Thanks for the help. There some diferances of my stands. I have opened them and there are 2 different boards inside. The right one which is giving me short message is stamped as jbc-r. Its interesting, maybe it is for jbc original handles.
No I don't think so, I'm using JBC original T245 hand piece and I have the second one (if you saw my post in other thread).
Have you checked for shorts with multi-meter?
Connector on the board marked JBC looks good to me.
But the T420D looks a bit marginal, like those solder blobs are very close to each other.
Still I don't see anything obviously wrong there.
If you have some 99% isopropyl alcohol and an old tooth brush you can maybe try clean up those boards to get the flux off.