Prusa i3 Rework problem

I have a Prusa i3 build with Arduino Mega 2650 and RAMPS 1.4. I have everything wired up properly. I have uploaded the Marlin firmware. I've read a lot of forums, ideas for fixing the problem but nothing worked so far.

* When I connect to the printer with Pronterface I can move the axes perfectly, but when I turn on heatbed stepper motors start to stutter, even when not moving.

* Another problem is heatbed wont heat up. It stops at 60C. Cables arent to thin, ramps and cables are cool. I have enough power. I have insolation undeneath.

What else could be a problem? What can I do? Thanks for all the help.

Power supply s probably dropping in voltage under load. Make sure it's still putting out 12vdc when the heat bed is on and there is 12vdc at the heatbed.

Thanks for your answer. You are correct, voltage drops under load to 9,2V. I swaped couple of thin cables because I thought they were the problem, but still power supply drops voltage. I have no idea what to do now.

What sort of power supply, what power rating?

I'm using ATX PSU Be Quiet, exactly the same as shown here [reprap.org]. After checking everything, soldering some cables again. I read 11,2V on power supply under load. What can I check? Where could be a problem? Any ideas?

What load did you put on the 5V line? The low 12V reading is characteristic of a group regulated PSU with the 5V line floating high. I use an old car headlight main beam.

My PSU is "Haswell compatible" or "Haswell ready". What this means in practical terms is that these power supplies will not shutdown, fail to start or lose regulation with very small power loads. The advantage for RepRap builders is that "Haswell compatible" ATX PSUs will also generally work without issues with the unbalanced (12V vs 5V/3.3V) loads presented by RepRaps. I just copied it from wiki.

Oh, cool. Mine is ages old out of the spare parts box. I guess there must be some other reason for the problem then.

Can you measure the current draw of the heat bed? It might be too high resistance and not pulling enough current to heat up to temperature. You should be pulling around 10 amps at 12V or 120W in order to get up to ABS temperatures on a standard size bed. You could try measuring the resistance of the heat bed heater but that will be difficult to do without a good meter since it will be in the range of one ohm.

You might still benefit from a 5V load even on a modern power supply. It's worth a shot to see if it will bring your 12V line voltage up.

What is the 12V line rating on the power supply? Are there multiple rails?

Bryan

Double check the wiring on the PSU. I think these are actually double PSUs with 2 x 12v output. The full power is only available if you combine the outputs - reprap instructions indicate you should combine all 4 +12v cables. However the standard controller wont handle all that power. Don't be confused by the negative 12v line - you don't use that.
I don't use this controller but you may need to adjust the current limit on the driver board.
Really need to check the resistance of the bed heater to work out if you can get to temperature but 60c is a bit low. You normally hear of people getting to 90C using a typical 1.5ohm heater which is at the driver current limit.