Ninja DCT401 Double oven/air fryer Possible microprocessor issue?

[tbsmith]

I'm having a random issue with this otherwise nice double oven/air fryer. It sometimes does not respond to the controls. Sometimes only the upper control buttons work, sometimes they all work and sometimes none of them work. The power supply is good as the control board has a steady 5V and 3.3V. The pushbutton switches are all good as tested with multimeter. When the buttons are working, there is about 2.5V to the high side of the pushbutton. I'm assuming pressing a button grounds the input pin to the microprocessor. When the buttons are not working, there is about 0.3 volts on the high side. Since some buttons work (with the 2.5V) at the same time as the others that don't indicates to me that the power supply is okay. Leaving it unplugged overnight sometimes helps, but not always. All solder joints seem to be good under magnification. I'm thinking there might be a bug in the firmware or a problem with the microprocessor due to the random nature. Does that sound like a possibility? I purchased an FTDI USB adapter hoping that I can check the firmware, but having never used one before. Is that something one can learn fairly easy, or is there a steep learning curve? The micro is an STM 32 ARM Cortex.

I couldn't find any youtube videos on repairing this model and no luck finding a schematic either.

[CapLeaker]

I am betting that the firmware on the STM32 is locked. Only thing you can do is to swap in a new board. I haven’t gotten a device with an STM32 that wasn’t locked. Look around on Ali… maybe you find a board for cheap.

[tbsmith]

I didn't find any control boards on Ali, but saved a search on eBay, something may come up. Thanks so much for sharing your experience with STM32's; that saved me a bunch of time trying to learn how to program or even access it! Worst case is that I will strip it for parts.

[CapLeaker]

Maybe call the manufacturer Ninja and ask them for a replacement board?

[lotas]

First, check the processor power supply for pulsation (or replace the electrolytic capacitors in the power supply), regarding the buttons, membrane buttons can "leak", that is, have a leak between the membrane and the contact pad and create internal resistance and a false triggering occurs, while no button will work (this does not look like a firmware failure).

[tbsmith]

First, check the processor power supply for pulsation (or replace the electrolytic capacitors in the power supply), regarding the buttons, membrane buttons can "leak", that is, have a leak between the membrane and the contact pad and create internal resistance and a false triggering occurs, while no button will work (this does not look like a firmware failure).

Thanks for the suggestions. The power supply was good. The push buttons are discrete surface mount and all tested good. After letting it sit on the bench for a few days, the behavior changed to where only the rotary encoder would power it on (but not in the way it is supposed to). Anyway, this convinced me that it is unrepairable so I stripped it for usable parts and trashed the rest.

Thanks to all for your help!

[momaka]

I 2nd what lotas mentioned in regards to the buttons - sometimes they start to partially short over time, and the symptoms could be completely intermittent too. I've had a few monitors suffer due to this and randomly pop up menus or change their brightness and etc. In some cases, it's temperature-dependent and nothing happens until the monitor heats up. Nearly pulled out all my hair with a 24" Dell some many years ago due to this. Just because the buttons appear to read OK when checked once with a multimeter does not mean they are OK. The best but probably most painful way to deal with this is to run some external wires from the buttons and have at least a few multimeters checking the voltage going to them. Or alternatively, remove the buttons and have external wires that you can manually connect... though DO be CAREFUL with that, as some cheap appliances do not use isolated power supplies and can be a HAZARD to work on without a ground isolation transformer.

Anyways, I know it's probably a moot point already mentioning this, now that you scrapped it. But just in case anyone else ever stumbles onto this.

[tbsmith]

I 2nd what lotas mentioned in regards to the buttons - sometimes they start to partially short over time, and the symptoms could be completely intermittent too. I've had a few monitors suffer due to this and randomly pop up menus or change their brightness and etc. In some cases, it's temperature-dependent and nothing happens until the monitor heats up. Nearly pulled out all my hair with a 24" Dell some many years ago due to this. Just because the buttons appear to read OK when checked once with a multimeter does not mean they are OK. The best but probably most painful way to deal with this is to run some external wires from the buttons and have at least a few multimeters checking the voltage going to them. Or alternatively, remove the buttons and have external wires that you can manually connect... though DO be CAREFUL with that, as some cheap appliances do not use isolated power supplies and can be a HAZARD to work on without a ground isolation transformer.

Anyways, I know it's probably a moot point already mentioning this, now that you scrapped it. But just in case anyone else ever stumbles onto this.

Thanks for the new info. I had the exact issue with one of my HP monitors. I had to remove the pcb containing several pushbuttons that controlled the menus. It wasn't a problem since I never have to change the settings. But based on the behavior of the air fryer, I'm not sure that was the issue. Too bad I scrapped it, or I would have done more testing. I would have just removed all the pushbuttons and used a jumper wire to test. Oh well. The other weird thing is that two previous units that were replaced under warranty had the exact same problem, but I don't see any others posting about it.