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Rusty
Rusty, Music Professional
Category: Electronic Musical Instruments
Satisfied Customers: 5533
Experience:  Sound Engineer, Electrical Issues, Tone Consulting
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Name is ***** ***** and I have been asked to repair a

Customer Question

Name is ***** ***** and I have been asked to repair a Kurzweil Mark 10 piano. The piano has loud popping in the audio, speakers, phones, and audio output jacks. Can't seem to isolate the problem and have no service manual/schematics.
JA: Thanks. Can you give me any more details about your issue?
Customer: The piano sounds do come thru, but the popping is much louder. The volume of the pops go up and down with the main volume control. When I unplug any of the I/O connectors from the audio board, the popping (and the piano sounds) all go away-nothing comes out.
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Submitted: 10 months ago.
Category: Electronic Musical Instruments
Expert:  Rusty replied 10 months ago.

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?

Customer: replied 10 months ago.
Hi Rusty,
There are AC power, DC power, Sliders, Phones, Left and Right Tweeters, Left and Right Woofers, and Audio I/O connectors on the audio board. Obviously if the AC or DC power is pulled you get nothing. By nothing I mean no popping and no piano sounds. When I pull the Sliders connector (Bass, Treble, Main Volume) it breaks the circuit to the audio amp section and I get nothing. The headphone jacks appear to be break jacks that allows quiet practice by disconnecting the audio feed to the amplifier/speakers. If I pull the Phones connector on the audio board, I get nothing. Now I am not sure about the I/O connector to the back panel. Thinking back, I don't think this connector had any effect on the speaker output or headphone output, since it is for external inputs and midi I/O.
I disconnected the speaker connectors and just used my headphones to verify that the piano sounds were being generated. They were, but the popping was much louder. But the tone generator/pitch bender all appears to be working OK.
I suppose it is possible for something to be loading down (breaking down?) to cause the power supply rail to either go to ground or momentarily spike to a higher voltage, causing the amps to pop. I could put the O-scope on the various IC pins to look for the pop, but without a schematic, I would be truly shotgunning. I like to take a more systematic approach to trouble shooting, but don't know where to start on this animal.
Expert:  Rusty replied 10 months ago.

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.

Customer: replied 10 months ago.
Sorry for the slow reply. The piano is still in the customers home. I may be able to get back into it tomorrow. Is the tone generator and pitch bender under the shield, I assume with the processor?
Expert:  Rusty replied 10 months ago.

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.

Customer: replied 10 months ago.
Rusty, I hope to get back into the customer's home tomorrow and dig back into this piano. I may need your further expertise after getting back into it. That is why I have not yet rated your help. So, please be patient with me. Thanks, Terry
Expert:  Rusty replied 10 months ago.

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.

Customer: replied 10 months ago.
Rusty,
I did extensive testing today. Found the pitch bender board and disconnected the flat cable and the cable going to the amp board. The popping continued. Also noticed intermittent 100KHz oscillation at the left tweeter output. All DC supply lines looked clean. Fed in an external test tone through the back and all amplifiers/speakers reproduced the tones cleanly with no popping. But when I tried to play the keys, the popping began again. Not sure if this piano has an actual bender wheel, unless it is under a protective cover. Regardless, I was able to trace the keys tone input signal onto the amp board. It did feed the right power amplifiers (both treble and bass), but it never made it to the left power amplifiers. Tried to isolate the actual preamp IC that the signal was being lost, but it appears the first IC has a combined external and internal signal present. Do the get mixed in the FETs ahead of the first IC? The IC's appear duals set up for left and right audio. After going through the first preamp IC, only the keys audio for the right side was getting passed on. It appears I am lossing the keys audio in the preamp stage and for some reason is causing an intermittent pop that drives it into oscillation. Sure wish I had a schematic. Sorry, but it was not the pitch bender.
Customer: replied 10 months ago.
Sorry, I meant to send pictures of the interior and the left side of the keyboard where the pitch wheel should be.
Expert:  Rusty replied 10 months ago.

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.

Expert:  Rusty replied 10 months ago.

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.

Customer: replied 10 months ago.
I connected the scope probe to the audio input from the volume control on the Sliders connector on the audio board. It appeared to be mono at that point. It did go up and down with the volume control and appeared to be clean regardless of the particular key or how hard it was struck. When I looked for the audio leaving the amplifier board going tot he headphone jacks, I saw the audio for the right side leaving on the Phones connector and returning from the headphone jacks. However, there was no sound leaving the board for the left channel nor returning from the jacks, for the keys. When using the external input, I saw clean audio leaving for both left and right and returning from the headphone jacks.
Expert:  Rusty replied 10 months ago.

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.

Customer: replied 9 months ago.
Rusty, after thinking about how I left the piano on my last visit, I am confused about the tone generator. Does it generate a stereo (left/right) output or is the tones mono going to the bender and then to the amplifier board? I left one woofer speaker connected to one side only and the piano plays clean (no pops) even with the master volume set to max. If the tone generator was causing the spikes, would they not be on this one amplifier/speaker? There must be a mixer on the audio amplifier board that combines the external input with the tone generator. That area of the audio board has a great deal of discrete transistors or FETs, so they are difficult to troubleshoot without being able to follow the board traces. Is it still possible that the mixer area is the place where my tones are being cut off and the popping is taking place. I also noticed that there is active filter sections to separate the treble for the tweeter amps from the bass feeding the woofer amps.
Expert:  Rusty replied 9 months ago.

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?

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