Printer and Photocopier Troubleshooting and Repair Collection : Motor driver blows fuses                
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Motor driver blows fuses

The following was in response to a dot matrix printer blowing the power fuse whenever the paper advance motor was driven. A 74LS273 was getting hot as well:

(From: Tony Duell (ard@p850ug1.demon.co.uk).)

I'm going to guess (based on what I've seen in other printers) that there's a set of power transistors (either H-bridge drivers or 1 per coil, depending on the motor) that drive the stepper. These transistors are driven from the printer's microcontroller via an output port - in this case the '273.

Now, if a TTL chips is getting very hot, then either something is drawing too much current from it, or something is overvoltaging one of the pins. A particularly unpleasant failure mode is when a PNP power transistor, with the emitter tied to the V++ rail (the 20V + rail that supplies the motors) decides to short and apply said voltage to the output of whatever device is driving it. If you're unlucky, the next stage is that the output port device breaks down as well, and the CPU data bus gets 20V or more on it. The result is blown chips all over the printer. Please don't ask how I found that out ;-)

My guess is that there's at least one shorted transistor in the stepper motor drive circuit. If the system uses an H-bridge driver (an equal number of NPN and PNP transistors) then if one transistor shorts, its companion is connected across the power rails. When it turns on, the supply is effectively shorted.

I think you'll have to trace out the driver circuit for the stepper motor. Figure out what drives what. Test the transistors, and then replace the defective ones _and_ that '273, which is probably now blown.