pump

Consumables:

• hot glue
• isopropyl alcohol
• desoldering braid
• (leaded) solder
• solder flux

Components:

• red 5mm⌀ through-hole LED
• ethernet wire (multi-strand copper wire)
• 10kΩ potentiometer
• toggle switch

Device Repair

Fortunately (or perhaps unfortunately for this article) the device needed very little fixing. It’s main faults were either cosmetic in nature or trivial to fix; and since the Stylophone was basically fully functional, other than the dirt and grime that had gotten into it over the years: there’ll consequently be little to say about it in this regard.

I will say that the sound it produced was a bit choppy (intermittent), probably due to dirty contacts on the keypad. This issue of dirty contacts applied to the two built in slide switches as well, as they where notably unresponsive. Flicking them produced unreliable results.

More specifically: flicking the power switch on didn’t always cause the device to power on. I had to toggle it a couple of times for it to operate as expected. Additionally I couldn’t even tell at the time that the “Vibrato” button wasn’t functioning. It was only until after it was cleaned and started working, that I realised what it actually did. It makes the sound of the notes “wobble”.

Since these slide switches were built into the PCB itself, rather than a discrete component than can be easily removed: I decided that the best course of action was to simply inject some isopropyl alcohol into them. Then work the switches on and off until they self cleaned. The combination of the alcohol breaking down the embedded grime chemically, and the friction from mechanical manipulation cleaned the electrical contacts.

Moving on. I cleaned the PCB keypad in a similar way. Dousing it in alcohol and scrubbing it with a toothbrush to remove any loosened debris. This simple cleaning fixed both the intermittent connection issues of the switches and keypad.

Another small but annoying issue I encountered was with the stylus’s wired cord. It was damaged. The cord had a deep nick on the inner side of a loop that it had to fold in on, in order for the stylus to fit into it’s receptacle. Because of the location of this damage, it meant that I couldn’t simply just put a layer of heat-shrink tubing over it and call it a day, as it would limit the cord’s flexibility. Which in turn would make it no longer able to fold up and fit into place.

Instead I decided to just replace the entire wire. This lead to the next issue. The stylus’s original wire is somewhat unique. It’s inner structure consists of loose bundles of stranded metal conductors (presumably aluminium) interlaced with plastic (nylon?) fibres. This made it significantly more flexible than any of my wire stock. This flexibility allowed the wire to fold in tight under the stylus when placed into it’s receptacle.

Since I didn’t have an appropriate substitute wire, I decide to just use the best that I had on hand. This consisted of a single line of pure copper multi-stranded wire that was salvaged from an ethernet cable. Since this wire was considerably less flexible than the original, it meant that I could not use the original cable hole to enter the stylophone, if I also wanted to use the stylus receptacle as well. This is because the new cable could not fold into the same tight space under the stylus the old one did. Prioritising the stylus’s use of it’s holder, I decided to just drill another hole for the wire towards the back of the stylophone’s plastic housing.

Now, with regards to the more cosmetic elements of this repair: firstly, I removed and straighten the bent metal grill top plate. I then scraped it and the plastic housing clean of all that yellow hard glue using a scalpel. Ultimately I replaced all the old yellow glue with hot glue after I cut up the grill to fit my additions.

Although as a whole, probably the most noticeable thing about this Stylophone: was the prominent yellowing of the plastic housing. This yellowing is caused by ultraviolet (UV) radiation in sun light. The more sunlight a plastic enclosure like this gets in it’s lifetime, the more yellowed (or even brown) it becomes. Ironically to restore these plastics to their original colouring, one has to use a UV light in conjunction with hydrogen peroxide.

I briefly considered whether or not I wanted to restore the plastics to their original white. And if this was a full restoration project I would have done just that. However this wasn’t the point of this project, since at the end of the day: I was planning to add some crappy home-brew mods to it.

Additionally, I actually rather like the yellowing of older machines. Computers especially. I find it nostalgic. It reminds me of a simpler time: of a young boy listening to the hum of a beige box as it powered on, and the clicking and chittering of the various drives as they promised quality escapism. Insert Sierra logo tune here.

Device Modifications

First things first: for anyone who might raise an eyebrow at my choice of components below, I wish you to know that I basically decided to modify this Stylophone with whatever junk I happen to have on hand at the time. I was unwilling (and somewhat unable) to purchase or salvage more appropriate components for the task.

Not a single part that I put into this machine works as well as they could if I did take the time to source (or install) things properly. However I think that for this particular use-case, such perfectionism is unnecessary. It was just a fun and experimental hack together; and ultimately a learning experience.

