Second extruder in my prusa turns on automatically with out any command when power supply is turned on.

I'm using printrboard Rev D with Dual extruder board and FW is marlin.. If no USB connection is established( USB is not connected to USB port ) and power supply is ON , second extruder turns on automatically and temperature increases without any limit. It already melted two of my j head hotend assembly. First extruder doesn't have any issue. But if the USB is connected to port (doesn't require connection with host, i.e., Repetier) and supply is turned on, then there's no temperature rise.

Supply USB connection Extruder 1 Extrude 2


Shouldn't second extruder be OFF in all condition unless it's turned by either command or manual control in Repetier

I made a video of what's happening

In the video, power supply is already on(not shown in video) As you can see when only the power is turned ON and no usb connection established(usb cable is not connected to usb port), second extruder starts heating up, when connection is made(you can hear the sound as I plug in USB cable) you can see the current extruder temp( extruder 1 is at room temp but extruder 2 is not ). But as soon as the connection is made temp increase gradually reduces and finally stops then slowly return to the room temperature. But if I remove the usb cable again, temperature starts increasing.

I changed the dual extruder board with new one also changed the flat ribbon type cable connecting Printrboard and Dual extruder board , re-flashed FW, even swapped hotends , but no matter what hotend connected to the dual extruder board heats up when supply mains is turned ON if USB is not connected.

I don't have a spare Printrboard to check..

Could this be due to any issue with Printrboard?

Anyone else had issue like this?

AS far I remember this won't happened before. Both extruders were off when the power is ON and USB is not connected(But I'm confused, I can't remember)
What I don't understand is second extruder doesn't heats up if USB is connected to PC USB port or Raspberry Pi, it doesn't require connection with the host software. But as soon as I remove the USB cable second extruder starts heating up without any limit until USB connection is made.

If I'm not mistaken USB is connecting to send serial data as well as powering the micro-controller(In some boards like smoothie).

But from the schematics there's a 5V regulator in printrboard

SO in the case of Printrboard it's sole purpose is serial communication(I guess)

I don't understand what changes when connecting USB.Does that mean my printrboard having problems???Like damaged pin or regulator. It doesn't take less than 10 seconds to reach 100 degree celcius from room temp. IF this was happened to me before I would've noticed since usb port at the printrboard side is quite loose, it often disconnects if i touch it accidentally.