Coherent I90 - Sagging Filament - Help required please?

[smogthemog]

I am hoping someone can help me here

I have an Coherent I90 with what looks to be a sagging filament heater.
I ran this laser up for the first time at the last UKLEM. It lased after an interesting walk in but only at 750mW at about 35A tube current.
When I look through the bore, after removing the HR & OC mirrors and putting a white card with a torch focused on it at one end, I can see an obstruction. When the laser is powered, before it strikes, the obstruction is glowing brightly and can clearly be seen through the bore as being the Cathode filament.
I am hearing that there is a method of inverting the tube and running the filament at over temperature causing the coil to sag back the other way. I intend in disconnecting the primary side of the filament transformer and applying 230V? to it via a variac, slowly raising the voltage to a required level in order to glow the filament. I wish to try this method but would appreciate some guidance in doing this please?
Can anyone help?
Thanks

[LaNeK779]

maybe this post helps? it is for a different type of laser though

http://www.photonlexicon.com/forums/...thode+filament

[mixedgas]

Assuming you could get the right temperature range, using say a pyrometer, this could take weeks or years.
I've learned a lot since then, and there are a few things that need stripped out of the FAQ.
Best to find a new tube.

Sorry,

Steve

[smogthemog]

Oh Steve, I thought you would have known me a bit better than that by now mate I will be trying this as I said but was just looking for a little guidance. If it goes wrong then so be it, I will chin up and move on but there is 'always' a chance of anything working out so I will press on.

After completely stripping down an ALC68 PSU and head to the bear bones, cleaning everything and then reassembling and being told by many that it would never work again - I have much confidence in this task.

If it works - great. If not then so what lesson learned for the next time - to do it again

[Carl B]

Being one of the group to see this beast emit coherent light at the meet last weekend I must say "don't give up with this yet Jim".

[tonyaztec]

I wonder if I turned the wife upside down for a while, it would help with her sagging?

It was good to see it fired up thou Jim.

[kecked]

if it goes wrong and shorts you'll blow the entire passbank and do a LOT of damage.

[mixedgas]

if it goes wrong and shorts you'll blow the entire passbank and do a LOT of damage.

Yeah, Concur. If you do this strip the cathode transformer and its resonant capacitor out and run it without the PSU hooked up.
According to a blurb in the service manual the 50 HZ connection of the cathode is a different tapping altogether from the US version.

Which raises a point, if the factory transformer is a resonant constant voltage scheme, yer gonna need a bigger boat, I mean another transformer, Chief.
(apologies to JAWS) As well as a variac....

Steve

[smogthemog]

if it goes wrong and shorts you'll blow the entire passbank and do a LOT of damage.

Obviously, I realise this but that's the same with any electrical item, if you short something your gonna knacker it/something/something else.
It's not an issue AFAIAC but thanks for the concern

Steve, I will have a look at what you are saying and take it into consideration. This is a laser that has been used in the UK for some time and I guess with it being an ex show laser it will have had some knocks and bangs possibly hence the cathode filament issue.

What is the nominal filament voltage/current rating on these heads please?

[mixedgas]

I'll need to do some digging, the manual is not so clear. 3.2V and 25-27 amps is what some of my tube data sheets say. It gives a range of acceptable amperage, never a exact figure. This is because of the refill system, allowing the service engineer to set up a tube for optimal power and life. As the gas pressure changes, the cathode will very in temperature when the plasma is lit, resulting in a change in the indicated voltage. So there is a range, not a "exact" current.

Steve