With that in mind, I made three simple mods. These were made with the aim to better facilitate my particular use case of this instrument. Something I will explain as I go on.

These mods are:

• a power indicator
• an internal speaker cut-off switch
• and a volume dial for the internal speaker

1) Power indicator

The necessity of an indicator was made apparent to me: when I first picked up the Stylophone to find that I had left it on between sessions, and that the battery was now flat. Now that isn’t to say that the Stylophone definitively uses power when on but not actually playing. I’d need to test whether or not that is the case to say for sure. Either way really, a power status indicator is needed on this device to remind me to turn it off when putting it away. Simple as that.

To install a power indicator, I just used a basic 5mm⌀ red through-hole LED paired with a 330 ohm resistor. I tapped into the 9 volt positive side just after the power switch, and the negative side to the common device ground.

Unfortunately, this resulted in the LED being far too bright for my liking; with a light output that is more applicable for illumination, than as a device power status indicator. I really should have ran some basic ohm’s law calculations on this. Instead I simply used the same resistor value that I was accustomed to pairing with these types of LEDs on the 5 volt circuits that I am used to. Even then, they were rather bright. Now they are even brighter. I should have used a resistor with a much higher value. 1kΩ would likely do for a status indicator on a 9v circuit.

RED 5mm⌀ LED amperage and brightness comparisons

• maximum continuous amperage: 30mA
• recommended continuous amperage: 20mA
• setup I am used to: 5v/330Ω=0.015A (or 15mA) –> reasonably bright
• current setup: 9v/330Ω=0.027A (or 27mA) –> too bright
• future amendment: 9v/1000Ω=0.009 (or 9mA) –> perfect for a status indicator

I should state that in my experience using ohm’s law like this is a good guide for component choice. However components are all variable. The vast majority of components all operate within certain tolerances of their stated values. Additionally they can also behave differently once within a circuit.

For example I tested a red LED with a 330Ω (actual value 329Ω) resistor in series on a breadboard and provided it with 9 volts. It’s current draw was 22mA. I don’t really know why. It should still be 27mA. I’m guessing that I am likely not adjusting for something, such as the inline resistance of the breadboard and it’s contacts. Either way, these simple calculations still allow a technician to set their general expectations with regards to component behaviour.

2) Internal speaker cut-off switch

In addition to the internal speaker the Stylophone also comes with an amplifier output socket (3.5mm audio jack socket). I intend to use this socket to sample the audio. Either directly, or via an intermediary signal amplifier of some sorts. This is an alternative to recording using a microphone as you would with an acoustic instrument for example. I think direct sampling like this would produce a cleaner signal, and ultimately better audio.

I added the speaker cut-off switch because I didn’t want the Stylophone itself emitting sound while I was sampling it using a computer. Additionally, since I am likely to be plugged in to PC audio using headphones during the process: the Stylophone playing to the room is unnecessary in this scenario. Hence it might as well be silenced in order to minimised noise pollution and/or disturbance to others.

To make this happen, I just added a switch to the line between the main PCB and the speaker. I decided to use a toggle switch because they are cool. Very simple stuff. That being said, I probably wouldn’t have bothered with a speaker cut off switch if I though of installing the volume dial first. This is because it effectively performs the same function. By lowering the speaker volume to virtually nothing, it does the same job of silencing the speaker.

This was however the first thing that I installed into the device, and I have to confess it was predominantly because I thought that toggle switches were rather neat. I like the tactile feedback of flicking a switch like this, and because of that, I then went looking for a reason to install it into something. I actually almost used two switches like this to replace the two built in slide switches; but decided against it when I saw how they where integrated into the actual PCB itself. Too much work for too little return.

As it is I did notice something interesting about this toggle switch. When flicked off, the signal outputted to the 3.5mm audio jack socket lowers in volume. I think this might have something to do with the cut-off switch taking the internal speaker out of the loop. Perhaps the lower impedance of the speaker coil draws a higher amperage. Which would provide a stronger signal which then would have access to the audio out socket: since it has been place in circuit parallel with the internal speaker.

It’s just a guess, I honestly don’t know why removing the internal speaker from the circuit would result in the signal volume on the audio output lowering. I’ll look into it further when it comes time for a full device overview of this Stylophone. Just for clarity, I should also mention that this does not happen when the internal speaker’s volume dial is set to lower the volume to zero. With it’s potentiometer adding ~10kΩ in series with the speaker in the process. It only happens when the switch cuts the speaker out of circuit entirely. Hmm. :/

3) Volume dial

This is probably the only add-on of mine that is actually an absolute necessity in my opinion. Simply put: the Stylophone’s default volume is too loud. It’s tinny high pitched notes can easily come across as obnoxious and irritating at it’s default volume. Especially, when the player is using it to learn by playing the same little tune again and again, and fucking it up half the time.

