I am rebuilding the electronics for an old Reprap with a RAMPS 1.4 setup.
If I plug in just the Arduino/RAMPS board into either 12 V or USB then it starts up fine, I can see this when I have the screen plugged in. However, when I start to add driver boards, the screen begins to dim. After I have three boards plugged in the power starts to drop to the point that the Arduino powers off and restarts, even when the screen is not plugged in. I have had this problem with two sets of RAMPS boards and Arduinos and with a combination of 7 different driver boards plugged into different combinations of location.
Has anyone else had any similar issues or have an idea of what might be going on? Any ideas are welcome, I am just about at my wits end with this.
Your power supply may not be able to provide enough current. Check it's output voltage, the input at the RAMPS and the supply voltage (both VMOT and VDD) on each driver.
If the supply is good, you can suspect the polyfuses on the RAMPS board. These are yellow rectangles, designed to protect against over-current. They age with stress, and typically fail to pass their full rated current (but you need to understand if there is a fault which damaged them). The link describes how to remove them.