My friend found a broken Coway air purifier for free on Craigslist near me and shot me message so I could pick it up. My roommate grabbed it and then all I needed to do was fix it.
After opening it up, I checked the power cord and fuse. Both of those were good. Next I plugged it in and checked the power supply. Thankfully, the power supply board is labeled with what voltages to expect on each pin. Everything was good there.Next was the logic board. I checked all the diodes, just because that's easy to do, and they were all good. I also found a reed switch used in tandem with a magnet to detect if the case is open. For the purposes of testing I disconnected it and replaced it with a jumper. Next I started looking at the ICs. There are three on board: a D78F0513 8 bit microcontroller, a ATMLH920 serial EEPROM and KIA7032AP voltage detector. I checked that the microcontroller had power and it did. Then I looked up the voltage detector and saw that it is frequently used to hold chips in the reset state until their power supply is stable. I checked the reset line and saw that it was indeed connected to the KIA7032AP.
Looking at the reset line voltage, it was about 1.8V, which is still a logic low for 5V parts. My theory was that the KIA7032AP chip had died and was refusing to release the reset line. To test this I removed it and manually pulled the line low while my roommate plugged the device in. After a couple seconds I ungrounded the reset line and the device powered up. I ordered some replacement parts (AZ7033), soldered them in, put the device back together and it worked.
Overall this was a pretty easy fix. I ended up using a slightly different part than the one I replaced but the only difference is that the new one turns on at 3.3V instead of 3.2V. The 3.2V version was out of stock at Digi-Key. The purifier's case is also a bit annoying to get back together, but at least they only used one type of screw. There's also a debugging header exposed which could be interesting to look at, but I didn't bother this time.
If you have one of these that's broken and need advice fixing it, drop me a line.