I scored a Haier HWR12XC5 window air conditioner on craigslist last weekend for $20. Owner said he had it for 4 months, then came the thunderstorm. He said lightning hit a pole near him and took out all kinds of stuff in his house. The insurance company cut him a fat check, and let him keep all his dead equipment.
$20 later and a quick trip to my house, here's the unit:
It seems previous owner had done some minor disassembly of the unit, as the bracketry and screws were lost. We don't care, screws are cheap and less work for me. Luckily, this unit is an RCA/GE original "works in the drawer" unit. A frame inside a frame, if you will. The guts just slide out after you take out like 3 screws (something they did with lots of appliances back in the day). I yanked the works outta the drawer, plugged her in, and found the unit was stuck on high fan. The unit wouldn't power up or anything. So I took out 1 screw and out came the control board.
Quickie check revealed main fuse OK, transformer OK but running hot, and hardly nothing after main rectifier bridge. So I unplugged it and started taking resistance measurements.
This unit is so simple, I didn't need a schematic to figure out what was wrong. I just looked up a few datasheets to verify main components. The brain is an 8-bit microcotroller, an 86C807Ng, flash programmed at the factory. It has 1 temp sensor, a thermocouple. An Ir sensor feeds right into the micro, for the remote control that was also lost. Muscle control is done by a ULN2003A, a package of digital switches. When the micro sends out 5 Volts to the switch, it turns on a relay. The other side of the switch can take up to 50 VDC at 1/2 Amp per switch. Easy peasy.
The power supply has 2 parts: raw DC and 5 Volt regulated DC via a 3 pin regulator. I checked the input and output for ohms and got 3 ohms on the input. The 5 Volt line had like 50K ohms.
Guess what the raw DC runs? a bunch of sugar cube relays and the 5V regulator. That's it. Since the unit was forever stuck on high fan, I removed that relay first. Lo and behold, lots of ohms on the raw DC rail now. I set he control board in a safe spot, so it wouldn't short out against anything, and plugged her back in. Voila, lights!
When these units are first plugged in, they light up like a car dashboard and beep. That was somewhat of a surprise. Then they go dead again for like 10 seconds while the micro initializes. After that, you can push buttons and do something. I set the unit to low cool and turned the setpoint down and it fired right up. I let it do its thing on low and took more readings.
The high fan relay control line had not changed a bit after setting the unit to low cool: zero volts and like 3 ohms. A few minutes after startup, the fan motor shut off unexpectedly while the compressor remained running. I unplugged the unit and didn't even need to touch the fan motor: I could feel the heat on my face when I looked close. It had shut off on high-temp limit.
Now tell me what's wrong, batman.