Spectra Physics 120 HeNe / 256 Exciter problem.

[Andrew Back]

Hello,

I recently picked up an old Spectra Physics 120 HeNe laser head plus 256 Exciter, and I'm not getting any laser output. However, the tube glow looks to be around the right sort of colour and I don't think there is air leakage.

I've yet to try cleaning the optics, but I have checked the tube voltage and current and I'm not sure if I have a problem here, as it's 2.7kV and 6mA, whereas Sam's Laser FAQ suggests it should be 3.7kV and 7mA. Also, after being powered up for a while the power supply seems to drop out -- I get a buzzing sound and the plasma goes intermittent, before stopping altogether. When it's intermittent the tube voltage spikes up to around 4-5kV (at least that's what I see it jump to on my DMM). So, perhaps a PSU component is on its way out?

I'd be grateful of any suggestions that folks may have. I'm hoping it's simply a case of resistor or cap that has not aged well.

Cheers,

Andrew

[Jon Elson]

Andrew Back wrote:

> Hello,
>
> I recently picked up an old Spectra Physics 120 HeNe laser head plus 256
> Exciter, and I'm not getting any laser output. However, the tube glow
> looks to be around the right sort of colour and I don't think there is air
> leakage.
>
> I've yet to try cleaning the optics, but I have checked the tube voltage
> and current and I'm not sure if I have a problem here, as it's 2.7kV and
> 6mA, whereas Sam's Laser FAQ suggests it should be 3.7kV and 7mA. Also,
> after being powered up for a while the power supply seems to drop out -- I
> get a buzzing sound and the plasma goes intermittent, before stopping
> altogether. When it's intermittent the tube voltage spikes up to around
> 4-5kV (at least that's what I see it jump to on my DMM). So, perhaps a PSU
> component is on its way out?

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

[Samuel M. Goldwasser]

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.

--
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Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

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[Samuel M. Goldwasser]

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. :( :)

[dave]

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!

[Andrew Back]

Hi Sam,

On Wednesday, 5 March 2014 21:16:34 UTC, Samuel M. Goldwasser wrote:

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

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

> 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.

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

[Andrew Back]

It still dropped out with the current set at 7mA. After a while a buzzing sound 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 dropped out unless some component is heating up and causing failure -- perhaps related to the voltage being down also.

Cheers,

Andrew

[Samuel M. Goldwasser]

These are not laser level type lasers. ;-)

[Samuel M. Goldwasser]

Andrew Back <arb...@gmail.com> writes:

> It still dropped out with the current set at 7mA. After a while a
> buzzing sound 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 dropped out unless some component is
> heating up and causing failure -- perhaps related to the voltage
> being down also.

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. :)

[dave]

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).

[Andrew Back]

On Thursday, 6 March 2014 21:39:29 UTC, Samuel M. Goldwasser wrote:

>
>
> > It still dropped out with the current set at 7mA. After a while a
>
> > buzzing sound 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 dropped out unless some component is
>
> > heating up and causing failure -- perhaps related to the voltage
>
> > being down also.
>
>
>
> If it's low even when the discharge is stable, then likely tube contamination.

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

> 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.

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

> 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.

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.

> (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.)

Right, good to know.

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

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

[Tom Miller]

"Samuel M. Goldwasser" <s...@repairfaq.org> wrote in message
news:u61nrd...@repairfaq.org...

> Andrew Back <arb...@gmail.com> writes:
>
>> It still dropped out with the current set at 7mA. After a while a
>> buzzing sound 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 dropped out unless some component is
>> heating up and causing failure -- perhaps related to the voltage
>> being down also.
>
> 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. :)
>
> --

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

[Jon Elson]

Samuel M. Goldwasser wrote:


> 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.

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

[Andrew Back]

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

[Andrew Back]

On Thursday, 6 March 2014 21:39:29 UTC, Samuel M. Goldwasser wrote:

> 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.

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:

http://designspark.com/eng/blog/breathing-life-into-a-vintage-gas-laser

> (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.)

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.

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

Never! :o)

Cheers,

Andrew

[dave]

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.

[Jon Elson]

dave wrote:


> 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

[Samuel M. Goldwasser]

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.

[Samuel M. Goldwasser]

Andrew Back <arb...@gmail.com> writes:

> On Thursday, 6 March 2014 21:39:29 UTC, Samuel M. Goldwasser wrote:
>
> > 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.
>
> 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:
>
> http://designspark.com/eng/blog/breathing-life-into-a-vintage-gas-laser
>
> > (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.)
>
> 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.

