SKR Pro 1.2 3.3V LED not lit - is this repairable, or am I buying new parts?

LowRider CNC Troubleshooting
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Well, I was on the very last step of rewiring my expanded LR3 build, and disaster struck! I had just finished installing the last piece of wire loom and heat shrinking the cables. I reconnected the wires to the controller and tested all of the end stops and motors, and everything was looking good.

As I was sliding the cover back on the controller, the LED screen went blank. I removed the cover, and noticed that the 3.3V LED was no longer lit (12V & 5V LEDs are still on).

I’m not sure what caused that, but all of the cables were still in place, and everything appeared to be connected correctly. The only thing that I can think of is that a wire must have moved or shifted as the cover was sliding back in place.

Fuses check ok, and I don’t see any visible signs of damage anywhere. It doesn’t work from USB either (5.0V lights up, but not 3.3V

I’m hoping that I didn’t release the magic smoke. I haven’t been able to locate a schematic or service manual for the SKR Pro 1.2 yet, only the User Manual, which doesn’t really have enough information to let me effectively test or identify the possible failed components.

Has anyone successfully troubleshooted (troubleshot?), diagnosed and repaired a problem like this on the SKR Pro 1.2? Is this likely to be repairable, or should I just bite the bullet and by a new board?

If the latter, I’ll have to decide whether to stick with Marlin and buy another SKR, or learn a new language and go with the Jackpot.

SKR considerations -

  • I have the LCD, drivers, etc (assuming that they weren’t toasted when the board failed)
  • I am familiar with Marlin
  • I am familiar with using the LCD controller to move, home, send gcode commands, etc.
  • I am happy using sneaker-net and SD cards to transfer files
  • Price is quite a bit steeper

Jackpot

  • I’m not sure that I really want to learn a whole new programming language
  • I don’t have a laptop that I want to use in my workspace
  • Price is cheaper.

Damn, I was so close to getting the machine back in service. Frustrating!

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Ok, I found the schematic

It looks like U4 is the likely failed part. $0.74 CDN (plus $8 shipping). Not sure if my SMT reflow skills are up to the task

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U4 is a bog standard AMS1117. I’m sure you will be able to do better than $8 shipping!
I would first off pull the fuse F3 and meter out the 3v3 line to ensure the thing is not short cct or low resistance (below ~10ohms). I think I would also replace D23 at the same time.

Recheck your limit switches are not wired across the 3v3 and ground :slight_smile:

A bench microscope would come in handy but if it looks like it is going to be too fiddly you could always use a 3v3 regulator with a bigger footprint soldered to wires that go to the board, 5V input and the 3v3 output are easy to get to at the fuses and ground can be found almost anywhere…

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Thanks @dart1280 for the suggestions.

That was the first thing that I looked at! I also double checked the recently soldered pull-up resistors for issues (solder bridge, inadvertent contact, etc.).

This is a SMT component, so it will take a small bit of effort, but I’ll give it a try

With F3 still in place, there is a dead short between 3.3 VCC and Gnd. De-soldering the fuse should at least help me isolate the short to either U4/D23 or somewhere further downstream.

I think that I’ll test my SMT de-solder skills by removing F3, U4 and D23. They’re all in close proximity, and the board seems pretty much unusable, so I’m not sure that I can do much more damage as has already occurred.

LOL - I live in Canada. $8 shipping is a true bargain! The other alternative is to ship to a US mailing address at the border and pay $4 to pick it up, which is only a hassle because of the lineups crossing each way.

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So my SMT soldering skills are definitely lacking. I wasn’t able to get F3 unsoldered, but I managed to dislodge an adjacent resistor. The board is toast.

I ordered a new SKR Pro. Decided to keep the LCD/Marlin thing for now, not ready to dive head first into the Grbl/FluidCNC/Wireless world quite yet…

2 Likes
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I just did a search for ams1117 on amazon, you can get alot of them for less than shipping. 10 for 7 bucks. If it is the same thing.

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LOL, then I would have 9 more waiting for when I blow up the board again!

I found that SMT is a lot more challenging than I first thought. While I like a challenge as much as the next person (and more than some next people), and I definitely tend to gravitate to the cheapest solution, I did a quick cost/benefit calculation in my head and surrendered to the “just buy a new f%^^ing board” argument.

Now if I ever inherit an SMT hot air repair station, the calculation might change.

1 Like
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If you haven’t de-soldered anything, you can also pull all the wires exvept board power and see if the symptoms change.

High resolution pictures might help direct you also.

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Yes, I metered between 3.3VCC and GND with the board completely removed from the LR3, and there is a dead short (0 Ohms) between the two points. I also applied DC power to the board with all limit switches and stepper motors disconnected, and the 3.3V LED is not lit (12V and 5V LEDs are lit).

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Front and back high-res pictures?