used different powerSupply now motors wont work

hello forum

I'am using a prusa mendel with:
- NEMA17 steppermotors
- arduino mega2560
- ramps 1.4
- A4988 motor driver
- pc power supply
and i have the following problem:

my printer already worked for a while till i decided to use a different power supply and added a heatbed to it.
old PS: 250W ; 5V/25A ; 12V/8A new PS: 400W ; 5V/40A ; 12V/18A
then happened this:
the motors wouldn't move anymore, the hotend didn't get as hot as it used to get, the mosfet transistors got hot, the tempreture curve of the hotend and the heatbed isn't smooth anymore but does steps of about 5°C.

I already tested the drivers and the arduino and they still work.
I also tried to use the old PowerSupply and reuploaded the firmware several times but still the same problem.

i don't know what to do from here so if anyone has an idea i would be very glad

(auch gern auf deutsch)

Have you put a dummy load on the 5v line?

---

_______________________________________
Waitaki 3D Printer

No i didn't. I'm not quite sure i understand what you mean.

I thought that maybe the transistors got too hot and broke, is this likely and what other parts could be affected?
Do i need to buy new transistors or a new ramp board?

What he means is that PC power supplies are meant to power PC's. Some of them have problems regulating +12V when they do not have a load on the +5V bus, which is usually how they are hooked up with 3D printers. The result is unstable power that could blow up your board. Hence, putting a hard drive or special dummy load onto the +5V bus to keep the +12V regulated:

http://reprap.org/wiki/PCPowerSupply

If you have not checked the input power voltage with a multimeter after putting a dummy load on the power supply, that should be your next step. It might not fix your currents RAMPS board, but it will certainly prevent you from blowing another one up.....