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. ;-)

> > Possibly time to install a modern laser tube or head. :)
>
> Never! :o)

Or a diode laser. ;-)

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

[Samuel M. Goldwasser]

Balderdahs. ;)

[Mike Tomlinson]

En el artículo <ede84062-bdd6-486e...@googlegroups.com>,
Andrew Back <arb...@gmail.com> escribió:

> http://designspark.com/eng/blog/breathing-life-into-a-vintage-gas-laser

Thanks for posting that, I'd been wondering what it looked like.

--
(\_/)
(='.'=)
(")_(")

[Jon Elson]

Mike Tomlinson wrote:

> En el artículo <ede84062-bdd6-486e...@googlegroups.com>,
> Andrew Back <arb...@gmail.com> escribió:
>
>> http://designspark.com/eng/blog/breathing-life-into-a-vintage-gas-laser
>
> Thanks for posting that, I'd been wondering what it looked like.
>

Looking at the photos, this MAY be an external cavity laser. The
typical HeNe has the mirrors cemented right on the end of the
central glass tube. External cavity lasers have one (or rarely
both) mirrors separate from the laser tube, and generally have
Brewster windows (thin glass plates mounted at about 56 degrees
on the end(s) of the tube.) The external mirror will have
some form of adjusting screws to align the mirror.

I used to be the master mirror aligner at our lab. My technique
was to shoot a working laser through the dead laser's two
mirrors, and observer the spot on a card at the far end. Generally
you could see a 2nd reflection on the card, and try to steer them
together. if the dead laser was being excited, you would occasionally
see short flashes of lasing when you passed through the proper
alignment. As you get closer, you had to make extremely small
adjustments and then take you hands away to let everything stablize
thermally. It is a very frustrating process, but if your laser
is, indeed external cavity, then you may need to do this.

Jon

[Phil Hobbs]

The other approach is to slack one adjusting screw off all the way, jam
a screwdriver in to let you rock that axis back and forth rapidly while
slowly turning the other knob. Once you start seeing flashes, you're
close enough to switch to using both knobs.

The auxiliary laser approach is certainly needed if you're aligning the
laser for the first time after a tube change, or if somebody's been
monkeying with both of the mirrors.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

[Samuel M. Goldwasser]

I should note that if the color of the discharge truly looks correct,
it should lase despite the problems with keeping it lit.

However, it's hard to tell from digital photos whether this is the
case. The proper color is bright unsaturated red-orange, sometimes
refered to as "salmon" color. short of spectral analysis of the
discharge glow, comparing it with a known healthy red HeNe laser
tube would be best. IF it's too pink or weak, then it's probably
leaked.

If the SP specs are to be believed, the tube voltage is way low, which
is another symptom. That really is the only way the tube voltage can
be low with the proper current. It's not a power supply fault.

I'm probably not quite motivated enough yet to measure the voltage
on the one I have here that was in a stasis field for 30-40 years and
works like new.

[Andrew Back]

On Saturday, 8 March 2014 21:04:52 UTC, Samuel M. Goldwasser wrote:

> I should note that if the color of the discharge truly looks correct,
>
> it should lase despite the problems with keeping it lit.
>
>
>
> However, it's hard to tell from digital photos whether this is the
>
> case. The proper color is bright unsaturated red-orange, sometimes
>
> refered to as "salmon" color. short of spectral analysis of the
>
> discharge glow, comparing it with a known healthy red HeNe laser
>
> tube would be best. IF it's too pink or weak, then it's probably
>
> leaked.

I've been looking at a low cost spectrometer solution:

http://www.science-surplus.com/products/spectrometers

Do you know which grating might be best for this sort of task, and perhaps also using with Ar lasers?

> If the SP specs are to be believed, the tube voltage is way low, which
>
> is another symptom. That really is the only way the tube voltage can
>
> be low with the proper current. It's not a power supply fault.

This I don't quite get -- how can the tube drag the voltage down if the current is correct? My guess was that the current would need to be too high.

Regards,

Andrew

[Samuel M. Goldwasser]

The VI relationship of a gas tube is highly non-linear and also depends
critically on the gases present. Add in low-excitation energy gases
like N2 and O2, and the voltage is lower at the same current. This
isn't a fixed resistor!

As far as the spectrometer, I've heard that they work well but the
resolution isn't that great. I use a Verity monochromator. It's more
of a pain to use being manual with a micrometer. But with a set of
narrow slits, it can easily small fraction of a nm.