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Thursday, July 27, 2017

Repairing the K40 LPS #1


It is quite common for the Laser Power Supply (LPS) in your K40 to fail. Although our knowledge of the LPS design has been dramatically increased it is still unclear why certain parts of the supply fail.

I continue to collect failed LPS's dissecting each to see if we can find a reason for failure and solutions to potentially extend their life.

The bad news is that the LPS failure rate seem to be quite high. The good news is that they don't cost that much. You can get a new supply from China vendors for 60-$70. It almost seems that the LPS just like the laser tube should be considered a "consumable".

K40 LPS are high energy supplies and it is very easy to experience cascading failures when making repairs. If you consider the cost the components you could easily spend as much on a repair as a new supply!

This post is still a WIP!


The stock K40 comes with LPS's in a few flavors. We have tried to categorize these supplies using the color of their connectors as a gauge. K40 LPS's typically have either all green connectors or both green and white connectors.


We have a fairly accurate schematic and have identified most of the replaceable parts.
Other posts regarding LPS information can be found under this search link:


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

I also collect failed LPS in an effort to better understand them and their modes of failure. If you want to donate comment below or PM me at +don kleinschnitz 

Typical LPS failures

My research on LPS failures has revealed three common component failures: 

  • The laser will not fire or fires at lower and often erratic current levels including arching inside the LPS
    • Replace the HVT, 
  • The fuse on the LPS pcb blows
    • Replace the AC Bridge Rectifier, 
  • The 24V or 5V power is missing along with other symptoms
    • Replace DC power supply PWM controller

I will discuss these three components and their replacement in some detail but first lets discuss safety and life threatening risks.

I do not recommend repairing your own LPS!

That said, removing and replacing components in a powered down LPS can be done successfully and safely if done correctly.

I am not guaranteeing that if you follow these procedures you will be safe.

The author does not make any warranties about the completeness, reliability and accuracy of this information. Any action you take upon the information on this site is strictly at your own risk, and the author will not be held liable for any losses and damages in connection with the use of this information.

Need some proof that an operating LPS is lethal.



  • Power up your LPS outside of a K40.
  • Power up the K40 with the anode or -L wires disconnected
  • Access the laser compartment with the AC power plugged in.
  • Access the laser compartment without first grounding the anode with a "chicken stick"* and its procedure" with interlocks in place.

Going About a LPS Repair

I haven't found a magic way to tell what part is bad. One or more of the parts cited below can cause one or more problems by themselves or as a catastrophic failure.

Here are some scenarios I have seen:
  • Scenario 1: F1 blown => Do repair #1= replace F1, BRI
  • Scenario 2: Fuse still blown after Repair #1 => Do repair #2 = replace F1, BR1 and PWM
  • Scenario 3: Arching, low or no power without scenario 1: Do repair #3 = replace HVT
I will add more scenarios as research continues.

I will refer to this picture for these repairs

Prepare the K40 for any type of LPS repair


  1. Remove the main AC power plug
  2. DANGER: In the laser tube compartment ground the anode of the laser using Procedure A below.
  3. Unplug all the connections to the LPS.
  4. In the laser compartment, pull (or cut if you have to) the sleeve from the anode. Remove any silicon insulation and remove the anode wire from its post. If the wire is soldered, un-solder it but be careful not to overheat the terminal in the tube as it may damage the tube.
  5. In the right K40 compartment loosen or remove the LPS hold down screws. Sometimes the front screws can be removed and the rear screws loosened making replacement easier. From inside the right compartment pull the anode wire back through the laser compartment wall and into the right compartment. Be careful not to chafe or abrade the anode wire as you pull it through orifices in the cabinet. 
  6. Remove the LPS from the K40 and place it on a work surface. "NEVER POWER UP THE LPS OUTSIDE OF THE K40!.

High Voltage Transformer (HVT): Replacement Procedure

  1. Test for bad HVT: position the head over a piece of mark-able material and then press the "TEST" button on the LPS.
  2. Verify Symptoms: the laser does not fire or fires at reduced power. In addition you may hear crackling noises coming from the LPS or the laser compartment
  3. By now you should have completed  "Prepare the K40 for any type of LPS repair": as described above if not STOP & DO IT NOW!
  4. Remove the LPS cover screws and lift off the cover. The fan will be attached...
  5. Unplug the FAN
  6. Locate and Remove the HVT: 
  7. Some HVTs are screwed to the frame and have a 3 pin connector. On this type remove the HVT from the frame and unplug the connector. Go to step 9.
  8. Some HVT's are bolted to the PCB. In this case you have to remove the PCB to get to the nuts.
    1. Use Procedure B to remove the PCB
    2. Remove the HVT
      1. Turn the PCB over and remove the nuts holding the HVT.
      2. Replace the HVT and its nuts.
      3. Reinstall the PCB into the chassis by reversing Proceedure B
      4. Go to step 8.
  9. Replace the HVT
  10. Replace the HVT in reverse of the order you removed it.
  11. Plug in the FAN cable
  12. Replace the LPS cover
  13. Reinstall the LPS into the K40
  14. Use Procedure C to reconnect the LPS to the laser tube
  15. Use Procedure D to test the LPS

