Breathing Life into a Vintage Gas LaserFollow article
Attempts at rejuvenating a Spectra Physics 120 helium neon laser and 256 exciter.
It's no secret that I'm something of a fan of classic computing and I have a modest collection of minicomputers that date from the late 1970s through to the early 1990s. However, this is by no means where my interest in classic technology ends and it encompasses all manner of things, from radio and telecommunications equipment — to lasers.
Nowadays you can pick up powerful solid state lasers for very little and they are far more compact and way more efficient than glass tubes filled with gas that is excited by high voltages. But at the same time they are nowhere near as much fun — provided that size, reliability and energy consumption are not primary concerns!
I recently picked up a Spectra Physics model 120 laser head and matching 256 exciter (power supply), a combination that was introduced in the late 1960s and which, if like me you appreciate solid engineering, clean lines and a dash of retro-futurism, holds a certain appeal.
I can't say for certain when this particular set was made, but it probably dates from the 1970s and this is pretty old for a laser.
The seller did state that the laser was non-functioning but that the tube glowed as it should do. Early designs such as this employ external mirrors at either end of the tube and he noted that these may need aligning in order to restore it to working order. Of course, it may take much more than this...
Other potential problems include loss of helium as this can permeate the glass, along with air ingress, and of course failure in the ageing high voltage power supply.
First power up
Signs of life but no red laser output.
On applying power the tube glowed, albeit without any laser output. Now I have to admit that, while being a casual enthusiast, I'm very much a novice when it comes to laser technology. So the first thing I did was to turn to the excellent Sam's Laser FAQ, which over the years has come to attain cult status among laser enthusiasts (and professionals too, I'm sure).
One suggestion for a non-functioning HeNe is to leave it powered up for hours or even days, and if you're lucky it may start to lase once again. However, after three or four hours of operation a buzzing sound could be heard and shortly after the tube went dark.
Fortunately, I discovered that if I removed the power, waited a while and then restored it, the tube would glow once more. So not a catastrophic failure, but something is obviously not right! Also, it was now the case that the tube would only glow for a few minutes before dropping out.
Suspecting the power supply, I decided to insert an ammeter between the tube cathode and ground (using 10kV rated wire!) A high voltage probe was used with a DMM to measure tube voltage.
The current should be 6mA and this is what it originally measured as, but a post to the sci.electronics.repair newsgroup resulted in a suggestion from none other than Sam of Laser FAQ fame himself, that I might try increasing it to 7mA to see if this stops the dropping out.
Sadly, increasing the tube current didn't seem to have any effect, even if it was turned up to just short of 8mA. With air ingress being suspected another suggestion was to use a fan to cool the tube to see if it held the plasma for longer — and if it did, this would confirm that failure mode.
I'm happy to report that cooling had no noticeable effect and the plasma was still lost after a couple of minutes of operation.
Sam did say that loss of helium is probably at least one of the problems and so I've ordered a disposable cannister of this, as used to fill party balloons. Amazingly, the solution for getting it into the tube is a “helium soak”, whereby you wrap the tube in a bag and fill that with helium. The suggestion being of one day soaking for every year of the tube's life. Which may sound crazy, but if helium can leech out over the years and the tube is below atmospheric pressure...
Also, the only specs I could find stated that the tube's operating voltage is 3.7kV, whereas I measured it at 2.7kV. That is until the plasma was lost and the power supply ramped back up to the starting voltage and at which point 7kV could be measured, and nearer 7.5kV with the tube disconnected. So, perhaps there is a power supply problem also.
Finally, it could be that the optics are filthy and/or the mirrors badly out of alignment. But the consensus seems to be to leave these until a last resort, as it's easy to make things much worse!
Thoughts so far
I have to admit that my expectations were not so high given the laser could be 40 years old and made only 10 years after the creation of the first ever working laser.
If it gets to the point of confirming that the tube is suffering from air contamination, there is probably little more I can do, as refilling it would require high vacuum equipment, an oven, glass blowing tooling and a lot of patience and skill.
Regardless of the outcome, it's been an interesting learning process and with its 1960s design, I would argue that the laser has some utility as a decorative object if nothing else!