Low power, Google ain't helping!

Well, i bought a k40 at auction. After a power supply, it fired up and produced a laser…nice.
Sad for me, at full power the ammeter says 8ma. Power stops going up when the knob is about halfway through its travel. I removed the knob to make sure it wasn’t just slipping.
Ohmed out at 481 from the ends, wiper goes from 0 to 481, stopping about halfway through. It’s stamped with 1k-2W.
Bad pot?

Hi Kenneth, the potmeter is normally a 5 kOhm pot. So I guess somebody has swapped it with a 1k Ohm. Hope this helps, cheers Paul

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Even if its a 1K pot it should read fully to 1000 ohms.
I would replace it with a 5k pot and see how that works.

First, thanks for the replies. I was expecting an email notification so I need to fix either my settings or my email. That’s easy.
The thing was completely disassembled in the box, all belts were loose, everything, so my guess was that it never operated. I suppose that doesn’t mean the pot is original, though!
I’m learning as I go, and the reported measurement was while it was hooked up. When I pulled the pot out of the circuit, it did, in fact, measure 1k. Now I can see that I wasn’t actually measuring the resistance across the pot because it was hooked up. Oops.
That said, I have a 10k ohm pot with unknown power rating. This thing is just sending a low power signal voltage, right? Can I try it out? Or should I just get the 5k and wait for the post?

I have seen both 1K and 5K used.
If the pot reads 0-1K then it is good.
10k likely will work also although I never looked at the pots application current requirement.
Yes, it is a low voltage signal going to the internal PWM generator.

The best way to test this is to measure DCV at the LPW IN pin.
When the pot is rotated the voltage at IN should read from 0 to 5V.

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Awesome. I’ll let you know how it turns out. Thanks again!

Ok. It does measure 0-5v on the wiper. So that’s good.
I also turned the knob back down to where the power JUST starts dropping below 8ma, and sure enough, 2.4v.
Any idea why the current would not go up if the voltage to the controller does?
This is probably a good time to note that the new power supply was different. Green plugs vs white ones on the old. I followed the labels on the board with help from pinout diagram I found on the web, and took my time.
Solid chance I borked something up?

So the net of this is?

  1. the IN voltage can be controlled from 0-5v
  2. the current stops increasing at about 1.2 way up
  3. this is a new laser power supply
  4. the tube is old

If the above is correct I suspect the tube…
Can you post a video of the tube firing.
Do you hear any sparking,archin or crackling sounds at or after the 8ma point

I think you understand correctly. I’m not sure what counts as old for a laser tube, though. Sticker claims a manufacture date of Feb 2019. I have maybe 30 or 40 hours on it trying different things. I picked it up in January, history before that is unclear, but I think it was a return that sat in the box after the original buyer couldn’t power it on (old shipping label was still attached).

Nothing weird, just a faint hiss. Can’t even hear it on the video, I think.

I guess I didn’t expect that the tube would draw less power as it failed, just that it would convert less of it into usable laser. Incorrect? Also, never got it past this point anyway, so nothing to compare it to.

Well, this is sorta hard to admit, but I guess I did suggest it could be my fault a couple posts up, right? Besides, I hate it when people never post back what the actual problem turned out to be.

I remembered I know a guy who builds co2 laser tubes for a hobby. From scratch. He suggested that maybe the ps had a limit pot on the side. So I pulled it out, looked it up and down, and what did I find?

Set for 230v. What a maroon. Flipped the switch and BAM full power.

Thanks for taking the time to help me out. I learned some things along the way, and you did help me rule out some possible causes.


Thanks for the followup that is a good thing to add to our arsenal of things to check on low power symptoms.

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