Atx fan is spinning with USB power and z and extruder motors aren't working

Help!!
My atx powersupply fan is running with just plugging the mega to laptops USB.and at first my extruder motor wasn't running I thought it's drivers fault I changed it and then still it wasn't working so I thought it's mosfets fault so I changed the ramps board and when I plugged the printer with my pc without turning on the atx the atx fan is spinning and all the printer fans are spinning now my z and extruder motors aren't working please help.what should I change mega? Or the atx? Are my motors burned? Thank you!!!!

My printer has fans that run when the board is hooked up to usb, that's normal. Not sure why your psu fan is running from usb though. I couldn't understand much of what you said, can you tell us exactly what is and isn't working right now. As for burning out the motors, I find that unlikely, they're probably the most bulletproof part of any printer.

When I try move my motors they are making very loud sounds and not moving at all but not all the motors though,my x and y motors are working my z axis motors and extruder motor are not working,the current now is 0.7a I tried to increase and decrease this but no use.and the old ramps extruder drivers mount was molten when I
swapped the driver out. Is this my atx doing or any short in my mega? Cuz I just replaced ramps and drivers so I'm sure they aren't the culprits. Thank u so much

Sounds like something overheated on the ramps there. I'm not the best person to ask but I'd have to guess there was a short somewhere, possibly the motor coils could be shorted either internally or at the connectors.

If swapping the ramps and drivers didn't fix anything check the motor connectors and motors, with a multimeter if you can. Maybe replace the mega and see if that helps? You could also double check the voltage the atx is supplying to make sure its not set too high or something. The ramps is only made to work with 12V so if you have a 24V psu thats an issue.

I've changed my Smps and now the fan of it is still spinning with the USB power all that left unchanged is Arduino mega will changing it solve my problem? And now my y axis isn't working the z extruder and y motors are just buzzing without spinning. I'm really broke to buy any new parts so any experts on which part is exactly causing my problem so I can change it with new.thanks

Have you tried to tune your stepper driver Vref? If the stepper drivers are underpowering the motors they won't move right. Buzzing is generally a symptom of power issues. It can also mean an issue with how the motors are wired (RAMPS connectors for the motors are IIRC A+, A-, B-, B+ -- reversing the connector reverses the motor direction.

As far as your fan running, that's largely going to depend on how things are wired. Normally the RAMPS won't try to put anything to the 12V line just because the USB bus is powered, but so long as it isn't overvolting the USB bus when the 12V power is on, I suppose that's not too harmful. Do you have a multimeter? You seem to have more than one power supply, so there should be some troubleshooting that you can do.

ATX power supplies power the fan from the 12V bus, so if there is power going to that bus from whatever it's connected to, the fan will spin.

Your RAMPS board has 2 points that connect to 12V. One is the line that powers the main controller, the other is for the heated bed. best would be to see which one is getting voltage when you power up the USB using a multimeter, because you'll also want to know if the USB bus is getting 12V when you power on the PSU. If so, that's a bad situation, and SOMETHING is broken. I wouldn't worry if you're not getting 12V on the USB bus when the PSU is powered on, unless you're getting 5V on the heat bed input, since that is supposed to be isolated from the main supply. (You'll get 5V on both if they're connected to the same PSU when you test.)

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MBot3D Printer
MakerBot clone Kit from Amazon
Added heated bed.

Leadscrew self-built printer (in progress)
Duet Wifi, Precision Piezo parts

Hi thank you for ur words and it's 5v that's going and coming out of the ramps and Arduino , when I connect via psu or via USB and it's over the microcontroller line than the heatbed one though I didn't check over heatbed port.any further trouble shooting

No nothing over heatbed line, thank you so much btw.

Now it isn't responding it's connecting but not taking xyz commands when I press the button to move it's just blank but when I turn the cold extrusion and extruder it's showing that filament is being extruded and when I press the button to move x or any axis it's saying echo busy processing

Okay the atx connection was loose but now everythings working expect for the extruder motor I'm sure with the hardware and cold extrusion I even changed the vref what else would be the problem