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Could try a pcb swap with another clone, actually, some of those "thor" clones will work with a Z-car controller (the ones that dont have the motor hardwired).
You might have one partially blown transistor there, if you have good reverse, and bad forward, that can be a clue, -(assuming froward was when that wire came off and touched something it shouldnt)
The transistors could be swapped out, but they're kinda tricky to get off, without messing up the board.
If you still have reverse, you could switch the motor leads around so reverse is now forward, and reverse is just "brakes"
There is a way to use a digital meter to test transistors, basically checking them like they were 2 diodes in one case. One lead is common or "base" and is positive or negative (the other leads are the opposite) Two kinds of transistor "P-N-P" and N-P-N", on a PNP, the base is negative, and on a NPN, it's positive.
You know you have found the base lead when you get .5 to .7v drop across the terminals, if you have it probed the right way.
When transistors go bad, they either short out or go open, so thet meter would give a real low reading (diode test setting used) like 0.1v or less. If it's open, it will be like there's no transistor anymore.
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