My wife's digital piano has a couple of keys that output distorted audio. Unfortunately, it's a Technics piano, and they're no longer supported. So, I'm doing some debugging. Walk with me through my hypotheses and see if you agree with my reasoning...

Initially, we suspected that it was something to do with the key switches, because it's only a couple of the keys that are defective.

The piano has various voices "Grand Piano, Upright Piano, E. Piano, Strings, Vox, etc.". If you switch to a different piano, the distortion remains, but if you switch to "Strings", etc. the distortion goes away. This suggested to me that it might be the waveform ROM that had been corrupted (on the assumption that all of the piano modes used the same waveforms but meddled with it).

However, with headphones, you can't hear any distortion, which suggests that it's actually the speakers that are at fault. But why, then, with only a couple of keys, and only with certain piano modes?

Further: in an attempt to work around this, and rule out the built-in speakers, I'm considering running the line-out to a separate amp and speakers, but I can't be bothered to move (or run cables to) my existing audio receiver. What search terms should I be using for a small basic amp that has line-in and twist connector output?

The piano's a Technics SX-PX554, incidentally.

For a cheap amp that sounds okay how about something like this one: https://www.amazon.co.uk/dp/B00SATAEEA/

I have one like that in the workshop, and a bare-board version of it in my study.

Cheers

Looks like it'll do the job. Thanks. Ordered.

If you’re absolutely sure that you can’t hear distortion on headphones, I suspect that your issue is that the speaker(s), or the amp circuit powering them, are bad in a small subtle way in which they only distort at certain frequencies corresponding with a couple of specific piano notes on the keyboard. Since string sounds have different frequency spectrum, that might explain why you don’t hear the issue with string sounds.

It’s even possible that the issue is that there is a dead bug or a piece of crud lying on one of the speaker cones. Do you notice the distortion mostly from one side? Try turning the piano up on its back edge (assuming it’s one that is small enough to do that) and see if that fixes it by making the crud drop out of the speaker cone. (I don’t know if it’s one of those keyboards that’s like a small stage synthesizer on a stand l, or if it’s a full piano sized thing.)

If you have an old pair of computer speakers in the garage, you can plug those in.

Make sure that the difference you’re noticing between headphones and speakers isn’t just a question of frequency response. For instance if you had a bad ROM and the samples for a couple specific notes were only slightly corrupted, maybe the distortion isn’t audible unless it’s on large speakers with more bass response.

Yeah; this is something that I hope to rule out by using a separate amp and speakers with the line-out.



It's not obviously one side or the other, but it's only obvious when you press the key hard (aiming for forte, rather than piano), and the speakers aren't that wide apart.



It's larger than a stage synthesizer, but smaller than a full upright. It's this, but in black, rather than "mahogany".

I found the service manual online, which explains how to dismantle the main body, and it doesn't look like too much work to get at the speakers and inspect them for damage / bugs.

All good tips, thanks.

I'm thinking you do have faulty keys, and the speakers, in certain modes, are shaking them just right. I saw this in a couple of old Mitsubishi TV sets, on headphones they were fine, but using internal speakers the picture went wonky. The speakers had pummeled (with sound) the pcbs to failure.

It looks like its speakers are vertical, so if that’s true then I rescind my theory about crud on the speaker cone. I give greater weight to my theory to either bad speaker (such as crumbling foam speaker surrounds) or bad amplifier circuit (perhaps a leaky cap from the early days of ROHS when caps had problems with leaks around the cap legs).

Actually I take all of that back. My new bet is on a rattling piece of the casing or wooden body. Something loose and buzzing similar to a rattling interior panel in an old car. That could easily sound like distortion.

Or maybe a loose wire inside the casing which vibrates at exactly the right frequency/amplitude corresponding to certain piano notes. Sometimes the wires connecting the pickup system in my guitar will buzz faintly, only to certain notes or chords played loudly. Fixed by reaching in and tweaking the positioning of the wire so it didn’t touch the wood.

A thought suddenly occurs: it's got MIDI-in. If I can pipe the relevant notes into the piano, without using the keys, that might be another way to rule out the speakers themselves.

Time to investigate MIDI adapters and software for the Raspberry Pi, maybe.

If it does turn out to be crumbling foams, replacing the foams is quite easy to do oneself. The foams are readily available for $2-6/pair from Asia (eBay), and a bottle of suitable glue can be picked up at any craft store -- I used "Aleene's Original Tacky Glue" on two pairs of speakers here.

Cheers

Regarding MIDI:

That would certainly rule out any issues with key switches or mechanics.

Is the piano new enough to have a USB port? Many keyboards do, with the capability to present themselves to the PC as their own MIDI device, alleviating the need for a third party MIDI interface.

Does this thing have fancy weighted/levered keys that are designed to feel like real piano keys? If so maybe there’s an issue where the key mechanisms of a couple keys are loose and rattling, but only when the keys are in the “pressed down hard” position. This might make a physical buzzing noise that sounds similar to distortion.

Does the noise appear to get quieter or go away if you turn down the volume? You could diagnose whether or not it’s related to frequency resonance in the speakers or body of the piano by fiddling with the volume.

No. If it was, it'd still be supported

Basic USB/MIDI converters (sufficient for this job) are about $8-10.



Yes, it does.



Yes. The piano also has a key sensitivity feature. If I set it to "firm", I seem to have to hit the key harder to trigger the distortion. That suggests to me that it's independent of how hard the key is pressed, and -- potentially -- of the key switch entirely.

Oh, here's an idea: the piano can record its own sequences. Could just use that. No need for MIDI. Doh.

BRILLIANT!

Really interested in knowing how that turns out.

The fact that the distortion goes away when you turn down the volume tells me that it's got nothing to do with the ROMs or the piano electronics, and it's got to be physical moving parts buzzing, or amp clipping. Everything is at the amplification stage or later.

Which notes are they? Low ones or high ones?

If low notes:
- Bad woofer, or stuff leaning on the speaker cone.
- Loose panels or objects inside the casing rattling with certain low frequencies.
- Bad amplifier for the speakers that is clipping at some threshold on certain frequencies.

If high notes:
- Blown tweeter.

"The D above middle C" says Jen. Whatever that means...



I just finished playing with the sequencer. The distortion's there in the playback, which strongly suggests that it's one of the speakers. I'll open the piano up later in the week and take a look.

Neither a particularly high or low note. Right in the middle of the range, slightly to the left of the exact center of the keyboard.

If you're absolutely certain there's no distortion in the headphones, then it's either the speaker, the amp, or cabinet rattling.

Try getting your head down between the two speakers if possible and listen to the playback on repeat. Try to narrow down exactly where the distortion is coming from.

If it's only coming from one side: Bad speaker or physical rattling/buzzing likely (or, less likely, just one channel of the amp).

If it's coming equally from both sides: either it's physical rattling coming from a different place than the speakers themselves, or, it's a bad amplifier.

It's one of the keys to the right of the keyboard, so apparently I misheard Jenni

The tuner app on my phone says "F5".

Bugger. The line-out also runs through the volume control, so I can't turn down the internal speakers to use only the external speakers.

The distortion seems to be volume-related as well.

Is there a headphone out that could be used instead?

Yes, he said that when he plugs headphones into it, that he does not hear the distortion.

However now that I think of it, the headphone output would probably be controlled by the same volume control as the main speakers. So he's probably not ever turning up the headphones very loud.

Also if you plug the headphones output to an external amp's line-in and then turn up the volume really loud, distortion is normal due to the the difference between headphone level and line level, and wouldn't necessarily indicate a problem. So that'd be a tough thing to use for diagnosis.