Laser fails to excite, arcs to body

It sounds like a bad tube:

  • Check for cracks, water leaks etc.
  • What coolant are you using?
  • Where is the tube arching from?
  • How old is the tube?
  • How hard do you run it?
  • Did anything out of the ordinary happen before you found this?

@ mcdanlj

Thank you very much!

I do have and know how to use a multi-meter. (I design and work with electronic systems as my day job).

I have taken all the precautions before inspecting things as well (letting the cutter sit, shorting the high-voltage lines over a resistor - and then reconfirming with just a heavy wire shorted to ground) before touching anything, making sure the unit is disconnected from mains, Laser UV safety goggles, etc. (I got a nice jolt from a HeNe laser supply I was playing with from one arm to the other back in my teens and did not enjoy that - suffice it to say I learned a lesson in due diligence.)

Thank you for your response!.. Short Answers:

  • I do not see any cracks and no water leaks
  • Coolant: Tap water, but was just for a test (in the past I bought distilled water)
  • Arcing from below the Anode insulation to the edge of the cutter housing
  • Tube is about 6 years old, with less than 3-4 total hours of use on it (hasn’t been used in ~5 years)
  • Never run above 50% (I would guess My average was around 30%)
  • The only out of the ordinary thing is that it’s been sitting unused for a long time.

Longer Answers:

My disappointing but leading theory, based on some forums I’ve read, is that the tube may just be dead. I purchased the cutter sometime in 2016… RARELY ever used it. The most use it got was a few test engravings here and there and then about 2 total hours of cutting of some thin, dense-foam sheets (I know… toxic and leaves residue on the optics :-() that I’d estimate, based on the position of the un-marked power knob, was no more than about 30% power (There is an onboard analog ammeter, but who knows how accurate that is.) Other than momentarily test-firing the laser at higher settings, I don’t think I have ever gone much higher than that. (I actually did draw a line by the knob for the ~30% setting that I was using at the time.)

I moved in late 2017, the laser came with me to our new house and has been sitting in the basement untouched until last week when I decided to dust it off and set it up.

For the test (I was literally just testing to see if it still worked), I made sure everything was clean, powered it up, ran regular tap water through the cooling system and hit the test button for a split second. There was nothing. I raised the power knob up to my marked position and still nothing… a little higher and the tube started to excite. (Purplish glow, high pitched “scream” sound.) I had a piece of paper under the head to see if it would make a mark and it did not, there was no laser.

I raised the power a bit more and momentarily pressed the test button a few more times… on the last press (probably at round 60-75%, I heard the “snap” of an arc. (I didn’t see where the arc came from at first as I was standing in front of the machine. Powered everything down, took a look at the power supply and high-voltage wires for any obvious damage and did not see any.

Standing off to the side of the machine, I repeated the test with the exact same experience and when it arced, I saw it come from below all the insulation / silicon on the Anode terminal up to the edge of the laser cutter housing. A few more subsequent tests resulted in the same. Not wanting to burn out the power supply, I did not press-and-hold the laser test button, but pressed it momentarily and repeatedly - each time, the gas would excite on the first two or three tests and arc on the third or forth. It seems like there was voltage potential being built up in the process.

Thought I did not see any obvious disconnections on either the anode or cathode wires, I did detach, reattach and attempt to re-insulate the anode and the result was exactly the same.

I have been googling around the web, came across several message boards talking about insulating the connections but also found a few comments (as I did here) saying that the tube is likely done for.

If that is the case, my question would then be, for a machine like this, is it worth trying to replace the laser tube (I don’t know what they cost) or should I just scrap it, use the steppers for some other project and consider buying a new cutter down the road.

Thank you both for your time and advice!


Unfortunately your answers to the questions suggest to me the tube it bad.
Laser tubes leak down over time and that is probably what has happened to yours.
When the LPS is unloaded [tube dead] it will arch to a near surface at the anode.

As far as should you keep it or not. Well, that depends. It sounds like the LPS ($70) is good which is usually a high failure item.

So replacing the tube might be worth it if:

  • You have a use for a laser cutter.
  • If you want to sell it as working
  • If the machine has a working controller and steppers

A tube like this cloudray one runs in the order of $180. You may find a cheaper brand if you look around.

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Cloudray is doing a 6th anniversary sale but I don’t know whether it applies to tubes.

