K40 losing power

Hi everyone , Im new here and found you while trying to solve an issue with my K40
All was fine one day and then the next I was having trouble cutting 3mm plywood
could cut it easy with 3 passes now it takes 5 or 6. , now the strange bit
I was using it at 40% but now anything over 30% and the laser seams to loose power. I tested it by stepping up the power 10% at a time and anything over 80% and the beam seems to flicker on and of.
I have stripped the machine clean everything reassemble it and realigned everything and still the same.

Anyone had anything like this or any ideas as to a cause

thanks in advance

My guess is its a bad LPS.
Here are the classic questions:

  1. Do you only have the digital control panel and no milliamp meter?
  2. Do you hear any snapping, crackling or arching when you turn up the %??
  3. Do you have any way to measure the cathode current?
  4. How old is the machine, tube?
  5. How hard do you run the machine?
  6. What coolant are you using?
  7. What temp have you been running it at?

Hi Don and thanks for the fast reply

  1. yes digital panel and no meter
  2. no noises
  3. no way to measure cathode current
  4. machine is 2 months old
  5. ran it pretty hard at first when trying to work out what was the best settings
  6. using distilled water and using ice packs to keep the temps down
  7. usually running between 16 and 20 degrees

I did once forget to put the ice packs in and the temp do go over 30 and that was for maybe 30 minutes before i noticed.
LPS was what I was thinking as increase power demand seems to increase the issue

Thanks for the complete and organized answer :), that helps.

Do you have and know how to use a DVM?


is that a digital volt meter

I have a reasonable understanding of how to use a volt meter

Yes DVM = digital voltmeter
Ok I will send you a sketch for measuring the current in the tube.
Can you post picture and model number of the DVM.

ok I tried the laser again tonight , at 30% it cuts but takes more than 10 passes to cut 3mm plywood , if I turn the power to 40% it looks like the laser keeps flickering on and off but it continues to follow the pattern, at 35% it cuts but still flickers on and off but if I change the power in any way or amount it starts again for a few minutes and goes off again , change power and back on.

It still sounds like your LPS is not putting out full power.

The attached diagram shows how to connect a DVM to measure the current in the tube.

Summary of the measurement
The tube’s anode is connected to the HV side of the LPS (stay away from this end)
The tubes cathode (the other end of the tube) is connected to ground at the LPS.
The cathode wire is typically green and connects to the FG pin on the left-most connector of the LPS.
We are going to put the DVM in series with the cathode side of the tube and ground and measure mA.

Setup the measurement

  • Disconnect the green wire at the LPS and connect it to the plug labeled mA on the front of the meter.
  • Connect a lead from the COM plug on the meter back to the place where the green wire was connected on the LPS. This should be FG on the leftmost connector of the LPS.

Set the dial on the meter to the DC amp scale (has an A with a dotted line under it).

The meter should read the cathode current in milliamps typically between 0 and 30.

Map your current output vs the % set on your digital control panel.

  1. Set the control panel power to 10%
  2. Close all covers, enable laser
  3. Push and hold the fire button and read and record the value on the digital meter
  4. Increase the digital setting by 5%
  5. Go to step #3

You will end up with a table like this

% power setting tube current
10 xxxx

An example table is here: https://donsthings.blogspot.com/search/label/K40%20Laser%20Power%20Supply
[ignore the rightmost column]

Cathode wire at the LPS:

Cathode wire at the tube:

OK great instruction , very easy to follow.
Your green wire is black on mine

readings as below , I did notice once I went over 70% there was no laser emitted

% mA
10 3.4
15 6.6
20 9.0
25 11.0
30 13.1
35 15.2
40 17.4
45 19.2
50 19.9
55 21.0
60 22.1
65 23.2
70 24.4
75 25.6
80 26.7
85 27.7
90 28.6
95 29.2
100 29.3

The LPS seems to be putting out more than enough power.
The % power vs current is pretty linear meaning it isn’t shorting or arching-over as the demand increases.

There is no simple way of eliminating the tube vs the LPS other than an expensive and dangerous high voltage measurement.

However, these symptoms suggest the LPS is good;

  • Usually, when a LPS is bad the output is erratic at high power %'s. This does not show that.
  • The tube can and does draw full current
  • There is no evidence (sight or sound) of arching at high power demands

Hints the tube is weak:

  • The supply puts out full current but the light output is weak
  • This tube dies at 70% with 24ma of current and no arc-ing.
  • The tube was driven overtemperature. 30 min is a long time to run at elevated temperatures.

Possible Causes

  • Alignment: Even a small misalignment can cause dramatic power loss. However that does not explain the loss above 70%.

  • Weak tube: The drop off of power at 70% points me to a weak tube.

What next

  1. I would check the alignment again carefully to be sure as I would hate to see you get a tube when it’s really an alignment problem.
  2. Post a video of the laser beam when firing at 50% and also at 70%
  3. You may have damaged the tube when you overtemped it. Did it run properly after that? Did it cut the same without increasing the power %.

After this is fixed you will understand first hand the value of a temperature protection interlock and
want to install one.:slight_smile:


1 Like

checked all alignment including the tube itself , checked gantry square and all ok
it worked fine the day after running it hot but not the day after that.

cutting at 40 / 50 / 60 / 70

Oops I’m sorry I meant to ask you to video the tube during those power levels.
I am wanting to see what the plasma looks like when it fires.


should it be doing this , the silver part inside the tube looks burnt in one edge

The plasma looks strange at the cathode end.
I am betting that it’s a bad tube but cannot be positive …