Old Spectra Physics 155 Model

[styojm]

Hi All
Our group has an old Spectra Physics 155 He-Ne laser in the storage room. I happen to found it sometime ago and amazingly it still works. The problem is that it will not switch on every time I turn it on---- like I have to try tens of time and there will be one click to fire the laser. I'm wondering if this is an electronics problem (like the starting circuit is loose somewhere) or is the tube is going bad? Much appreciated if someone can help me sort this out.
Attached are three pictures of the laser.

P.S. there seems to be some rust like stuff in the tube but I am not sure what they are...

 

[Hemlock_Mike]

That is the same as my OLD 155. It has the side tube like this one.
Mine lights up but the plasma is deep red like the H2 leaked out.
No beam on mine. If yours will lase, run it a long time to heat it.
That might work -----------__________
HMike

 

[styojm]

Hi HMike
Yeah, I've been keeping running it for days. Still, once I turned it off, next time it is very hard to turn it on--- That's why I'm wondering it might be electronic problem.

 

[Hemlock_Mike]

If you get plasma and no lase, it likely
lost some gas. I'll try mine again.
HMike

I light mine up again -- The plasma is blood red - not right.
It might be fun to put it in a tube and add He pressure at ~200 PSI
to see if it will re-gas.
I haven't tried measuring the current (electronic problem) but the
plasma appears bright -- wrong color :-(
HMike

 

[Sigurthr]

If it starts at all it is unlikely it is an electronic problem, but without measuring the HV output to see if a 10KV+ pulse is applied every starting attempt it is hard to tell. In low power HeNe's the click is usually an effect of the tube capacitance discharging down the bore once the arc is initially struck.

Run it a LOOOOOOOOOONG time. It could be just that it has lost some He and will never recover beyond this state, but sometimes long runs will force He back into the envelope which helps to lower the internal pressure and make starting easier.

Edit: Mike, I remember reading about revitalizing old HeNes which have lost their He but not come up to air by putting them in a pressure chamber fed with He from a tank. Several atmospheres of pressure was used and I think they got the tube back to as-new condition after only a few days.

 

[Hemlock_Mike]

Sig -- You must be as old as me
to know the old tricks on HeNe
tubes.
HMike

 

[styojm]

Hi Sig
Thanks for the advice. I'll try light it up longer to see if there's any change. Maybe I'll need some special tool to measure a 5000V pulse than just oscope or multimeter, right?

 

[Sigurthr]

Hehe, Mike, I'm but a whippersnapper in chronological age. My biological age is sadly a multiple of my chronological age, but fortunately my intellectual age matches that of my biological. More to the point I'm just an autodidact with a voracious appetite. You can see my picture on my website (it's in my signature).

Styojm; You would need a suitable very high impedance probe (or an impedance matching unit and HV probe, or suitable voltage divider) and a fast enough scope. I'm not sure what the rep rate or pulse width for that model's starter is but you don't really need to measure the starting pulse, just confirm that it is there. At the very minimum I would suggest a high voltage rated resistive voltage divider consisting of at least a billion ohms series resistance.

There's always the practical tests too but we wouldn't want to change damaging the circuitry since it IS actually starting up at least sometimes.