Similar or the same direct from Cloudray for $120 plus shipping:

Or look through what they have:

720mm tube from Lightobject for $170 (two-week backorder right now though):

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Thank you again for the information!
I definitely want to use the machine - I own a few 3-D printers and CNC machines including a Komo industrial CNC at my shop, but I had some finer-detail projects I wanted to use the laser for. I have no intentions of selling it and as far as I can tell the motion system is working (I haven’t connected it to a computer since I fired it up, but the gantry does home-itself and the power supply (at least for now) does seem to be functioning.

My concerns over it being “worth” repairing are the cost of the replacement laser tube (I didn’t know how much they would be) the likelihood that I would get bad tube or it would be damaged in shipping, or the chances that there might be something else wrong with the machine other than the tube itself (and thus I’d be throwing good money after bad).

I like the idea that the Cloudray tube you linked to on Amazon comes with the wiring already attached - seems simple-enough to do, but I know the proper insulation for that high-voltage connection so close to the housing is not that easy.

I will ponder what I’m going to do and make a decision. I really appreciate your input. When I first saw the arcing, I wasn’t sure what to make of it. A “bad tube” didn’t occur to me as the tube was not physically damaged.

Again, thank you,

Hi Michael, thank you for these resources. I had no idea the laser tubes would be this “easy” to buy and I had no idea what they might cost. (In fairness, I had not looked much into it until just recently so I hadn’t looked that hard yet.) But I am going to look over the options you have linked to and figure out what I want to do.

Thanks, again,

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Some of the older cheap tubes had a reputation for losing their seal after a while, regardless of use. The Reci and SPT tubes (at least) have a better reputation for longevity.

The K40 Intro at the top of every page links into the #k40:intro category that has lots of useful information, including on replacement parts.

Connecting the HV line is pretty straight forward and you can use RTV to insulate it. If you need help when you get to that point we will be here :)!

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Again, thanks!

I guess when the electricity has no place to go (eg a bad tube that’s not “conducting from end-to-end so well”) it needs to go somewhere. So even though the insulation on my current machine was adequate up until recently the potential was enough to find a way out and to the housing. In the last few days I saw so much online about improperly insulated the high-voltage connections causing the arc that I may be making more of it than it really is.

I haven’t decided which / where, yet, but I am going to buy a new tube and give it a go. I would like to get the machine going again if I can. I will look around the info as you suggested to learn a bit more as well.


When the LPS is unloaded it will find another path. Anode arching is very common symptom of a bad tube. There really is no way to prove a tube is bad we generally rule out all other causes.
Although faulty insulation can result in arching I generally don’t find that to be the real cause. Yes, folks try to fix the insulation when in reality its the tube.
In any case the tube will eventually wear out so you really just starting with a new consumable :slight_smile:

There is information on this site and my blog on replacing the tube.

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with tap water(how conduct is that) in there, is there a chance it’s arcing through the water jacket in the tube then out to metal?

My general rule with HV is that “all things are possible”. However I have never seen conductive water cause an arc. Based on high LPS failure rates it does apparently stress the LPS.

The OP had said tap water was put into the system and for all we know it could even be well water which is super high in mineral content. Wasn’t thrilled to see that corner cut since distilled water is so inexpensive.

I don’t want to jinx it but my original K40 laser from 3/2018 is still working good. Same voltage setting(2.4V) and I’ve only had to go from 12mm/s @ 60% to 12/mm/s 65% to cut through 3mm plywood in one pass. I don’t use it a ton but do use it every once in a while and still with the original distilled water( pretty much a closed system ). But if tubes die just sitting on the shelf no doubt mine is about to fail too.

I am aware that tap water is not ideal, but when i saw the arcing, it came from the base of the insulation on the anode connection every time. Certainly not (obviously, anyway) through the glass by way of the water.

We have municipal water here, not perfect, but not well water, and (i presume) relatively free of minerals. I know dissolved electrolyte in water could potentially conduct electricity, but I am 90% sure that wasnt the case here.

When I used to run the machine when I first got it, I actually had used the distilled water simply because I knew it was cleaner and wouldnt leave mineral deposits inside the tube’s water channel. (The potential for conductivity through the glass into the water wasn’t something I thought about at the time.) I HAD used tap water the first few times I ever tested the laser because, well, thats what I had :-). At that time, everything worked fine, though.


