Hello, my name is Rusty. Here is the block diagram of the piano:
Keys --> Tone Generator --> Pitch Bender --> Amp circuits
You said something about the "audio board", which I/O connectors are problematic? Are you able to play the piano with the I/O circuits disconnected?
Yes, and the problem is usually in the pitch bender circuitry. If a capacitor starts to leak, it will start to pop. I want you to disconnect the output of your pitch bender just before the amp circuits, and test for DCV on the output. If you see spikes indicating the pops, reconnect the amp, and disconnect the pitch bender from the tone generator. Look for those same spikes. This will tell us if the input to the pitch bender is clean, to determine whether it's coming from the tone generator or the pitch bender.
In my experience, it's usually in the pitch bender circuit.
The tone generator is connected to the motherboard, power supply, and pitch bender. The pitch bender is connected to the tone generator, power supply, and amp circuits. It's the smallest board in there, with all of the resistors, and it's connected to the pitch wheel. The processor is on the motherboard.
We need to check at the pitch bender input and output for spikes.
I'll be in and out today. I have to stock up on audio gear for the year, but I'll check in periodically. Run the tests I suggested, and use the process of elimination to locate the pop.
No, the bender wheel would be in plain sight. I like what you've done here, and now we have it isolated to the tone generator. That's not a good thing.
Does the popping remain consistent on each key? How about does it make a difference how much popping occurs by how hard you hit the keys? I'm checking to see if it's a blown capacitor in your tone generator. Since you could insert tone without the popping, that tone also goes through your pitch bender, so the problem is now isolated to the tone generator.
I like what you said about the preamp stage as well... Check for spikes, the tone generator is quite a bit more to troubleshoot than the pitch bender.
Disconnect the tone generator from the pitch bender. Check just the output of the tone generator. That should generate the spikes. With the lack of a service manual, you'll have to trace it back.
On that particular model, do you have two or three signal wires coming from the tone generator to the pitch bender? I believe it sends a stereo sound, but I could be wrong on the Kurzweil.
What about when you reconnect that speaker, and disconnect the other one? Does it play clean?