I've recently built my Ender 3 and am trying to get it to warm up for my first print, but it's failing to heat the nozzle/hot-end. The temperature slowly climbs, but gets to about 135degrees before a 'heating failed' screen shows and I have to turn off the printer. It seems to be heating very slowly by this point, about 20 seconds per degree increase. Everything else seems to be working fine the motors work the bed heats up etc. I did have a minor incident the first time I switched it on, and I blow the stock power supply having it on the 115v (I'm in the UK) as everything on the internet suggested this was the 'right' switch - I blame a lack of sleep the previous night for not realising this daft mistake. I've bought a replacement power supply off amazon (KINGPRINT DC 12V 30A 360W Universal Regulated Switching Power Supply for 3D Printer) which all seems to work fine - same size, just fit as a straight replacement etc. Any ideas as to what may be going wrong? I've tried heating with Fan Speed set to 0 (the secondary fan), it was a self-build (but modular, nothing involving more than an allen key), the wiring on the Hotend/Extruder looks fine.