TEM01 mode on my tube ? :(


My K40 laser cutter was working perfectly well.
Then suddenly, about 4 months after buying it, and only few hundreds of hours of service, cutting power decreased a lot. It’s now impossible to cut 3mm plywood even at 10mA with 2 passes.
I got this image on a piece of plywood placed just in front of tube exit (in front of mirror #1

Do you confirm it’s a dead tube working in TEM01 mode ?
What could be the cause ?

I have to admit that I put tap water in my cooling system with some bleach to avoid algae.
And I turned also my tube 90° to facilitate bubble elimination and at the beginning some electrical arcs where present between anode and K40 as the distance was shorten a lot due to rotation.

If not the tube, could it be due to the K40 power supply ? I read on other streams that should not be linked to PSU.

Thanks for your help !


Yeah, looks like the tube…

  • TEM01* is “donut mode”
  • TEM01 has two spots
  • TEM02 has four spots

I’m guessing you have some TEM01 and some TEM02 but that distinction is academic; once the tube shifts mode it is dead.

Have you seen the “K40 Intro” link in the top bar? It has our recommendations on cooling.

Also, do you have an analog ammeter?

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Thanks for quick answer.

If you refer to that stream : Cooling the K40 laser tube, yes I’ve read it … Too late of course :smiley:

I have analog ammeter installed on the machine.
Before tube death, I was able to cut 3mm plywood at 10mA (mesured in mA) in one pass
After, I’m just able to engrave the plywood, it’s almost impossible to go through it completely

So, in your opinion, it’s a dead tube ? Or could it be linked to the PSU/LPS ?

Yeah, just didn’t want to leave you set up to cook the replacement tube as well. :grin: Also, another page in that series has info on how to wire new tubes safely.

@donkjr is the resident expert on evidence of LPS death, but you didn’t mention weird sounds.

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Unfortunately I don’t know what is weird sound :slight_smile:
From the beginning, especially at low power, there was a high frequency rattling noise (like air ionization, I don’t know how to explain). This sound was vanishing at higher power.

No need to say that I never went above 10mA
I was wondering if electrical arcs when I turned the tube could have led to LPS breakdown …

New tube if on its way from China … Hope to receive it by end of May.

I will take a look at this thread later today or tomorrow.

First, let me add some color to the answers previously provided:

I can be one or the other or both. The output mode you have shown makes the tube the primary suspect.

A bad LPS will not usually destroy a tube or cause it to jump modes. Mostly you just get low performance.

It’s very difficult to safely determine which is bad a tube or LPS. I usually evaluate the health of each component and then decide which part to buy first. Keep in mind whatever you replace, even if it turns out to be the wrong part, will not be wasted as these are consumable parts… i.e. they both eventually will fail.

Tap water over time will damage a LPS depending on how conductive it is.

Repeated arcing can damage a LPS. Usually arching from the anode means that the tube is dead. Rotating a tube to where the water jacket inlet is directed upward is not usually a problem unless it is very close to a ground plane or has poor anode insulation.

Note that your tube is not dead as much as it has shifted modes. Mode shift is usually caused by damage from overheating which changes the physical characteristics of the tube’s optics.

A hissing sound (from LPS) is common at low powers at or below 4ma. It will stop as the current is increased. At higher powers, you can tell if the LPS is arcing internally as the ma meter readings will bounce with the arcing.

Conductive water will also cause a hissing noise and/or a hot bucket.

From the output behavior of your tube, I would guess it is bad. Was it overheated?

The LPS also could have failed from arching or from conductive water.
Try this test while you are waiting for the tube to arrive.
Set the current at 20 ma [that may be about 70% if you have a digital panel] hold the fire button long enough to read the current on the meter and note if the meter is jumping (from 0 to its setting). LPS’s whose HVT’s are failing will hiss and the meter will not be steady.

Consider adding these important upgrades:

  • Safety interlocks on all covers… I know you will tell me you have laser glasses… not good enough
  • Add temperature and coolant flow sensors into the LPS interlock circuit.

If you need help adding safety for you and your components let us know and we will help.


First, thank you very much for this very detailed answer.

Never. i have added from the beginning a water flow indicator with temperature meter integrated. My water temperature is always between 16°C & 18°C

So in your opinion, tube won’t fail because of conductive water or arching. It’s more LPS that could fail.

I will ! But old tube is already on my desk …
I need some time to install it again with proper anode insulation before testing.
What I can say is that at 10mA, power on my analog ammeter was very stable.
I will try at 20mA

As far as I have well located the sound, hissing noise was coming from the tube, not from the LPS but I can be wrong on that.
I experienced this noise below 6mA in general

I experienced arching as soon as I have turned the tube. I didn’t managed immediately to understand from where was coming the sound because of insulation of anode that seemed quite good to me. When I unmounted the tube yesterday, I saw 1cm of Silicon above the top of the anode post. Compared to the video from the other thread on this forum, it seemed not so bad to me.
After adding a piece of 3mm plywood between anode and ground plane, no more arching

Already done :white_check_mark:

Other thing I didn’t mentioned earlier.

