Some time ago I got a NAD D3020 V1 amplifier. It's a decent class D amplifier, and very compact.
Like many D3020 V1, mine had dim LEDs that made the display basically useless (the more a LED was used, the dimmer... so for example the never used inputs still had some luminosity. I found that Inolux IN-S63AT5UW is a replacement for the original WR-PB1608UW-M58 (which is not available), so after ordering 30 of these
https://www.mouser.com/ProductDetail/Inolux/IN-S63AT5UW?qs=fAHHVMwC%252BbjOUaZe%2F%2FIGRQ%3D%3D and some SMD soldering, the display was as new.
But there is still a very annoying problem on power on, a high frequency audio tone that increases in frequency until it's inaudible. That seems to become worse the more I use the amplifier. If I stop using it for a few days, the problem is less obvious. Sometimes it goes away completely, sometimes it returns after a while
I tried measuring the output signal, and noticed that with no input and only the speakers connected, both channels have a 522-533 KHz sinusoidal tone at around 480 mV (I used a 1M Ohm scope probe, 1x)
I have only another D class amplifier, but that one has no high frequency tone on the oscilloscope. I know that D class amplifiers might have high frequency components well above the audio range, but an almost 500 mV tone on the speaker output seems really wrong.
The amplifier is based on a UcD102 module from Hypex, for which no schematics is available, as far as I can tell. I included the amplifier service manual with schematics and the only document I have about the UcD102.
The headphone output is fine, but that is not using the Hypex module. So I suspect a problem with the SMPS, but I wouldn't know what to check either
I know nothing about class D circuits apart from a general understanding of the basic principles, so I have no idea where to start troubleshooting. I'm hoping some kind soul here can provide suggestions on what to check, otherwise I'm stuck with just an headphone amplifier