I was putting my pcb v2 (cytron/nano) in tractor and very foolishly connected Battery v12 incorrectly. After turning on I heard a click (probably the relay) and then slight burnt smell. I see no obvious burns on pcb but now I can’t power up cytron and don’t know where to start troubleshooting. Don’t have a ton of spare parts either .
Any thoughts on parts to replace? Any help would be great !
Haha ! I’ve done same stupid thing a few months ago , check this Tips and tricks on how to assemble PCB v2 - #31 by Tooki57 . If you are lucky , just repair the track with a wire .
Thanks for the tip! What did you end up doing to fix? I tried going from onoff switch to d3 but keep having same issue.
I wonder if it is something to do with power in? Where are you drawing your power from? I am taking from tractor battery but am wondering if this is too much power for the board or something. I keep blowing fuses.
I have just the lines between on/off and d3 , but i justed reverse polarity of power .
check every connection , the polarity of diodes, condensators and voltage regulators , then check ,and re check , there s probably an error somewhere …I use a direct wire from battery. If everythings good , a 15A fuse should be enough .
If you use cytron md13s it might be burnt as it is not protected against wrong polarity, so take wires off the cytron and try again, if everything else is correct.
Hi everyone! i started with my pcb v2 and the same problem happened to me. Wrong polarity. Could anyone send me a picture of how to fix this ? i think the relay burnt. Should i work without on/off switch/relay ?
I don’t see any picture in this subject. Is it too old?
Thanks !
Yes too old.
But check if your cupper line between D3 and on/off connector is burnt, that was what pictures shows.
Replace with a wire and voila pcb works again.
Off course with correct polarity 
Relay is not harmed by wrong polarity.
Thanks for the answer. I checked the lines and found no one burnt. Is it possible that if i connect the arduino directly to the fz-g1 tablet and at the same time put the pcb on 12V the fuse on 12V blows ? 
should i have only to connect to the usb hub ?
I’ll try again step by step connecting things one to one
Or perhaps is d3 diod dead ? but the line is ok!
You must send pictures of the whole pcb, so we can help to check the components and their orientation.