PSU troubleshooting

[printbus]

Has the long time to bring the heat bed to temp been a problem all along, or a problem that just started? EDIT: Please clarify the warm up time. One part of the first post says 30 minutes average. Another part of the post says it takes 15-20 minutes to heat up. That's a big difference. Do you have a heat bed relay installed? That would speed up the heating at least a bit since a relay has less voltage loss than the on-board FET normally controlling the heater. Heftier gauge wire in the path from the power supply to the heater makes a difference, too.

Sure, you can measure the output of the power supply with a DC voltmeter. Put it across the + and - outputs of the power supply, or the power input pins at the RAMPS board. You should read about 12V DC. If the power supply is providing an output but it's crappy, the DC reading may be misleading since your meter will provide an interpretation of what it is seeing. You could also set the meter for AC volts and measure that. Any substantial AC component (maybe more than 0.2V?) suggests the quality of the power supply output is suspect. If the AC component is low and the DC voltage is low, try adjusting the trimpot on the power supply by the LED while you are monitoring the DC output voltage for the desired 12V. If you didn't do it, this should have been done as part of your build.

On my i3v, I've noticed that the RAMPS and LCD will power up if the printer is turned off but connected to the computer via USB. What I don't know is how much current the printer will try to draw from the USB in this case, or possibly if the printer power supply is on but set too low or has issues. There's a limit to how much 5V current should be drawn from the USB.

If it were me, I'd disconnect the power supply from the printer and work with it by itself until I felt I could trust it. To test the power supply under load, you could do something like power some 12V automotive light bulbs. They tend to draw a lot of current. I'd also leave the USB unconnected until you understand what is going on or you risk damage to the computer you're connected to.