r/AskElectronics • u/CyberCow3000 • 1d ago

USB output problem with MIDI keyboard

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I have an Alesis Q49 MIDI keyboard, in which the USB port doesn't work, but the normal 5 pin MIDI does. It had an issue which is very typical to this product, that the USB type B connector broke off. I soldered in a new one, checked all the connections, but it still doesn't work. The computer says Device couldn't be recognised. I have traced back the data+- lines (shown on picture), it leads through two resistors to two output pins on the microcontroller. When plugged in, both lines have 3.14 V on them, unfortunately I can't check the signal since I don't have an oscilloscope. Is there any way I could fix this?

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u/Lostdotfish 1d ago edited 1d ago

Close up clear photo of your port rework please.

Circa 3.x V is expected on the data lines (logic level operates around 3.3v)

Is the device powered by USB or does it use an external power supply?

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u/CyberCow3000 1d ago

I can send a photo of the rework, but it's not really visible, since I had to use jumper wires, and covered the whole thing with hot glue to make it stronger. But there is continuity in all lines, so that shouldn't be an issue. The device is powered with USB or external supply, in both cases the same problem persists.

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u/CyberCow3000 1d ago

https://ibb.co/60Jh3BK9

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u/Lostdotfish 1d ago

Your problem is there, almost undoubtedly.

Is the USB socket on the same PCB as the MCU or does it connect from a separate board via wire connector?

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u/CyberCow3000 1d ago

It's on a seperate PCB, and it connects with a wire connector.

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u/Lostdotfish 1d ago

Any chance you accidentally crossed the data lines?

If it were me, I'd take a known good USB A cable cut the end off and expose the 4 wires. Solder, the wires to the underside of the wire connector on the MCU board and test. Crossing D+ D- isn't usually harmful, so if you're in any doubt, you can try them both ways

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u/CyberCow3000 1d ago

Just done that, unfortunately that didn't fix it.

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u/Lostdotfish 1d ago

at this point - I'd suspect MCU damage and probably wouldn't waste any more time on it.

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u/CyberCow3000 1d ago

Yeah, likely that's the cause. I just find it weird, that the MIDI communication works, which is also done with the MCU. So that would mean only the USB output pins are gone. Thanks for all the help!