It happened my k40 died after a year and 3 months

It happened my k40 died after a year and 3 months. I am assuming it is the power supply. It worked fine last weekend, but this past Saturday the power supply was squealing at 42% power, usually I only heard that noise at below 22%. Plus, it wasn’t engraving at 42%, (usually I use 28-36%) so I started increasing power until it started engraving, which was at 68% (highest I ever had it). I was able to do a few small pieces at that power, then while watching it engrave I saw the dot almost disappear to barely noticable. At that time increasing the power had no effect. Is it possible the tube went, or is it the power supply.

Most likely the supply’s HVT. What kind of coolant?

@donkjr just distilled water in a 5 gallon bucket.

@donkjr Will I be able to tell if it is the HVT that went bad by examining it?

Disclaimer: Warning, laser High Voltage supplies have extremely high voltages and can hold a lethal charge even after the power is removed. Do not attempt repair of these supplies unless you have certified training and experience in High Voltage electronic equipment and testing and repair procedures.
Any attempt to test or repair a HV supply is at your risk.

You usually cannot tell because the internal diodes go bad. You can try taking the cover off the LPS and running the machine in the dark to see if you can see corona or arcs around the HVT.

You can also tape a grounded wire on the tube with a bared end within a <1/2" of the anode. Then run the laser and see if there are violent arcs when you pulse test. Pulse briefly do not hold the test for any length of time.

You can replace the HVT but lately I have seen pretty cheap supplies @ about $40 so it may not be worth it.

I feel like this may be what I’m currently experiencing, but not entirely sure. I was thinking it was the tube going bad. The power drops off from the moment it initiates a vector cut and doesn’t engrave anywhere nearly as deep as it did a few weeks ago. Was going to open my own thread, but wanted to chime in on this one to make sure I should. I’m seconds away from ordering a new tube (if necessary), but how to diagnose? The problem described here seems the same as mine, from first glance.

@CPUnltd If it suddenly dropped off it’s most likely the power supply. Happened to mine recently. Does it make a screeching / squealing sound when it’s engraving/cutting?

It does make a noise, but I feel like it always has made that noise since I got it. Which power supply do I need to replace? The sound was always different spending on the power output.

You might be able to find them a little bit cheaper, but cloud ray is a good company.

Alternatively Brandon has one 50W LPS left for sale at $40 + shipping as part of his Hobby-Fab Liquidation. See line 153 on the second page of that spreadsheet.

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Going to look into what you suggested, but are you sure it’s that power supply and not the 5V?

Well you can check the voltage between the Ground and In pins and see if you get ~5V with the power set to max.

Also here is @donkjr 's power supply troubleshooting guide.

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This is the kind of noise that the LPS makes when it is not working properly. Mine just started to go bad I think. Listen at about 7:00 sec.

LPS hissing arcing HVT

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That seems like the sound mine made if I ran it under 20%, then right before (18 months later) it died, it was making that sound at 25, 30, 40% etc… until I got no power at all( a few hours laterj Replacing the power supply didn’t seem to fix it, and now I am installing a new LO tube.

I ran a profile on mine yesterday (IN voltage vs tube output current) and I noticed the same thing in that the hissing is at the low settings not the high settings, which surprised me?

Attached is the profile of control voltage vs current for my supply.
I am carefully evaluating the conditions of this machine as I bring it back to normal with the hopes that I will take away some data that would allow us to practically sort out a bad laser tube from a bad supply.

My working theory has been:

If the supply is bad and the tube is good the control voltage vs tube current will be abnormal in these ways:

  • Nonlinear
  • Intermittent especially at high control voltage settings
  • Low power (no way to measure yet)

If the tube was bad and the supply was good the control voltage vs tube current would be:

  • Linear as compared to the model
  • Lower than normal current values at control voltages as compared to the model.
  • Low power (no way to measure yet)

The Attached Graph
The orange is the “Ideal” Model profile as taken from the analysis of @NedMan 's new tube and LPS. That model was used to calculate an ideal curve for my machine using my control voltages.
The white curve is the data that I measured at the machine using the dvm on the pot and the laser current meter.


  • For control values below 1.5V the Tube Current is abnormal and the supply hisses.
    • Note that the lower end currents are near the lower ionization levels of the tube i.e. 4ma.
  • After that point the LPS operates above normal until about 4.5v at which it outputs higher than the ideal model.
  • If the LPS had a bad HVT that was breaking down I would expect the current at the higher control voltages to be less. These results suggest the opposite.
  • These results indicate that the supply operates HOT between 1.75 and 4.25v but that the supply can support a load near normal at max control voltage.
  • Why is the tube not ionizing sooner as the model would suggest. Ionization (tube current >4ma) should occur at around a control voltage of .7v. On mine it is somewhere between 1.2 and 1.5vdc.
  • Why does the current drop to 2ma at 1.5 on my machine?


  • I know that an unloaded LPS will tend to arc more than a loaded one. An open HV lead on the bench will jump an arc gap of nearly 2".
  • Is it possible that at low control voltage the tube is not fully ionized because it has not reached breakdown. It is not providing much load (4ma) and therefore the HVT sees a higher voltage and hisses (corona just before breakdown)? At these low levels the tube is not ionizing and therefore the firing voltage is high for a longer period than when the laser full ionizes and rapidly drops to its lower voltage and higher current.
  • Could be that at 4ma (pre-ionization) a normal LPS will hiss? @NedMan can you check on your machine?



Yes at less than 4ma there is some hissing not present at higher currents.

Thank you… so perhaps hissing is not a good indication of a bad supply.

The k40 I purchased more than two years ago always hissed. My friend who purchased it it is still using it to this day. I actually purchased a spare power supply just in case and never used it.

That definitely sounds familiar! The frequency of the sound changes depending on the power settings. I’m pretty sure it was like that from day one, though… But don’t quote me on that. I can’t remember a time when it didn’t whine while firing.

Curious, would i be better if fixing this power supply or buying a new one? And if I buy a replacement, can I get one rated for 50w and drop it in… Or would that require some modifications?