Another Newbie here, surrounded by a lot of smoke

I am building an antweight with my son using parts which seem to be standard (DasMikro 2s6A, n20 6v 600rpm, gearmotors, lemon rx, tgy-306 servo and 7.4v lipo).
We soldered them up and did the binding with a dx6i. Initially all was good. Motors ran (with both sticks) and the servo moved using the gear switch. So we patted ourselves on the back.
THEN (while driving the motors) SMOKE and then SMALL FIRE from one of the motor chips on the DasMikro. The other motor and the servo are still working.
I have another DasMikro (as we were going to build 2 robots) but don't want to burn that one out as well.
Any ideas on what could be the cause.
My thoughts
We didn't solder on capacitors
We didn't wire the motors up any particar way round (+ and -)
Dodgy motor (although both were running at the same speed)
We have the servo (for a flipper) connecter to the 5v out on the DasMikro and to the gear control on the Rx (is this normal?)
A search for "DasMikro burn" didn't come up with any similar problems (surprisingly) so any help would be appreciated.