Want help with HeNe tube.

[Mattronium]

I have a HeNe laser tube that I am currently working on.
It is from MWK Industries, 1mW max. I can get it to light and hold the discharge, but there is no output.
I tested it using a 77.1K Ohm ballast resistor with 1500V @ 4mA and 1450V @ 4-6mA power supplies and got the same results.

From Sam’s FAQ it looks like if the tube is getting more than 2X-3X the correct current lasing will stop, but that would be ~8-15mA and the PSU’s only does 4-6mA.
AFAIK if the current is too low there should still be lasing until the discharge stops at 1/2-2/3 normal current.

And if it is not gas intact I don’t think there should be ANY discharge.

So I think that the only other possibility is that it must be the mirror alignment.
I built a wood “mount” to test if the mirrors are only slightly misaligned using this method: Sam's Laser FAQ - HeNe Laser Testing, Adjustment, Repair
But before I try to bend anything I want to check with you guys to see if I am correct with what I’m thinking the problem is.

It also appears like there are collar type mirror adjusters at both ends. Should I do anything with these?
Link: Sam's Laser FAQ - HeNe Laser Testing, Adjustment, Repair Different adjusters listed, including the collar type.

More pictures soon.

 

[ultimatekaiser]

That's odd that's a rather large tube for 1mW. What color is the discharge? It should be a salmon color. it looks like a melles griot tube, so it probably runs optimal at 6.5mA and closer to 1800V+ballast if it's a 9" tube. Mine is the same profile and does about 2KV with a 75k ballast, that might not be the proper power supply for it.

As for the no output, check the mirror alignment, sometimes they get a little banged up in shipping. Just apply gentle pressure to the side of the cathode mirror mount at 45° angles and see if you get anything.

 

[Mattronium]

The discharge looks fine to me. Yes, the tube is ~9-10 inches long.

I would not be surprised at all if it is a MG tube considering that I didn't find ANY info or reference to MWK Industries, and Sam's FAQ said that "most or all higher quality Melles Griot laser tubes used to come with a locking collar permanently installed at one or both ends...excluding barcode scanners and newer tubes."

It is definitely not the right power supply, because I have absolutely no idea what it needs, except for rough estimates based on size. Although I just realized that I do have a 2.5KV 3.1-6.5mA PSU for my GreeNe that I could try.

I'll see if I can fire it up with the other PSU tonight and then maybe try testing the cathode side mirror.

Pic of it on.

 

[ultimatekaiser]

now that I can see it closer like this, it looks an inch longer than my P-122, which is running on a 2.1KV supply @6.5mA with a 75K anode ballast and a 10K cathode ballast.
but if you have stable discharge then its at least working. Going off of that if I knew its exact length and diameter in mm I could likely tell you which tube it is....It looks like an 05-LHP-097 which is a wider bore tube. so it runs more around 1600V @ 4.5mA if I remember right. The pot is standard left to right, low to high power. you can probably ballpark the current doing that to settle it in proper.

but as its not emitting a beam, its very likely a mirror alignment issue.

 

[Mattronium]

It is approximately 10.5in or 266mm long, I'll see if I can also measure/calculate the diameter.
BTW, are HeNe power supplies constant current or constant voltage?


EDIT: The diameter I measured is 35 mm.

If I use my other PSU (2.5KV 3.1-6.5mA) should I just add a 75K ballast to the cathode side?

 

[ultimatekaiser]

It is approximately 10.5in or 266mm long, I'll see if I can also measure/calculate the diameter.
BTW, are HeNe power supplies constant current or constant voltage?

Constant current, with voltage change as necessary to sustain the discharge arc. (hopefully within the compliance range or things may go 'pop' )

interesting...I don't think i've seen any tubes that are that long, that run that low except my brewster tube (1500V @ 6.5mA). 1500V is not much, so if it's running at that low, it must have a very wide bore, or the ballast is making it happy at that low current, while taxing the power supply.

It would be fun to put that on my variable and find where it's happiest. Are you getting that value measured? or is that just assumed? the power supplies can run out of their range to some extent, so it may not necessarily be in compliance and still work fine. if it's happy it's happy though. i'm not arguing with it, and 2.5K is probably a bit much.

I digress though... the point is moot as the mirror mounts are likely bent, causing no beam. Low current can sometimes cause a weak beam, but usually it'll drop out long before you lose it.

 

[Sigurthr]

HeNe supplies are constant current, but have a very high voltage ignition pulse. Gas discharge tubes exhibit a negative resistance characteristic, where an increase in current actually lowers the voltage drop across the tube, which is what the ballast resistor is there to counteract.

It looks like a solid discharge of the right color. I suspect a minor mirror alignment issue.

It looks like a pretty standard MG tube, and has the adjusting collars as well. The way these collars work is the set screw is slightly larger in diameter than the joint it fits in to, so it wedges that area apart slightly upon insertion.

Follow what Dr. Sam suggests and use a rolled up sheet of paper as a rocking tool to gently apply pressure to the mirror mounts (and the mounts only!!!!) in a systematic manner. Since you have no emission, you'll know when you get emission easily (trying to align for that last 500uW is another story!). Be patient, take your time, be careful and deliberate with movements. Remember, over-adjusting the alignment collar can rip off the mirror mount.

 

[Mattronium]

Yah! it lases, I rolled up a paper and applied pressure to the anode side (no room on the cathode side) and when I applied pressure in one way it LASED I didn't measure the power but it looked like roughly 1mW.

Picture! The dot is kind of hard to see, it is on the paper proped on the left.

Closer up.

 

[ultimatekaiser]

Congrats sig summed it up nice. I couldn't type that up without being late for work. The anode is likely planar, and the OC concave, so you can use it as your fine adjustment.