I have developed a shield for ESP32 dev board in order to use lightburn + GRBL.
This shield has a voltage shifter for the 3.3 PWM coming out of the ESP32 to 5V to go in the IN input of the LPS.
PCB has been manufacturing by an EMS and I’m using a Texas TXS0101 for voltage shift.

It seems working well, with ability to manage everything from my computer (movement, speed, power, air assist, safety sensors, …)

I have set PWM frequency to 15khz (I saw yesterday that recommendation is more between 20khz and 50khz) and I have calibrated my power on lightburn with my analog ammeter.

However, yesterday, I asked myself if this shield could have damaged the tube or the LPS.
So I have checked the PWM signal with a scope.
When my shield is NOT attached to the LPS, PWM signal is perfect.
When my shield IS connected to the LPS, I can see a peak of 5V signal then 3.3V for the remaining 99% of the high state.
Assuming that the LPS is just “interpretating” an average DC value out of the PWM signal, I assume it’s not a big deal.
Do you have other opinion ?

English is not my mother tongue. If not clear enough, don’t hesitate to ask me to rephrase :ok_hand:


@Avataar120 , Welcome among us.
Your English is good no need to apologize. Add a flag in your profile, to let us know where you come from (handy for time zone).

Please tell us more about your Esp solution, Open a new post for it.
Can’t wait to read it ;-))


I just made the test.
Power on the ammeter set to 20mA for 3sec → No noticeable change or movement of the needle
However, very high frequency hissling noise was comming out the machine (probably from tube)

You are one of the most carefully prepared new forum members. Kudos on having done so many of these fixes that the K40 vendors leave off. Very few people have installed interlocks as well as flow and temp sensors before arriving at Maker Forums. This is why it has become a reflex to suggest adding them. :relaxed:

Your English is very clear. (My small amount of French is very much less so.) I don’t know what “EMS” means here and would like to learn.

I see that @donkjr has a schematic for what looks like a PWM driver circuit at Don's Laser Cutter Things: Replacing the K40's Digital Panel with an Analog one and at Don's Laser Cutter Things: Repairing the K40 LPS #1 links to reverse-engineered schematic at https://www.digikey.com/schemeit/project/k40-lps-2-EFKO7C8303M0/ from which this image is excerpted:

If I’m reading this right, the PWM in has a current sink within the TL494 PWM driver (I don’t know how much current) which would make sense for your logic level shifter not not actually providing 5V at whatever current it is drawing.


EMS stands for Electronics Manufacturing Services.
When I have surface mounted component, I prefer to buy my PCBs through one of them, it’s quite cheap, quite quick & quality is top !


I cannot imagine a way that your shield could damage the LPS.

That shows the loaded and unloaded state of the driver. Those drivers are not really intended to drive much load.

The simple answer is that that signal is filtered at the input to the control amp which creates a voltage (integrator) that controls the LPS internal PWM. If you have calibrated your ESP PWM for power then you should be ok.

That said, I think driving the L pin with an open drain driver is an easier approach and it leaves the opportunity to add a pot to IN for local control. There is an extensive explanation on my blog why I chose this approach. Adding this driver to a esp32 is not such a big deal.

That sounds like a good power supply to me … although it’s still possible that something else is weird with the LPS :grimacing: We will find out after you install the new tube.

One thing we never asked for on the failed tube, which would be interesting, is a video of the tube firing so we can see what the plasma looks like. No big deal now that you have a new one. Just still wondering what caused the tube failure.

Also, at night you can turn out the lights and fire the tube to discover where the hissing is coming from.
It may be coming from inside the LPS and not visible but sounds like it’s the tube.

Of course, it could be that the tube had a manufacturing defect from the start…


Thanks very much for all this material :slight_smile:

@mcdanlj, I read a lot of forum & watched a lot of youtube videos while my K40 was on its way to my house :slight_smile: That’s why I was already aware of safety of K40 machines !!


Please see the two videos. I took end of the tube. Hope it was what you expect.

Looks clean and normal …

Yes, that also what I thought.
Did your hear the hissing noise at the very beginning of the laser firing ?

After you mentioned it I do hear it.
Sounds like what I have always thought was the supply HVT corona.

You’re fully right, I confirmed yesterday that noise is coming from the LPS not the tube as I thought.

Don’t be shocked if the LPS gives up after you install the new laser … :frowning: