Glad to hear that you got it working again. Fixing an intermittent problem is one of the most difficult problems to solve. You have to wait for it to fail in order to find the fault. As they say, "you can't fix it if it ain't broke".

You can try to trigger the fault by applying heat and cold, vibration, tapping and banging. Certainly, applying heat with a soldering iron may have been significant. We can try to narrow down the area to somewhere after the MCU, digital signals to the DACs and analog signals. It could also be caused by faulty power supply lines. Other suspects are bad solder joints, PCB traces, an intermittent component. Capacitors are more likely to fail rather than resistors.

Try to see if the fault is common to both the speakers and the headphones output. Next time it misbehaves, try plugging in the headphones and compare with the speakers.

/MUTE signal is still a prime suspect if the failure affects both speakers and headphones.

While it is still working, get a feel for what the signals look like on the oscilloscope, particularly at R104, R105, DAC-L, DAC-R.
In post #49 you had posted input voltages that were not quite +15 V. You may want to revisit this.

Hey MrChips,


it’s been a while since I last wrote in this thread. After we got the piano working again, I played it for a few days, but life got busy — and since I was only about two months away from my wedding, I didn’t have much time to keep troubleshooting or even to check what the real root cause was. Months passed and the piano stayed in our office. A few coworkers probably tried to play it now and then, but otherwise it was untouched.


When I returned after our honeymoon and finally had time to look at the piano again, the loud buzzing sound had returned. I knew I had to revisit the problem and go through everything we had done earlier. A few days ago, I decided to analyze everything myself using the knowledge you taught me. I checked DAC-L and DAC-R with the oscilloscope first, but all I could see was noise. Since nothing interesting appeared there, I went back to L204 — the place where we originally discovered something was missing.


I thought maybe the solder joint had become weak and needed to be redone. But after working on that area, the sound completely disappeared again, which made me confident that the problem must be related to L204 and CN9.


So I went back through your old posts carefully. With a bit of help from ChatGPT, I realized that the resistor we originally placed on L204 (47Ω) which I had laying around at that time, should actually be replaced with a 100Ω SMD resistor. I found a 100Ω SMD on an old RC car board We had at laying around at the office and replaced the previous resistor.


After reinstalling the PCB, I turned the piano on — at first the buzzing noise came back, but after a few seconds I started hearing sound from the demo I had running. A few seconds later the buzzing disappeared completely and the piano played normally again.


I’m not sure if this second replacement at L204 is the final permanent fix, but for now it’s working, and I really hope it remains stable. I wanted to give you an update and also apologize for not replying for such a long time. I’m incredibly thankful for your help — even months later, your explanations allowed me to diagnose the problem again and fix it. I’m really happy that I asked my question here.


Thank you again for all your support.

 

Thanks for coming back with this and thanks for the kind acknowledgement. Intermittent problems are some of the most difficult problems to solve. As they say, "You can't fix it if it ain't broke!"

I will have to take some time to reread the thread and study what we did.

Hope your honeymoon was sweet!

Edit: Ok, I reread the entire thread to see what had transpired. I didn't know I already said the same thing in post #80.
The fact that the buzzing sound returns and the disappears is suspicious and annoying. I think this behaviour is important and it needs further investigation. I have to have a closer look at the schematics in order to give you some test points to examine with the oscilloscope.

 

If the keyboard continues to work without problems, leave it alone.

For future reference, here is a summary of what we know.

1) The keyboard presented with no sound on speaker and headphone.

2) No +3.3 V was present at MASTER VR, master volume control connector CN9 pin-1. There was no connection across L204.
What happened to L204? A 47Ω resistor was put in its place for testing. Sound returned.

3) New symptom - noise on first power ON, goes away after a few seconds. (2) and (3) could be unrelated.

4) For future testing, remove the resistor at L204 and determine and confirm the track paths from +3.3D leading to R252 and CN9-1.

5) Examine digital and analog signals at the DAC circuit when the intermittent noise persists.
Is the noise present in the analog signal from the DAC?
Is the noise present in headphones as well as speaker output?
Is the noise the same regardless of the VOLUME control setting?

Reference: <Page 2: N-3>

 

Hey,
I was testing the piano over the past week, and the buzzing problem occurs every single time I power on the piano for the first time of the day.

What’s strange is that if I leave the piano plugged in but turned off for a few hours, the buzzing sound comes back when I try to play it again. If I start a demo, it changes the character of the buzzing sound, every time I change the demo piece.

My current workaround is the following:
I start one of the demo songs and let it play for a few minutes, then turn the piano off, unplug the power cable, press the power button for it to discharge and then power it back on. After that, the piano often works normally again.

However, this is not consistent — sometimes it works, and sometimes it doesn't.

Another odd behavior is that in some cases you can hear both the buzzing noise and a very faint piano sound at the same time in the background.

Maybe the resistor at L204 is still not enough?

 

We can do some partial diagnostics without opening the piano.
Is the noise present in both the speakers as well as headphones?
Is the noise the same regardless of the volume setting?

 

We can do some partial diagnostics without opening the piano.
Is the noise present in both the speakers as well as headphones?
Is the noise the same regardless of the volume setting?

Hey sorry for the late response.

Both speaker and headphones have the same Noise.

The Noise is the Same if I Play with the volume Setting, only thing that changes is it gets louder.

It’s a bit weird, sometimes I just have to wait for it and let a demo play until it just works by itself. Here a video that shows the issue perfectly: https://drive.google.com/file/d/127Z7-4yxA9gn08Kjl0zcxvSOgVHEyJTG/view?usp=drivesdk