Bottom side of PCB showing alternate HVT mounting

Bridge Rectifier: Replacement Procedure

BRI: note its orientation. + of the bridge is on the right in this view
  1. By now you should have completed  "Prepare the K40 for any type of LPS repair": as described above if not STOP & DO IT NOW!
  2. Remove the PCB using Procedure B: Removing and Replacing the PCB
  3. Locate the Bridge Rectifier BR1 on the front of the PCB
  4. On the back side and using "solder wick" and a hot iron suck all the solder from the joints of BR1. Alternately you can cut BR1 out from the top side of the PCM and then pull the remaining legs out while heating. Finally suck the solder out of the joints leaving open holes for the new part.
  5. Replace BRI by inserting a new part and re-soldering its three legs. You can replace BRI with a direct replacement or substitues.
  6. IMPORTANT: Insure that the new BRI is oriented with its "+" lead to the right as this picture shows. The "+" marking is not shown on BRI in this view because its on the other side . You can see the "+" silk screened on the PCB. Just in sure that the replacements part "+" is aligned with the "+" on the PCB.
  7. Locate F1 and remove using the same soldering method as #4. 
  8. Replace F1
  9. Replace the PCB using Procedure B: Removing and Replacing the PCB
  10. Plug in the FAN cable
  11. Replace the LPS cover
  12. Reinstall the LPS into the K40
  13. Use Procedure C to reconnect the LPS to the laser tube
  14. Use Procedure D to test the LPS


Procedure A: Discharge the machine

Make a discharge stick ["chicken stick"] (see photos's below):
See picture below, I think it is self explanatory. Mine is about 2ft long. You can use a dowel or PVC like mine. BTW I can sell you one for $200 ...:).
Note: I recommend PVC as wood can have high moisture content.

Ground the end of the wire opposite the taped end of the wire to bare metal on the cabinet. The terminal post on the back of your K40 is a good place after you insure that it is really grounded to the cabinet. You could replace the alligator clip on my example with a banana jack to make it more convenient.

Hold the end of the stick at the end opposite the taped wire. Put your other hand behind your back do not touch anything else with any part of your body.


Probe the bare wire end in and around the anode to discharge it before you enter the compartment.

If you see a spark just silently say "thank you Don, that woulda hurt!".

Procedure B: Removing/Replacing the LPS PCB

    1. Remove the 3x A screws from each corner of the PCB.
    2. Remove the 2x B screws that hold the power FETs to the chassis
    3. Remove the screws holding the 5V Reg and low voltage PWM controller to the chassis and associated heat syncs.
    4. Retain all screws and thermal insulator pads
    5. To replace reverse steps 1-4, insuring that you include the heat sync thermal pads under PWM and 5V reg

Procedure C: Connecting the LPS to the Laser

  1. Replace the anode wire in the same way it was connected, twist, solder or screw.
    1. If soldering use minimal heat. Some recommend using Teflon tape to hold wires that are twisted see video below.
  2. Route and restrain the wire in the same way. Usually tie wraps around the tube moving away from the anode end toward the cathode end.
  3. Flow silicon around the anode wire connection (use the white tube that came with your K40) 
  4. Push the silicon tube over the wet silicon filling the tube. If you had to cut off the tube you will need to replace it with a peice of silicon tubing.
  5. Add more silicon to the top of the silicon tube if needed
  6. Let dry for 24hrs before using the machine
Silicon pot-ing materials: 
  • Permatex Blue RTV Gasket Maker. Available in auto and big box hardware stores.
You can use a plug-able HV connector to connect the laser HV lead to the LPS. I recommend this approach because once done you can avoid disconnecting the anode connection when troubleshooting or replacing the supply or laser. Also consider buying a laser tube with the anode pre-connected.

HV plug

The videos below show connection and disconnection of the tube from its supply and can be used as reference example:

LPS and/or Laser Tube Replacement Kit

Procedure D: Testing the repaired LPS

Unfortunately there isn't a safe way to test a LPS outside of the K40, you will have to reinstall it in the K40 and test its operation by checking while actually marking. If you repaired this supply as a spare verification will have to wait :(.


Enjoy and comment

Maker Don


  1. Hi, I bought a K40 second hand and the owner saw need to replace the tube... I hear noisy inside the power supply mainly in low power.... 25%... 15%.... but in 70-80-90-99% not noisy... finaly the power supply deads... I check all and the HV transformer was inside the ferrite a arc hole... buy new one in aliexpress same model... change and it works but noisy still.... and now, along the HT wire (when pass a metal hole) it was arcing.... only cut some pieces.... it´s so strange...
    I checked the tube with lights off and at high power (when no noisy) I can see the "ray" along the whole tube... when it´s working at low power (with power supply noisy) I see in the center of the tube the "ray" is too weak than in the tube sides....

    But in low power I can cut 3mm plywood (on 3 times) at 25% 15mm/s...

    I´m affraid..... I use distilled water.... put frozen water in water tank to has the tube temperature at 20 degrees (turn on and off to keep the temperature)....

    I don´t know if the tube is damaged... I can cut in 1 time a 4mm acrylic at 90% and (I don´t remember well... 10mm/s)...


  2. Why? "Don't ever... Power up the K40 with the anode or -L wires disconnected"

    what happens? I disconnected my tube to work on my unit and powered up the unit to test the gantry and now the power supply blows the fuse. Thank you in advance. CommsPrepper at gmail