PSU troubleshooting

[printbus]

...Sorry but I didn't understand what you are talking about regarding the AC voltage.
"RAMPS and LCD will power up if the printer is turned off but connected to the computer via USB." Mine does this as well.

All I was getting at with the AC measurement is to put the meter in AC volts mode and look at the same power outputs from the power supply. It's not a good test, but it might reveal something. Ideally, the 12V DC output should read 0V AC. If the meter shows a considerable AC (ie, not fixed like good DC would be) component, the power supply might have issues.

My point on powering up RAMPS and LCD via the USB port is I wouldn't do that. I'm not saying you are - it's just one way that people can have unexpected current flowing through the USB.