Broke my K40

Hello everyone,

I bought K40 from Aliexpress about a few months ago. I had problems with mirror alignment and finally, I was able to align them well enough.

This is my first laser so I didn’t have enough knowledge of how to use it :slight_smile:

  1. I plugged it in not grounded socket.
  2. Use water from a tap.
  3. Few times tried cutting with ~80% power
  4. Every time if I tried to go over 50% power it started screaming… I didn’t know that it was not good… I thought it was supposed to sound like that.
  5. Every time I started cutting and touched the water it electrocuted me, but I thought it was normal for a Chinese laser.

The engraving was fine from the beginning, but with cutting, I always had a problem. Sometimes it cut at 25% with one pass, but mostly I used 45% power, and two passed so the small cut part would fall out by itself.

A friend said it was not normal, and it should cut 3mm of plywood with one pass.

Finally got my first bigger order, so I cut for few days 10 hours a day. Last night it started to cut not ass deep, and this morning I noticed that the cutting line became much wider. I tried cleaning mirrors, adjusting the focus point, but nothing helped, and finally, I noticed that laser beam in the tube arcing.

I google that can it be, and found that I need to use distilled water, so I changed tap water to distilled, I installed a new grounded socket, I grounded the laser itself. The laser became like a new machine, no extra noises, I could touch the water when it’s cutting, didn’t get electrocuted…

But it didn’t help with arcing… So PLEASE can you gays help me out to figure out if it’s possible to fix this machine o do I need to change the tube or PSU…

By screaming do you mean like a hissing kind of sound?

Where is/was the sound coming from?

Has it made that sound from the beginning?

If you have an analog meter when the screeching sound is heard what is the meter doing?

After doing the above does it still scream?

What do you mean by:

Please post;

  1. Video of the tube operating/arching
  2. Picture of the control panel
  3. Pictures of wiring in and around the LPS.
  4. Picture of a dot on “post it” taken at the 1st mirror.

The first thing I’d do is clean that tube…cycle a solution of distilled water with a few drop of Dawn through it several times then follow that with a few changes of distilled water.

I think yes, quite high pitch sound.

I can’t remember where it was coming from. Now there is no more hissing after I changed water to distilled and installed new sockets.


I have digital.

No. And no more electrocuting myself than I touch the water.

House I am living in is quite old, and I put my laser in the “pantry” in witch had old sockets without ground. So I installed them (with ground now). Also at the back of K40, there is the place to connect ground wire, so I did it also.

what is this? I found that viniger does quite good job, using it now for cleaning.

…if the arching you are referring to is that jump the beam makes at the anode ring shown by the red arrow …

then … that can be normal.

Why did you include this picture?
Is it arching there…?

The output mirror of the laser looks discolored but it is hard to tell, at this wavelength, if it affecting anything. Perhaps it’s just a stain in the water jacket that is making it look like that.

My tubes output mirror:

What does the beam look like going into the first mirror?
Image a spot on a PostIt at the first mirror and post a photo of the result here.

I hear hissing in the background at about 1:02–1:03 in the attached video. This may suggest a bad LPS.

It is hard to tell if the tube and/or the LPS are bad in these situations.

  • Suspicions are that using tap water damaged the HVT in the LPS.
  • Screeching/hissing and a ticklish water tank are normal symptoms of overly conductive water.
  • The poor grounding made the problem more obvious but IMO it did not create the failure. The water did.

We constantly warn users that conductive water can damage the LPS. Most ignore that warning…

If I were to guess as to what is wrong it would be that the LPS is bad and the output HVT is arching. The arcing is less now that grounding is improved and the water is distilled but it’s still a problem.

I don’t think the tube is bad but I could be wrong. Usually bad tubes are associated with violent arch from the anode, unusual plasma characteristics and low or erratic cathode current.

I suggest:

  • Show us a picture of the output beam and if it is properly formed then that is further proof the tube is ok.
  • Ensure that the black wire on the cathode is connected to ground. It should be connected via the leftmost pin on the LPS usually labeled -L. I would simply verify that the cathode wire (black) is connected to the LPS and with a meter verify that that pin is actually ground.

If the output beam and cathode connections are ok the I would buy a new LPS.
It is not wasted money even if it ends up being a bad tube instead. The tube and LPS will eventually fail it’s simply a matter of when.

Some longer-term suggestions to improve the machine, its safety, and your ability to troubleshoot problems. [come back here if you need help doing these things.]

  • Add an analog 30ma meter in the cathode circuit of the tube. This meter will help you understand what the laser current is doing.
  • Add interlocks, temp and flow sensors to improve you and your machines safety.

but sometimes it hits dead center and I can see power increase when it hits flywood.

don’t know, wanted to provide as much information as I can.

sorry took a picture but didn’t upload it.

Unfortunetly I didn’t found this warning…

… it’s hard for me to see that spot to tell if it’s normal.

I am looking to see if it burns as a solid dot or a doughnut?

Can you do a 1-sec burn on PostIt note then take it out of the machine and post a picture of it?

Yes, it has the shape of a doughnut… BUT at first, when I started cutting it had also, but it mysteriously disappeared and became a round dot.

We need to see a clear view of the dot to be sure.
That picture being at an angle I can’t tell.

I am sorry sir, picker wong picture…

It was few seconds burn at 9% of power

Do the same at 40% power please.
9% barely ionizes the tube…

Doesn’t seem good… But like I said sometimes it hits dead center, but I can take picture of that spot because it rare thing.

if its arcing like that either you are grounding a wire or tube is dead cracked and looking at last photo of arcing i would diagnose dead tube . just check your cables

I am not sure, the tube and/or LPS is bad.

I think I would start by replacing the power supply

LPS stands for Laser Power Supply? if yes what is the difference between LPS and power supply?

I have a cheap multimeter, but I don’t know to correctly check if the cathode grounded. I checked all wires and they look fine.
I also tried to pug in the old socket without ground, and it did not sound as back as it used to. So probably like you said my tap water was poison.

Yes LPS stands for laser power supply. Laser Power Supply is the “Power Supply” that powers your K40.

With the power to the machine OFF:

Put your meter on the lowest Ohms (omega) scale.

  • Put the black probe on the leftmost pin on the LPS.
  • Put the red probe on the screw that is fastening the LPS to the floor.
  • The meter should read close to 0hms

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Thanks, it shows 0.00

Ok that says your tube is properly grounded.

I am not positive its the, or only problem but I would start by replacing the LPS.