USB Powered Music Device issue (turns on but not always works)

Thread Starter


Joined Dec 20, 2023
Hello everyone!

I'm currently having what I suspect is a power issue with a musical device (small drum box "Akai MPX8").
It's just 2 years old, used only a few times and very well preserved (almost new). A while ago it sporadically started glitching, and as it's something you hold in your hands, I blamed the power cable (being accidentally moved, not making good contact, etc.)

Now the situation escalated, to the point that it will switch on, respond properly to all interactions, its screen will show perfectly fine; but all button lights will constantly blink fast (the blinking is not noticeable at all unless you really pay attention, but it's there) and a buzzing noise is audible through the speakers. Also the output volume of the machine is very very low when this happens.

How to reproduce the issue: Just plugging the machine to a PC or power outlet (USB cable is the only option) and switching it on with its slider switch.

How to workaround the issue: it's possible to fiddle with the ON switch, moving it to a certain intermediate point where the lights will stay still. Once that point is found, directly unplugging / plugging is the best, not touching the switch, so next time it won't glitch (when working good, output volume is really good, lights don't flash, buzzing noise is not present).
Now, what is really interesting is that this workaround only works with 2 of 4 or 5 USB cables I tried. With the others it's not possible to find this sweet spot with the ON switch.

The machine has a separated small board for the power input and MIDI connectors: it basically has an optocoupler, a USB microcontroller, a diode, 1 electrolytic 10uf 25v capacitor (measured it with ESR meter, seems to be ok; visually good as well), the USB-B connector, 2 mini jack connectors (for jack-DIN MIDI cable), several SMD resistors and capacitors and not much more.
I don't have a lot of knowledge on power supplies unfortunately, but I suspect the microcontroller, or the switch... (the optocoupler is probably there to isolate from other instruments that may be plugged through MIDI cable?)
The strange part for me is that it "sometimes works" (it ALWAYS turns ON in the ON position, and ALWAYS turns OFF in the off position). And even weirder to me: it depends as well on the cable.

Could you please help me with this? Any advice, hint or recommendation will be appreciated! I can open it again and take photos, share component codes, etc.

Thanks a lot in advance!
Last edited:


Joined Oct 2, 2009
Welcome to AAC!

I sounds like a power supply problem.
Can you post sharp, well focused photos of the circuit boards? The photos have to be sharp enough so that we can read any markings on the ICs.

Thread Starter


Joined Dec 20, 2023
Thank you for your reply,

I hope these are good enough:

There are 3 boards, as I understand the little one is the power (with USB input). Then a brown one that I guess it's for the output (audio out plugs are there) and the big one must have the logic, UI: buttons, screen (yellow flex?), SD card reader, etc. I suppose there are a few audio chips, a couple multiplexers (74HC?)

All looks brand new, no strange smells, caps show no bulging.

I can record a video of the machine when misbehaves if necessary, or try to provide any further info