Hi
So my K40 stopped doing much. If i place paper in front of 1st mirror and test fire all I get is two outside edges of a circle about 10mm in diameter.
the amp meter show 25ma to get it to do that anything less i get nothing.
is it a dirty laser tube, a bad laser tube, bad power supply, or a bad pot? any help greatly appreciated.
Post a picture of the test paper, please.
You can also compare to pictures from other folks who have posted about tubes having shifted mode, typically to TEM01* (“donut mode”) or to TEM01 (two halves).
https://forum.makerforums.info/search?q=tem01
If your tube has shifted into a higher-order TEM than 00, it’s irreversible, and the tube is dead. A common way to reduce the lifetime of a laser tube is improper cooling. There is a lot of misinformation about cooling out there; this article may help:
wow, THAT is very high for a K40. What’s been your typical power level on the amp meter when you’re cutting things?
typical around 8-10mA but i get nothing at that power now
The dot pattern indicates the tube is at EOL and as long as you’ve not exceeded ~18ma and never operated it above 25C it’s likely you’ve had a long and useful life from that tube.
The fact that you can get a steady 25mA from the power supply is a good indicator that it is operating fine and you just need a new tube.
Just incase you, or others, aren’t aware, the stock tubes shipped with K40 machines are of vary low quality in general and while they are claimed to be 40W lasers they can’t be operated at 40W of output without greatly degrading the tube chemistry. Most every K40 FAQ should tell you not to exceed roughly 18ma for a longer tube life. The other thing which will greatly degrade the tube life is the water temperature entering the tube. It must be below 25C with 18-20C being ideal and 25C the point at which to stop using the machine.
Good replacement 40W tubes are often tested to 50W and can operate at full 40W without excessive tube wear. So often a new tube will result in better-then-new capabilities but the installation is not like changing a light bulb. Thankfully there are lots of HowTo’s around the process.
I am having similar issues with my K40 except it’s not sending any laser beam from the tube as far as I can tell although there is a nice purple beam inside the tube when it’s supposed to be cutting. I have put countless hours on this machine and my guess is the tube is just “used up”. By the way I use this machine for one step in my manufacturing process and it’s actually my second machine after the first had a similar issue. Sadly I never took the time to repair the first and now the second is dead, and the third machine is on the way. It’s hard to argue with a brand new machine for less than $400 especially since I rely on it for my business and can’t afford to be down long. But I promise I will order a tube and fix the current machine so I have a backup. Sorry for the long read 
Can you post a picture of the burn at the output of the tube?
Dan, that was not a long comment by any means and the more information you provide the better we can help. After you post your picture of what the beam shape looks like out of the tube( between the tube and before mirror #1 ) and we get to the bottom of what condition your tube is in let’s include a discussion about what features your machine has and how you use it to see if we can’t help you get a bit more life out of them.
@donkjr and @dougl thanks for the replies, I am traveling this week but will try and get a picture next week. I do know I passed a scrap of paper by the end of the laser tube when it was supposed to be firing and there was no burn mark that I could see. But I don’t know how focused the beam is supposed to be at that point. The tube did have a solid fat purple beam in it when firing and as I recall it curved at the end where it’s supposed to exit and looked to be going into a metal piece near the end inside the tube. Not sure if that’s normal.
Picture of the end of the tube while firing would also be useful.
If the laser was active and you had something interposing it at its output position it would get burnt.
Update; I had a chance to look at the laser. I powered it on and pressed the laser test button and it fired properly. This made me think maybe it was working again. I set up a job and started it but the laser did not fire. I put a piece of paper in front of the tube right before the first mirror and hit the test button, it did not burn the paper. I do seem to hear a crackling from either the tube or maybe the power unit…? The tube still has a purple beam in it when I press the test button.
The purple beam isn’t the laser. It’s the plasma that excites the laser. The wavelength of the CO2 laser is way longer than the human eye can see. In fact, it’s so long it can’t pass through your intact eye though it can burn your eye up from the outside.
If it doesn’t burn the paper, you now have a plasma bulb but not much of a laser any more. 
There can be plenty of reasons for that. If the tube has ever run hot, the CO2 may have become permanently CO and that doesn’t lase. Microscopic cracks might have poisoned the gas mix.
The fact that you have plasma should, as far as I know, mean that you have a working Laser Power Supply. 
Tell us about how you have cooled the tube when the laser has been in use!
shouldn’t be so… open the tube bay door and open the LPS compartment door, turn the lights off and test fire the laser and look for arcing.