This is a write up from my work shop point of view, to help you make decisions when you consider buying a digital piano, whether new or second hand. I have serviced pianos from all kinds of brands, expensive ones, cheap ones.
Maybe you found this page because you were looking for a test or review. This blog post is not about testing pianos of brands like Altana, Classic Cantabile, Clifton, Cronenwerth, Fame, Gear4Music, Hemingway, Mc Crypt, Medeli, Schubert, Stagg, Steinmayer or Thomann. Instead, I will provide general information about pianos by cheap brands based on personal service experience.
Generally speaking the cheaper ones need maintenance earlier, sometimes after just a few months of playing and often because mechanical parts fail. More expensive pianos enter my work shop entirely because intense usage throughout many years makes e.g. the filt dented or electrolytic capacitors need to be replaced due to life cycle ending after some 20 - 30 years or so. Still, they are easily repairable while cheaper pianos will not even last that long for many reasons. Here is why.
1. Failing keys
This happens either due to dust or dirt on contacts because the contacts aren't encapsuled properly, or too little conductive material is applied to them. I've even had a case where corrosion in a cheap connector plug broke the piano after just 3 years. But since dust is enemy No. 1, as a conclusion, lay something on your keyboard while you are not using the piano. Hopefully you have a cover installed (and make use of it).
Or there is a mechanical problem. I've had torn off and broken hammer rubber and plastic, worn out keys, bent axes. I've even had a Cronenwerth Piano with every single hammer broken just because someone played on it for a couple of years. Sadly, the original importers usually refuse to keep spare parts for more than two or three years so you either have to improvise or you have (or add to) a collection of spare parts from trashed pianos. While the European Union has started to force manufacturers to provide spare parts for a certain period of time from 2021, manufacturers of digital pianos are not (yet) among them.
There are brands that have products with really good keybeds, though. Classic Cantabile for example sometimes use keybeds by renowned italian manufacturer Fatar. Usually this is mentioned by adding to the type name of the piano, so in this case at least you'd get away with something sustainable.
2. Failing capacitors
It's electrolytic capacitors that are failing mostly. While good pianos are built on quality capacitors with good headroom (e.g. tolerating significantly higher temperatures and voltage than expected) the cheaper ones are made of parts that just meet the absolute electrical must, well, if this is ever met at all due to large manufacturing tolerances (~20%) and unselected parts. Since a cap life cycle is measured in hours (e.g. 2000 h) they can loose capacitance early and start to fail even if you just left the instrument switched on over night repeatedly. Loosing capacitance sometimes means leakage which leads to more issues. Symptoms could range from low volume on a stereo channel, crackling static noise or low voltage in critical parts of the circuit resulting in complete failure. In the worst case dying capacitors lead to shorts.
I've made the experience that SMD* caps fail more often than through-hole-components although they could be the better choice if selected properly. I have even seen exploded SMD caps due to lack of determined breaking point. You guessed it, SMD components are often cheaper than through hole equivalents. And if the manufacturer decided to use multilayer PCBs (Printed Circuit Board) SMD components are often a must for practical reasons.
I observed that in newer products even cheap brands tend to return to through hole components at least for some parts of the circuit which makes maintenance a lot easier. It would be better if they made this a general rule in my opinion, though.
One more word about a problem familiar even to high class manufacturers: (Before) some of them like GEM (†), Roland, Technics (†) and even Yamaha use(d) rechargeable batteries to buffer memory when the piano is switched off. These batteries tend to leak after many many years of usage, damage can be dramatic so it's a good idea to keep an eye on that if you own such a piano or plan to buy a used one. If you have a memory function and there is no standard battery installed (e.g. CR 2032) then it's likely that you have something rechargeable instead. Watch out and have that component replaced after 10-15 years latest.
3. Failing pots, faders and switches
This applies generally to all manufacturers of electronic devices. But you can use cheaper or better components for your products. While better pots often can be revived by cleaning them, cheaper ones have their conductive material lost and are only to be replaced. What's more, sometimes it's not easy or even hopeless to get spare parts with the same specification and dimensions.
4. Poor manufacturing quality
In (junk) industrial manufacturing all parts are preconfigured, like predrilled housing screw holes or automated placement not only of circuit board parts. Despite the fact that robots do a lot of the manufacturing work, controlling is essential. Whenever I came across a cheap brand piano I spotted indications of inattentive controlling. Missing or misplaced screws. Badly placed or even wrong parts on circuit boards (while the boards itself fulfilled function testing somehow). Displays and LEDs not fitting their panel spaces (low quality displays by the way that become pale just after a few years). And hot glue on spots where it does not make much sense like on connectors that are self securing or wires that have dedicated supports anyway, always glue, glue, glue. My image of a junk industry controller is of a person holding a signing pen in one hand and a glue gun in the other.
Still, it's kind of sympathetic that the IC's in the above image are marked LOL.
5. Built not service friendly
Especially chinese manufacturer(s? Plural? See below!) seem to save costs whenever possible (presumably pressed by western online- or chain stores). Sadly, this policy applies also to certain connectors between circuit boards, most of all panel boards. So if some replaceable component failed you have to disassemble all of the boards because all wires are soldered AND GLUED (!) onto the boards. Why do engineers do that? Don't buy that. Nobody wants to repair it.
6. Poor sound quality
These cheap ones sometimes have tons of sounds and functions onboard. But they don't sound good at all. Speaker types and filters are not well trimmed to the housing which often results in a certain frequency range (often low mid) being too dominant. And they tend to have strange dynamics. They are noisy. Even their transformers hum. D/A conversion often sounds like there was no improvement since 1983. Take a pair of headphones and listen to a single note while it sustains, how long does it sustain? When is it getting noisy? They're trying to paste up noise with reverb, right? Reverb can't be switched off permanently, right? You got it.
My rule of thumb:
If the total price per button ratio is below 20 € (or dollars), don't buy it. This means a 400 € piano should not have more than 20 buttons/controls.
What kind of digital piano should I buy?
If you don't have much money, rather buy a 20 year old Yamaha, Roland, Technics, Kawai, Korg or a later Casio, than one of these cheap brands that are supposedly manufactured all in the same factory run by Medeli in Zhuhai/China. If there are issues with second hand pianos from the above name brands they are often easily fixed, as long as it's not a leaking-cap-problem (see above). And often you still get spare parts for them even after many, many years. I have revived digital pianos from the 1980s and early 1990s that still sound better than nowadays 500 - 600 Euro China products.
If you really need to buy a chinese piano, never buy it second hand. Prefer buying it in a physical shop and ask for guarantee, service and sustainability. If you really need to buy it online, I don't recommend buying on a site that is not properly translated or missing imprint and contact address, or on Amazon or Ebay. Someone will have to service the instrument sooner or later, and it's not unlikely the money you saved when buying will be spent on service (and transport). If service is possible at all.
Questions? Additions? Comments? Insults? Feel free to leave a message below.