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.
https://drive.google.com/file/d/1yAGfqw3VVKW91qEpCVFOuAwYoyI7PPuh/view?usp=sharing
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.
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
- 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.
Observations:
Musings
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?
Not sure I thoroughly understand your situation before making recommendations on how to fix it. Have you determined it is a bad LPS?
would i be better if fixing this power supply or buying a new one?
These supplies are pretty cheap and a new HV transformer costs 1/2-2/3 of the cost of a new supply. That assumes your HVT is what is bad, if not you will spend more $ and a lot more repair time. Purchase is a less risky albeit a little more expensive option.
And if I buy a replacement, can I get one rated for 50w and drop it in… Or would that require some modifications?
You can get a 50w supply but probably not “drop in”. Depending on the supply you currently have vs the one you get you may need to rewire. I would go to a 60-80W if your going to go through the trouble of changing the supply type.
These choices are more budget driven …
I have a mostly stock K40 with the M2(?) controller. The version I have has a ebrake button (big red button) and a digital readout for the temp as well as the laser power output. Machine does run, but laser is very weak (power drops off sharply). How do I test the 5v (any youtube videos I can be linked to)? I can provide the tube info you asked for if I am shown how to find that. I am fairly adept at figuring stuff out if I have instructions to work with. I don’t wanna go in blindly and possibly make things worse because I’m not quite sure what I’m doing. Would rather wing it with tutorials vs wing it without. 
Good to know about the HVT. I’m still pretty new to the repair side of things, but can pick up on things if I’m given enough info to work with.
Thanks for the info about going the route of upgrading to a more powerful power supply. That also gives me a good idea about future plans as well.
If you machine does not have a ma power meter characterizing the tube will be more difficult as you will have to hook up a DVM for the voltage and a DVM for the current.
If you are up to that I can give you instructions otherwise you can buy a new LPS and try that then if that does not fix it get a tube. Worse case you have an extra supply if the tube ends up being bad.
I have the meter, but I hacked up the mounting plate, so I wanted to get the laser up and running to make the plate out of acrylic.
I know I should have the switches installed, but I have been very good up to this point with my workflow that would prevent me from starting a job with the lid open. Basically, and don’t laugh, but the wireless mouse needs to be placed on the lid to get signal, and I cannot start the job unless the lid is down. Yes, it doesn’t prevent me from opening the lid after it is started.
I’ll take the instructions anyway. I’m going to try to get this figured out in the next couple of weeks because I want/need my machine back up to normal functionality. I have a backup option that I’ve been working with, but I don’t wanna get used to my backup option, nor do I wanna let my machine sit unused/unusable for too long. I’ll weigh all my options and go from there. Do we have any links in the forum to YouTube vids detailing how to replace any parts on stock/semi-stock K40s? I’d love to watch a video or read a blog with accompanying pictures if I can get my hands on either to help me follow some sort of proceedure to minimize the possibility of me screwing things up worse by winging it. It all may be way more straight-forward than I’m imagining, but I’d rather not risk it (at least until I get a second machine that I’m willing to hack and possibly break -unintentionally- that wouldn’t upset me 
because I made a mistake).
Sorry, I lost track of where we are on this …
Do we have any links in the forum to YouTube vids detailing how to replace any parts on stock/semi-stock K40s? I’d love to watch a video or read a blog with accompanying pictures if I can get my hands on either to help me follow some sort of proceedure to minimize the possibility of me screwing things up worse by winging it.
What can we do to help … what do you want to repair?
Perhaps this blog can be of help…
No worries. I’ll read that link over and see if I can fully understand what’s going on there (gave it a quick glance and didn’t absorb enough to be sure if I’ll fully understand it later). As far as the YouTube videos, that was a secondary curiousity/question and hope that something like this had already been started and could be linked to. But I’ll be doing some YouTube searching myself and collecting links in the not-too-distant future. I may have ran that all together and came off confusing. Sorry about that.
What exactly would you like a video of?
In the grand scheme of things… disassembly and reassembly of everything. 
But if I had to pick something specific, I’d have to think on it and would likely lean toward the things necessary to fix my current problem. What those things are, I still need to make time to clearly narrow down (which would also be nice to have… some troubleshooting guides).