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I have worked on a couple of projects (not laser related) where I was using 20kv and 40kv inverter coils as an ignition source and it was always surprising when the voltage would decide to take the least obvious path.

One time, while holding a device, the voltage went through insulation into my hand, through my hand and out to one of the terminals despte the electrodes being VERY close to each other at the end. Perhaps I had a salty dinner the night before :-).

(I realize this is now the second time I’ve admitted to shocking myself… I swear I take precautions, lol. :-))


So I am looking to purchase a replacement laser but could use some additional input - the tube I have now is exactly 700mm. I honestly don’t think there is enough room to install a 720mm tube as, even if it would fit, it would have to sit right up against the left-side of the machine (from the back) to clear the mirror on the right and that would put the anode VERY close to the edge of the housing.

I see other tubes on Amazon from companies like “TEN-HIGH” that are actually quite expensive compared to the 720mm tubes from CloudRay or SPT ($200+).

Do you know anything about these other brands? Are any of these brands ones I should stay away from or notably better (ie more / less reliable; more / less likely to be defective or die quickly)?

And again to the money question - I am not worried about “saving money” (not that I’m independently wealthy, but I am willing to spend a bit more for a better outcome) but does it make sense to spend $235 or more on a laser tube of unknown quality vs purchasing a brand new machine (of unknown quality) for less than 2x more?

Thanks again for your thoughts!

Especially since Lightobject doesn’t seem to have any 700mm tubes in stock in a US warehouse, I’d buy the cloudray tube if I were replacing the tube on a K40 today, at least if shipping weren’t too crazy expensive:

I can’t find an SPT T30 in the US right now.

Here’s a 700mm tube on ebay for $150 — OMTech has plenty of satisfied customers here.

Welp, FedEx just dropped off my 700mm OMTech laser… I’ll be heading out to pick up some distilled water and some supplies a bit later and hopefully I’ll be operational over the weekend. I will let you all know how things turn out :-).



Sorry for the delay, I’m sure everyone was sleepless with suspense :-). (Actually, I managed to catch Covid and was sidelined for a week - been getting back up to speed on work, etc the last 10 days.)

The new tube works!!!
I installed the laser tube, wired it up and got as far as testing. The new tube seems to fire just fine when using the test button (even at very low settings). So, I am going to have to conclude it was definitely the old laser tube gone bad that was my issue. I appreciate all of you steering me in the right direction!!!

Unfortunately the hardware holding in the original tube was a pretty crude setup incorporating some pieces of rubber padding and layers of E-tape under the sheet-metal straps. I had to cut it off the old tube to remove it and I have taped it back onto the new tube, but but new diameter of the finished “cushion” is apparently not the same as the previous setup so my laser is not sitting in the right place. I will have to spend some time adjusting the laser height as well as the mirror positions till I get it right.

I’ve done some adjusting so far but the laser is still coming off the second mirror at a slightly downward angle so it eventually misses the final mirror as the gantry moves to the right. I wish I had low-power visible laser light I could shine through the system to make testing easier (and not have me worry that I might blind myself). It would be more efficient than using the tape and momentary test-fires, but I will get it done. (I actually do own a handful of tiny red diode lasers that I may figure out how to rig up in front of the end of the tube, but in reality I can’t guarantee I’ll position it at the exact same angle as the real thing).

Anyway, again, thanks for the input and suggestions about the tube being the problem and sources to replace it. I think I’m headed towards getting this running again (and it was a lot less expensive than replacing the whole thing).



Glad to hear you are back on your feet.

Just to make sure you have this information, here are our recommendations for alignment, from K40 initial setup and regular maintenance: Alignment

The path that the laser takes along three mirrors and the lens needs to be precisely aligned. Here are three resources that may help you understand how to do this.

If you prefer video instruction:

Ep6: Laser Beam Alignment. The DIY CO2 Laser Cutter / Engraver Build Series

If you prefer written instruction, here are two guides:

Note that if the third mirror does not align the beam with the middle of the lens, the beam will not be focused correctly and will not be aligned with the hole in the nozzle. You may wish to use a device that helps you align using a red visible laser. One kind uses a “beam combiner” that slightly reduces beam power, and the other is a tool used only while performing alignment .

That link at the end is for a visible laser alignment tool that you might be interested in.