Stepper motor driver dead?

Question

I own a cheap 3D printer FLSUN Delta, which was bought 6 years ago. Several structural and other upgrades were done, such as additional frame links, a new heated bed with external MOSFET.

In the past week in the middle of a print, the printhead came crashing into a printed object and bed. I made an emergency stop and started to look for the problem. I noticed that one of the stepper motors acted weird. Below is a more detailed description and the malfunction is presented on homing example.

1. First I have disassembled the belt system so that stepper motors can spin freely to prevent more collisions of the printhead with frame and bed.
2. I have also tested all of the end stops with the M119 command and they all work perfectly.
3. Then I have set jog speeds to 5 mm/s in the Simplify3D's Machine Control Panel and sent G28 to home all axes. Two stepper motors did the job right as I was manually pressing the end stops, however, the third one rotated with much faster speed. This is more understandably explained with the help of the two following videos:
   • First one (video for the X-axis stepper motor) is showing the movement of the working stepper motor - X-axis motor. When G28 is sent, the motor starts rotating until the X end stop is pressed and is then accordingly to the Marlin re-bumped again. The clicks can be heard. The noise is coming from the second stepper motor.
   • Second one (video for the Y-axis stepper motor) is showing the movement of the faulty motor (or faulty driver) - Y-axis motor. When G28 is sent, it starts to spin very fast. The first two clicks are from the X-axis end stop. The first click on the X-axis end stop stops the Y-axis motor, the second one continues its rotation until it is reversed at the first press on Y-axis end stop - the re-bump takes place and it is stopped at the second click on the Y-axis end stop.
4. Then I have switched the X and Y axis motors and the problem moved to X-axis, so I suspect that one of the stepper motor drivers is faulty. Correct me if I am wrong.

The printer uses the MKS Mini B V1.0 control board (see picture) and Marlin V1 (output from M115) firmware (parameter MOTHERBOARD is defined as 33, RAMPS 1.3/1.4 for extruder, fan and bed).

What do you suggest to resolve the problem:

• Is it possible to replace the faulty driver? I am more familiar with programming and Marlin than with actual PCBs and replacing their components.
• Is it better to replace the whole board and if yes, do you suggest getting an identical one or a different one, like MKS SBASE V1.3?
• Any other suggestions?

Answer 1

You can fix this.

The current switching portion of the driver appears fine, as does the motor.

First check the improbable, but easy to fix

Although I think it is unlikely, something could have been changed in your configuration file. It would be work a quick glance to see if there has been any obvious damage.

It is probably a board-level (not chip level) problem

Assuming the configuration is good, it is likely that there has been a (repairable) failure on the controller printed wiring board. This could be the driver chip, pull-up resistors, traces, or (perhaps most likely) solder joints. Given failure rates of working boards, the solder joints may be the most likely culprit.

This is my reasoning:

The driver chip is a DRV8825. This is a "step and direction" driver chip, where the controller pulses inputs to the driver to change the position by a fixed rotation. The DRV8825 has three input pins that define the degree of micro-stepping. These pins are named MODE0, MODE1, and MODE2 on the schematic, and change the operation from full step, one-quarter step, one-eighth step, one-sixteenth step, and one-thirty-second step.

If there is a problem with these pins, or with the solder connections on the parts which connect with them, the motor speed will be wrong, probably by a factor of some power of two. For reference, here is a link to the datasheet for the driver chip: DRV8825 datasheet.

Defects in the power driver transistors cause other effects, such as no motion, jiggling, buzzing, or even reversing the direction. Since this is not the problem you are having I think the power driver transistors are OK.

A problem with the MODE pins would cause the size of the rotational step to be wrong, which would affect the rotation speed, which is the problem you have.

You could probe those pins with the power on, looking for the voltage. Since this is a Delta machine, the three X, Y, and Z drivers should be symmetrical. Each of the inputs should be close to 0 V or close to 3.3 V.

If the corresponding pins are the X, Y, and Z axes are the same, then the problem is probably in the driver chip, which would require replacing the surface mount component. Here is a link to the electronics distributor DigiKey where individual driver chips are available: DigiKey link for DRV8825. If the voltages are different, try re-melting the solder on all of the pins on the circuit.

[Note, I can't tell without the schematic of the board, but the 3.3 V output pin from the DRC8825 could also be poorly soldered, so check pin 15 (V3P3OUT) also.]

Good luck!

Delta machines are wonderful, and the symmetry serves you well to help diagnose this problem.