Hello,
thank you for the detailed report. I have a EC310 which makes crackling noises sometimes when it's played. I opened the piano to try to find the error. It looks like it's "just" the right speaker. When I unplug the left speaker the sound is very quiet even with volume full on. When I keep one key pressed for some time it starts the crackling and after I release the key and play the volum is loud and it plays as normal. But sometimes I can hear some crackling. It looks like the voltage is dropping and afterward slowly ramping up. Before I started the measurement I thought I ask you if you have a tipp where I should start to search.