Disaster

[Raypat]

I received my controller board for the igaging digimax scales a few days ago. After carefully mounting two mini usb breakout boards and the controller board in a plastic box I soldered them up checking and re checking the connections. When it came time to power it up I was confident everything was connected correctly. I am only using two outputs because I do not have z axis dro at the moment.The only doubt I had was the polarity of the power supply as I had to source a different connector to suit the board. I connected positive to the centre of the plug and tested it with a meter. The voltage polarity seemed correct with a positive reading so I powered up the board. Everything worked as expected with the three leds flashing. Just to be sure I put a meter across the output voltage to the scales. With the red lead of my meter to 3v and the black lead to ground I got a negative voltage at this point. Thinking I had the polarity reversed I quickly unplugged the power supply and reversed the leads to the plug. You can guess by now what happened next. On power up I got instant smoke and the board is now dead. My question is can the board be repaired by me (a machinist rather than an electrician) or have I made a hundred buck mistake? Also is it to be expected to get a negative voltage at the output to the scales? Thanks in advance to anyone who can help.