Bought a RAMPS 1.4 combo off of Amazon, with A4899 drivers and a handful of endstop boards. It powers up with the USB connection, but nothing with the 12V power supply. I don’t see any fuses, so I’m worried I burned something out.
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You probably toasted your 5V regulator.
Make sure those big transistors don’t touch. They shouldn’t touch.
But those regulators have gotten cheaper and cheaper and sometimes the load is just too much. Ryan used to sell replacements.
Well, crap… Looks like I need fix up one of the dead computers I have lying around, because the mega clone I have has a SMC regulator, and I’m really not comfortable trying to replace something like that… I may have to ditch the LCD as well. I’ve been seeing quite a few restarts in knob turns and button presses. My fault for getting cheapo knockoffs. 
Now I just need to build up some bank and goodwill with the wife, then I can order a new controller from Ryan. But I’ll limp along with what I have for now. Builds character and all that…
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It is surface mount, but probably the most forgiving you will find. But a new mega is like $10 or you can just power it through 12V and USB.
FYI the square yellow plastic things are “polyfuses” that supposedly interrupt the circuit wen they get too hot, then reset when things cool down. Some folks replace then with regular fuses, either because they dont trust them, or especially when converting the RAMPS stack to run on 24 volts.
