LCD Doesn't turn on unless connected to USB

[nzhu]

I have the i3 Prusa that comes with an LCD screen+SDCard reader.
The board is a MKS base 1.2.

In addition to a million other problems I'm having, there's some other weird stuff.

The LCD only turns on when the board is plugged into the USB.
The LCD doesn't turn on when the PSU is powered on and connected to the board WITHOUT the USB connection.
When the USB is plugged in with the PSU off, it turns on.

Is it normal for the LCD to only respond to the USB? That seems unreasonable because that defeats the purpose of the LCD screen + SD card reader which is meant to print without being plugged into the PC

[Starlord]

Yes I had that problem with my Rostock Mini. The problem was an intermittent power plug. The controller was not getting power from the power supply. But when connected to USB the controller and display was powered. But the motors and heater were not

[curious aardvark]

yep sounds like it's wired up wrong.

[Marm]

Sounds like when I tried to change radios in my '85 Chevy Blazer. I messed up the wiring and the headlights would only turn on if I also had the radio on.

[morgue427]

Could also be blown aurdino voltage regulator blew 2 gettingy prusa up and running. Clip the axis if youose it the bed sliding is enough to blow it I found