I have a Series 1 Boss 5 cnc mill and the Z axis is nuts. X and Y are fine. Z acts like it has dead transistors. Changed them, to no avail. Looked at the drive signals to the bases of the transistors and they are all bad. I disconnected the power transistor bases and instead, drove some diodes to simulate driving the base emitter junction. Looking at the drive signals, I should see square waves with about 0.6V and 0V levels. Instead, I see 0.6V and negative 5V! I swapped out another SMD board and got the same result. The other SMD board is unknown condition, so maybe both are hosed up. I have a whole stack of manuals, but nothing for the SMD boards.

Is there a typical failure mode for these? Any clue what to check, next? The SMD output from the other axis (X and Y) are fine. What could be causing negative voltage to come out of the SMD board? Thanks. cnc1000