To install a volume dial, I placed a potentiometer in series with the speaker. That’s it. In this case I used a 10kΩ pot as that’s what I had to hand. Once it came to testing however: it became apparent that I was using less than a quarter turn to effectively move the volume from 100% to 5% volume. With the other approximate two quarters moving the volume from 5% to 1%. Interestingly, the volume never does go down to zero. Even with the full 10kΩ of inline resistance: I can still hear the notes coming out of the speaker faintly. (For context: this pot only rotates to approximately 225 degrees; i.e. to a little under three quarter turns.)

I think this may be the reason why older devices’ volume dials ended with switches. For example with mono-sound CRT televisions: they’d work the volume level with a potentiometer, and then once the volume was below a certain threshold the dial switch would click on to either mute the volume entirely, or switch the device off all together. With that in mind, it makes the unnecessary speaker cut off switch sound almost useful. Eh?

If I were to redo this add-on: I would probably replace the potentiometer, with one with a smaller resistance value range. Maybe a pot that caps at 2500Ω. This is because only the first quarter of the current 10kΩ pot is in effective use, as it represents the most dramatic change in resultant volume.

The main reason why I may want to use a smaller value potentiometer is because it will increase the amount of incremental control the user has over the volume. This increase in precision is caused by adding a larger number of degrees that the dial needs to be rotated in order to increment the volume. Ideally this will result in a full turn of the pot corresponding to the volume scaling accordingly (100% to 5%). As opposed to the current setup of 0 to 90° rotation representing a 100% to 5% volume level, with the other 160° of rotation essentially going to waste.

Another way I could possibly achieve this is by using the same 10kΩ potentiometer and pairing it with a fixed value resistor in parallel in order to bring it’s effective max resistance value down to around the 2.5kΩ I desire. I am honestly not sure how that would work out, as I am only thinking of this while writing. I will experiment with putting resistors in parallel with the potentiometer when it comes time to revise this device.

Before & After

Before

After

Video Demonstrations

Mod demo #1

Mod demo v1:

• internal speaker output
• vibrato function
• speaker cut-off switch demo
• volume dial demo

Mod demo #2

Mod demo v2:

• power LED
• internal speaker output
• vibrato function
• volume dial
• speaker cut-off switch

Sound output demo

Sound output demo:

• external output plugin
• external & internal speaker dual output
• internal speaker output
• internal speaker cut-off switch w/ external output demo
• internal speaker volume dial w/ external output
• volume dial unable to mute completely
• volume differences on external output w/ using speaker cut-off switch

Music demo (internal speaker)

Music demo (external speaker)

Closing thoughts

Ya’know reading back on this: it really is funny how much I could say about so little. At the end of the day all I did was purchase an old Stylophone, clean it up, and then stick a bunch of bullshit in it.

Now, some people may be mad that I did this to such an old device. I noticed that it was made in the 1970’s; and honestly it’s age did give me pause. However I paid very little for this Stylophone, and bought it for the express purpose to tinker with. Additionally, it was literally the cheapest one I could find. Spares and repair condition, economy delivery, no returns accepted. You know the drill.

Also let’s be honest here: not everything old is an antique (e.g. your mum ;)). A mass produced low price point item like this Stylophone is not going to be worth much any time soon. However my innate preservationist did have to hold his breath while I butchered this wee lad. I’ll say that much.

I will be revisiting it, mostly to repair my repairs. To lower the power LED’s brightness, to decrease the volume potentiometer value, and to look into the utility of the speaker cut-off switch. I did have a few other mods in mind as well for it. Such as a 9 volt DC barrel jack socket and power source switch. That way I can run it off a wall charger in addition to battery.

That’s right you heard me. If I am going to butcher a beloved piece of British history, I am going to go all out.

Thank you for reading.

Term glossary

CRT – Cathode Ray Tube
DC – Direct Current
LED – Light Emitting Diode
PCB – Printed Circuit Board
UV – Ultra Violet

Links, reference, further reading

https://en.wikipedia.org/wiki/MIDI
https://en.wikipedia.org/wiki/Stylophone
https://en.wikipedia.org/wiki/Synthesizer
https://en.wikipedia.org/wiki/Voltage-controlled_oscillator

https://www.youtube.com/watch?v=VU7vXMezW_I