Spectra Physics 120 HeNe / 256 Exciter problem.

How old is it? Most glass-case HeNe lasers slowly leak the Helium away. You still get the pinkish glow from the He-Ne mixture, but there isn't enough He left to develop the population inversion of excited states to produce laser action. if the tube is over 20 years old, that is the most likely cause.

Are the mirrors separate from the HeNe tube? If so, this is an "open cavity" laser, and the mirrors need VERY careful alignment to get laser action. If the mirrors are glued to the end of the tube, then no alignment is possible.

Jon

Hi:

The tube probably IS at least one of the problems assuming no one has messed with the mirror alignment.

There's a range of current that would be OK. I'd suggest increasing it to 7 mA. That should stop it from dropping out. I don't think there is anything wrong electrically but the tube has leaked. If you can run it for a few hours or days, it may recover at least partially.

This ia an SP-120, probably from the 1970s. And, yes, those old tubes do leak due to the Epoxy seals. Modern tubes do not leak.

The dropout current is higher than 6 mA, so he should increase the current to 7 mA and let it run. It may recover, at least partially.

If no one has attempted to adjust alignment or clean the optics, they are probably OK. Else, all bets are off. It could be that someone found it didn't lase and really mucked it up. :( :)

You actually repaired SPs? We were a dealer and they wanted to do all the repairs themselves. Remember the 3600 rotator? Now you can get a rotatinglaser level for way under a hundred bucks! I don't miss the sewer business, don't get me wrong!

Ah, so I'll look into sourcing a bottle of helium.

I've turned the current up to 7mA and will see how it gets on. Any idea why the voltage is only ~2.7kV and what I could try to get the exciter output back up to 3.7kV?

Looking into the end of the tube the optics do look grubby, but I get the impression that attempting to clean them may not be helpful.

Thanks for the advice, it's much appreciated!

Regards,

Andrew

It still dropped out with the current set at 7mA. After a while a buzzing s ound develops which becomes increasingly low frequency until the plasma is lost, and sometimes it will then strike back up again. Not sure why it's dr opped out unless some component is heating up and causing failure -- perhap s related to the voltage being down also.

Cheers,

Andrew

hy the voltage is only ~2.7kV and what I could try to get the exciter outpu t back up to 3.7kV?

impression that attempting to clean them may not be helpful.

These are not laser level type lasers. ;-)

If it's low even when the discharge is stable, then likely tube contamination. I doubt a He soak will help. Unfortunately, it's not uncommon for SP-120s to have tubes that won't stay lit even at 8 mA though the color looks good.

One test to confirm it's the tube and not the power supply is to put a fan on the tube. If it's the tube, that should enable it to stay lit at least for a longer time.

(This assumes that the 3.7 kV value in the Laser FAQ is correct. It's from the spec sheet but I have not measured a healthy one.)

Possibly time to install a modern laser tube or head. :)

They aren't self leveling but otherwise they work great for drop ceilings and poured floors. SP had a cool alignment range (box of mirrors).

Interesting. Aside from stopping lasing, what does air contamination do in terms of plasma stability / tube voltage?

Ordered the He now, but I am sure it will find other uses and I'll at least give it a go.

So air contamination means it heats up and when it gets too hot the plasma drops out?

When I first got it (only a few weeks ago) it stayed lit for 4 or 5 hours. Whereas now it drops out after being powered up for 30-45 minutes.

Right, good to know.

To be honest I only picked this up because I like the design of older gas lasers and what might be regarded as classic tubes. Oh well.

Thanks again.

Andrew

Don't these power supplies have a several watt carbon comp ballast resistor? They are well known to drift up in value and/or become unstable when heating up.

Just a guess.

tm

I hate to dispute a known expert in the field, but as you say this laser may be almost FORTY years old! Yes, the crummy old tubes with sloppy epoxy seals may have leaked down in a few years, but even the best HeNe all-glass tubes will eventually lose their He. It diffuses right through the glass. And, the partially aluminum tubes leaked much faster, still.

Jon

Sam said earlier that it _will_ have lost some He and so that will be at least one of the problems it has. The question then was whether it has air contamination also.

I'm going to try giving the a helium soak, but if it came to having to break the seal, evacuate it, bake out and refill -- well, that's probably beyond me.

Regards,

Andrew

I tried this and it didn't seem to have an effect. A write up of experiences so far, along with a few photos, can be found at:

This still has me wondering and in particular when I look at those old resistors and caps etc... Helium ordered so will try a soak in any case.

Never! :o)

Cheers,

Andrew

I was told the gases become un-mixed. The Helium and Neon separate when the tube isn't used. You have to use them weekly to mix the gas, if nothing else.

That's quite humorous! Gases will not spontaneously separate out by gravity, you can probably find this in a physics textbook. While theoretically this could happen at VERY low temperatures, it is impossible at room temperature. I have glass HeNe lasers that are about 20 years old, and they developed the laser beam instantly when turned on. I also have some other odd lasers including an external cavity one, that will not lase no matter how long you run it, and a He soak was attempted without success.

Jon

Not on any time scale that matters. :) New/MOS HeNes from the early '80s with hard seals and even before when hard-seals were first introduced will still work like new. So, yes, everything leaks eventually but they have pretty much nailed the He diffusion through glass thing. That's not to say every tube will be like that but it is more common than not.

Please keep us informed. If you have a spectrometer/monochromator you can take a look at the discharge spectra and see if there are any O2/N2 linse, and whether those two yellow He/Ne lines show up and are about equal in intensity. See the Laser FAQ. ;-)

Or a diode laser. ;-)

Ah come on, no one will know. "You're getting 250 mW from that head???". :)

Balderdahs. ;)