Airthereal APH260 mcu appears to constantly reset

[vgamesx1]

My air purifier does this weird flickering/resetting, at first you might rightfully assume it's a power supply issue, however as you can see in this video I've uploaded a demonstration of the problem where I'm injecting 5v to the board directly, also double-checking while powered from mains, the 5v supply looks solid and since there aren't many components, I've checked nearly all of them for shorts but nothing appears to be wrong with the support circuitry at least as far as I can tell, so my best guess at this point is the MCU firmware is corrupt, or it's partially dead, and it crashes as soon as it tries to boot, any other ideas what could be the cause of this? The name of the chip is sc91f844p but the datasheet is mostly in Chinese.

One noteworthy difference between mains vs injected power, is it turns on or hops between all the triacs for L/M/H rather than leaving just one open and after several seconds it seems to enter sleep mode, as the lights will turn off, it stops clicking with the buzzer and the fan spins on its lowest speed rather than sort of jumping between speeds.

[stj]

Re: Airthereal APH260 mcu appears to constantly reset

if those springs are touch-switches i would resolder the transformer and replace all the caps with good low impedence ones.

any electrical noise could be picked up as a "touch" on the springs!

[vgamesx1]

Re: Airthereal APH260 mcu appears to constantly reset

Huh, for some reason I hadn't thought about noise on the switches but that makes a lot of sense, thanks!

Wait but wouldn't the noise have to be from the caps then? Because the transformer shouldn't have any power when I powered it from my supply, right? Well anyway I don't have any good caps just a bag of cheapos and I can't replace the 400v ones either but I'll replace the ones I can and update if it works later.

[stj]

Re: Airthereal APH260 mcu appears to constantly reset

i'm not concerned with the 400v one,
it's the smaller ones that may be dealing with high frequency's

if you want to try an experiment,
ground all the springs and see if it still does random stuff.

[sam_sam_sam]

Re: Airthereal APH260 mcu appears to constantly reset

I could be wrong but I think I see a very small capacitor under all those capacitors grouped together if this is the case for now just replace this one and see if this makes a difference or not ( it is on the left side of the board )

[vgamesx1]

Re: Airthereal APH260 mcu appears to constantly reset

I know it isn't the best photo but there are a couple, could you be more specific? C5? C6 (it's the one next to R6)? C6 and C11 appear to be a part of the power line btw, not sure what C5 is doing (maybe feedback? I see 2.5v on it).
Wait you're not thinking EC2 is a cap are you? Lol if that's what you were referring to sorry about that, it's the regulator.

Edit: Maybe it was something I did while poking around, but now I'm seeing a short on C6 and C11 or something else there is making them appear short...
Actually, it seems to show a short until it charges up then it goes away, then if I flip the leads same thing, how odd this is new for me.

[vgamesx1]

Re: Airthereal APH260 mcu appears to constantly reset

if you want to try an experiment,
ground all the springs and see if it still does random stuff.

Update, I tried your experiment by placing some metal tweezers over the springs with a croc clip to ground and it'll sometimes kinda pause for a moment as if it tries to boot then it crashes and goes back to the previous resetting, I also removed C6 and the electrolytics surrounding it, then bodged a through hole cap in its place, this hasn't changed its behavior either.

[vgamesx1]

Re: Airthereal APH260 mcu appears to constantly reset

So I replaced not all but most of the caps on the 5v line and not much has changed, it will pause and occasionally do some funny flickering of the main display, but it'll soon go back to clicking away, I'm not so sure that this going to work.

(Note the electrolytic caps I removed have been replaced on the under side of the board as it's more accessible.)

[sam_sam_sam]

Re: Airthereal APH260 mcu appears to constantly reset

What is on the other side of the board ?

[vgamesx1]

Re: Airthereal APH260 mcu appears to constantly reset

Just a few test points, but they aren't useful for me as I don't have an oscilloscope, so the only thing I can see is that it does have a clock, and it's around 60khz, although that said if I can find pin INT0 then I could hook up UART and see if it displays anything.

*Gasp noises* I only just noticed on this page that the chip has Low voltage reset, so maybe if it or normal reset were repeatedly being pulled low it would cause what I've been observing.

After SC91F844 is powered on, before the client software is executed, it will go through the following process:
◆ Reset phase
◆ Transfer to the information stage
◆ Normal operation stage
It means that SC91F844 will always be in reset condition, until the voltage supplied to SC91F844 is higher than a certain voltage, the internal will start to be effective

[vgamesx1]

Re: Airthereal APH260 mcu appears to constantly reset

Well I've replaced some of the remaining caps and read a bit of that datasheet for ideas, but I don't think this is getting fixed...
Of course, I'll keep the purifier since the motors in these things are super simple, so I've just hardwired it to its lowest speed and later on I can wire in a 4 position switch or should I feel compelled to do so, it doesn't look like it would be too hard to desolder the mcu and hack in my own.