(iMac Late 2013 - 21.5" 2638) I have had a lengthy repair starting with trying to upgrade to a SSD and upgrade the RAM; I purchased both from iFixit.com. Unit starts up; i.e. I can hear the fan spinning and after several seconds I hear a start-up chime, but the screen stays black; no apple logo. I have checked and the LED lights on the motherboard are all lit (except for the top / 4th one). I think this is happening because there's no OS installed on the new SSD, but I don't think I can do the internet OS install as I have a wireless keyboard / mouse that aren't paired to the computer yet which I think you use to initiate the OS download process. Thanks for any guidance!