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Tuesday, August 29, 2017

Repairing K40 Optical Endstops


It is common for the optical end-stops in your K40 to fail. Usually, the interposing tab gets out of alignment and the gantry slams the optical sensor into it doing damage to the sensor and connectors.
There is only one active component on either of the end-stop daughter cards and that is the optical sensor, TCST 1030. So if you think you have a failed end-stop you probably have one of these problems:
  • A ribbon cable plugged in backward
  • A bad ribbon connector or cable
  • A bad optical sensor 
I have not found a source of new sensor daughter cards. Therefore you have no other option but to repair these on your own.
With basic de-soldering and soldering skills along with careful attention to replacement part orientation,  you can repair your own. 
If you are uncertain about removing, replacing, and soldering parts in a PCB, Google it, there are lots of "how to solder" videos.
I am not in the business of K40 repair but I have been known to repair these for community members provided you pay the postage both ways. Contact me on G+.


Please consider donating (button to the right of this post).
Your donations help fund additional research, tools, and parts that I will return to the community as information.
For other information on the K40-S build use the K40-S BUILD INDEX with schematics

More Information & Other Repairs


If you feel the need to probe the board or understand the circuit refer to K40-S BUILD INDEX for schematics of the end-stop cards found in a stock machine with the white flat cables.

What you will need to do the repair


  • Soldering iron
  • Side Cutters
  • Solder wick
  • Solder (rosin core with flux)
  • Toothbrush
  • Alcohol
  • Something to hold the PCB while you solder


Remove the Daughter Cards

Most users claim that the gantry must be dissembled to get to the X Sensor card.
The Y sensor card is accessible by pulling the gantry to the front.
Later I may provide more detailed disassembly instructions.

Inspecting the Assembly.

Inspect the assembly looking for:
  • Poor solder joints
  • Broken, loose, or incorrectly oriented ribbon cables
  • Cables plugged in backward. Look into the ribbon connectors and you will notice that only one side has pins. Ensure that the ribbon cable is inserted with the bare lands toward the side with the pins.
  • Loose or bent Optical Sensors. 
The photos below were taken on an assembly where the X end stop was suspected of being bad. I suspected that since the optical sensor was out of position it was impacted by the interposing flag and damaged. 

Testing on my tester showed that the Y sensor was fine but the X sensor turned out to be bad.

Replacing the Optical Sensor 

This procedure and pictures are for an X end-stop repair but in principle, they apply to the optical sensor on the Y daughter card as well.

I always take pictures of everything before I start a repair so that I can check part orientations when I replace the parts, I suggest you do the same.

The Y daughtercard, Optical Sensor was not mounted straight but in this case, the sensor was not damaged. I straightened it anyway.
The X daughter card

The backside of the X daughter card is held in a soldering clamp

The top side of the X daughter card

The x daughtercard, note the crooked optical sensor (black).

The X card after solder is removed with solder wick. Part not yet replaced.

The proper orientation of the optical sensor (before replacement)

The Optical Sensor is removed and the holes tinned. Note that the top holes lost part of the copper pad so the coating was scraped back down the connecting land so a bridge could be soldered 

After the new part is installed. Note the solder bridge created on the upper and lower left pins

Replace the resistor with a new value: The 1K resistor R1 has been removed using solder wick and the pads tinned for replacement

A new resistor (100 ohm)  is installed, soldered and cleaned

The replacement of the Optical sensor is complete and ready for installation in the K40. Note the white lettering on the sensor is facing out.

End-stop Tester

If you want to verify that your end stops are working either in or out of your machine build yourself an end stop tester which is found in this post: Middleman-board-interconnect

Enjoy and comment
Maker Don


  1. Where can I buy this full green board

  2. could I replace with an ITR1608 instead of the sensor described? as the tcst 1030 is very hard to get hold of in the uk?

  3. Hey Katie, Thanks for sharing valuable content, keep it up...