RAMPS 1.4 without D1

Hello,
I'm the (almost) happy owner of a prusa i3 with RAMPS 1.4, that has never wanted to work. I was having some weird voltage issues where arduinos would get fired if I connected them to my PC, so today I tried a different approach. After reading quite a lot on the topic (and boy, was it difficult to gather information...) I carefully desoldered D1 so that I could power my Arduino straight from my computer without experiencing magic smokes or similar.
Since I was full of old RAMPS and fried Arduinos, I did my experiments. Everything seemed to work fine, at least no smoke was rising at any stage. So I connected RAMPS to my working arduino, plugged the 12v in and connected the Arduino through USB to a Mains adapter, just to see if everything worked fine. The printer turned on, no smoke, no fire. Then I connected the USB cable to my PC. And the LCD (I'm mounting a Full Graphic Smart Controller) started flickering on and off, without displaying anything. Also, the USB cable (the same I was using before) started to slowly heat up.
So I don't know what I should do... I'm still not able to talk to my printer. Also, if I try to use the display controller while on mains and tell it to home all axis, nothing will happen, it just freezes.
What could it be? A broken Arduino? Broken RAMPS? Both? Or is it just me completely missing the point here?
Thanks in advance!

You can't power the electronics with GLCD just from USB you also need a power pack, if you have removed D1... and maybe not even then as ramps has a marginal voltage regulator ...

If your USB cable i heating up there is probably a voltage differential, either your USB or the mega is not generating 5v correctly. Or you just trying to pull to much current threw a crappy usb cable.

I would check out your voltages first.

Just with a mega plug in the a dc power pack and check the 5v line with a multimeter, also check the 3.3v lines

Now remove the dc power and plug in the mega to the usb and check the 5v and 3.3v line again

They both need to be a clean 5v and 3.3v

see [yourduino.com] for where you can find 5v and 3.3v pins

Secondly I would remove all pololu drivers while testing.. as a dead one of those can cause all sorts of issues, also make sure you have them in the correct way, different type of pololu modules goes in opposite ways (is your looking at the location of the vref pot)

now add the ramps board only, no lcd, no pololu modules and re test.

Then add in each pololu module, one by one and re test after each one is added

If you have everything in and working without the LCD... some one else will have to help you with that.

Edited 1 time(s). Last edit at 09/18/2016 07:47AM by Dust.

Thanks for the reply! I have done as instructed, and in fact I'm getting very weird values...
I get 4.24 V on the 5V pin and 0V on the 3.3V pin when connected via dc power (12V), so this isn't consistent at all with what it should be.
If I connect it via USB i get a better 4.87V on the 5V line and still 0V on the 3.3V line.
Does this mean I have to buy another Arduino Mega? There must be something wrong with the power regulator, right?
What if I input 5V in the Vin pin? Does that make any difference?
I still don't understand why it would be more under load when the USB is plugged into the PC and not when its off the mains...
Thanks again

I would say the voltage regulator on the mega is dead and is pulling the 5v low even when not in use

Something else is also dead to kill the 3.3v line...

When connected to the pc over usb, the usb provides 500ma, when plunged in via power pack on the board regulater provides more than 500ma (about 800 from memory)