Series 1 Boss5 SMD board signals messed up. Help!

[cnc1000]

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

[HillBilly]

If you are measuring across the junction what you are seeing is forward and reverse bias. If you make your base V reading with your black lead on ground you will see the voltage never goes negative.

Darek

[cnc1000]

The other two axis behave properly and the base-emitter voltage is what you would expect, namely +0.6V and 0.0V, with no negative excursions. The bad axis (Z) is the only one showing illegal voltages. These are NPN power transistors being operated as switches, and should never ever see reverse bias. Something upstream is not operating properly, in order to get reverse bias on the Base-Emitter junction. I have limited documentation and can't really diagnose it from here.

[HillBilly]

Here is the drive schematic. There are no negative power supplies used.

http://www.cnczone.com/forums/attach...5&d=1176638444

I would check your fuse 14 and compare its voltage against fuse 12 & 13. That will make sure your motor has power and ready for the transistors to ground the windings.

[cnc1000]

I checked that fuse and literally every other fuse on the machine. All good. Your suggestion of a dead supply is a good one. That might explain how I am getting negative voltages, because if any of the phases are moving the stepper, then the stepper could be generating positive and negative voltages itself, because steppers also work as generators. That voltage could be backfeeding somehow.

The machine I have has BOSS 5 controls, but I'm not sure my documentation is correct. Back to square 1.

Thanks for the schematic. That helps a lot. I see those fuses called out on a separate board somewhere. I'll look for it but I don't see a designation or location for it. Hope that is the problem.Thanks!

[HillBilly]

Here is another helper

http://www.cnczone.com/forums/attach...0&d=1215725875

Darek

[cnc1000]

Thanks. Found it. Fuses are all good, dangit.... Don't know what else to try. cnc1000