[DEAD] Repairing broken HeNe laser

No, the Uniphase is just fine, don't worry

Update #3: Fuse popping problem solved; waiting on parts. Still investigating stray voltage leaking to laser case.

Update #2: Laser tube is confirmed working! Except now I am reading 93 volts AC between the host and ground! See my second post, below for details.

Update #1: It might not be the tube, turns out one of the components has a heatsink which is connected directly to the host. The board is otherwise loose inside the case. When the laser hit the floor the leads on that component broke. The component is clearly loose. Heading downstairs to warm up the soldering iron. Stay tuned!

This post concerns a non-working Metrologic HeNe laser, 632.8nm. No idea what model number or what the actual output was, there is no label on the outside indicating any of that. There's the usual safety sticker of course; class IIIa laser, according to that. Very, very old, 1980's I figure. This has got to be a really nice one; it has a video and audio input on the back.

I got the laser when I was in 11th grade, back in 1998. I was 17 at the time. This is the first laser I ever owned. I never did find out if the a/v modulation worked but it put out a typical TEM00 red beam.

Then, I broke it by accident. It fell off my bed and dropped a few feet onto a carpeted floor, and after that it never lased again. I'm not kidding when I say I'm klutzy and tend to drop things without warning, and this laser clearly paid the price for that. The memory of it is still crystal-clear and it's always sort of bothered me, in a way. I figured the tube was damaged and that it wasn't fixable, or at least that it was beyond anything I could do for it. Ebay didn't exist at that time and neither did LPF. I had no idea where to find a replacement tube, and I assumed it would have cost 100's of dollars anyway. So, I put the the laser away and it's sat in a box in my basement ever since, 15 years now.

Until today! Nowadays we have LPF and ebay, and between the two maybe I can bring this laser back to life. So, I pulled it from storage, opened it up carefully, and had a look to see if something might be done.

The power light comes on when I flip the switch, but the tube doesn't light up at all. Not even the pink plasma discharge. Nothing rattles around inside the tube. It's hard to see into the tube as there is a metal can covering most everything, but I'm pretty sure that the central bore tube is cracked. Never actually saw a HeNe tube in person until now but I'm pretty sure the bore tube is supposed to be a single piece of glass all the way through, and when I shine a laser into it (flashlight batteries are currently dead) it looks like the bore has a crack in it. Would that cause it to not light up at all though?

The board looks fine, can't imagine why it wouldn't be, so I'm going to cross my fingers and assume that that isn't the problem. I'm not sure how to test it for high voltage output since my multimeter only goes up to 1000 volts. No spark or anything after I unplugged it and shorted the HV leads, but maybe the caps have resistors across them to drain off unused charge. The ground lead broke off the board due to a really poor solder joint, but that's a trivial repair. Advice on how to test the board would definitely be appreciated.

What do you all think? Probably just killed the tube and all it needs is a new one, I hope?

Photos:

In the first two I tried to get a shot of what I think is the crack in the bore tube. The metal can covers up pretty much everything and the viewing angle is therefore awkward. The automatic focus wasn't cooperating much either:


Bore tube attempt #2:


Tube serial number. It matches the number on the tube's actual label, which is hidden below this hand-written one:


The board, bottom side. Looks OK to me. Lots of pretty caps. Nothing missing or broken, except the ground lead on the right:


Top side of the board:


The back panel, with A/V jacks:


Aperture panel:


Closer look at the host/casing. Is it still called a host when it's a gas laser? White enameled aluminum:


Safety sticker, old and faded:

Re: Repairing broken HeNe laser

If you decide you want some diagnostics done I can test both the tube and the power supply, but you'll have to pay shipping both ways. I can do basic oscilloscope probing, short circuit current measurements, high impedance HV measurements, and RF excitation testing of the tube to determine fill gas health and if it will lase or not. I'm not equipped to test the A/V modulation linearity but can inject a test signal in and check for it in the laser's output in case I do get it lasing.

Re: Repairing broken HeNe laser

Any update? Have you repaired the loose connections?

Eager to see how this works out!

:beer:
-Matt

Re: Repairing broken HeNe laser

Yes, the laser is working again! When it hit the floor it broke the leads on the voltage regulator (I think that's what it is). I resoldered them and the laser came right on as soon as I powered it up.

I didn't plug the laser straight in. First I turned on the laser's switch, then plugged it into a power bar, which was switched off. Then plugged in the power bar and used its switch to power up the laser. It worked, the laser came right on and the beam was exactly as I remembered!

But now I have a new problem: I'm reading 93 volts AC between the host and ground. I don't trust old electronics in general and I especially don't trust anything I've worked on until I prove that it's safe. Good thing! I put my meter between the host and the kitchen sink and sure enough, the host is definitely live. Not sure what is causing that or how to fix it just yet. I resoldered the ground wire to the board but the outlet in question is 2-prong, not grounded. I'm hesitant to try a grounded outlet because I'm worried that something will short out and really kill the laser. More likely it would just pop the fuse, but still....

I only read the voltage when the laser is on, so maybe it's that voltage regulator component; maybe I didn't do something right. Really do not like working with mains voltage/current at all so to me this is a much more serious issue.

But at least the laser is working! My first "real" laser repair, on my first laser no less Now I just need to find out why the host is live!

ETA: I have confirmed that it's the component I re-soldered. For some reason, its heatsink is live to ground. I need to find out why that is.
Maybe it's some sort of leakage from that component; the resistance between the mains leads and the host is very high, about 1500k ohms, and actually I had to work at it to get the meter to read voltage at all. But eventually I got a solid connection and it read a constant 93 volts AC. Both the live and neutral give me resistance values, ~1800k ohms and ~1500k ohms, respectively. However the live wire only reads when the power switch is on. The neutral reads regardless of whether the switch is on.

edit 2/28/14 at1:43 AM - I was wrong, the live wire reads ~1800k ohms to ground and the neutral 1500k ohms. I have edited everything below to reflect the corrected information:

For all I know, maybe it was always like that. The last time I had the thing turned on was 15 years ago and I had handled the unit while powered many times. Never a problem. So I don't know if this is a new problem, or if it was always that way, in which case it evidently wasn't an issue.

But there shouldn't be any readable resistance at all, should there? Between the host and the mains leads? At 93 volts RMS that would give me a peak voltage of about +/- 131 volts. Since we mainly care about the live wire, then according to Ohm's law, 131 volts divided by 1800k ohms resistance *should* mean that the peak current is limited to no more than 72 µA = 0.072mA = 0.000072 amps = 52µA RMS.

However I am not satisfied with that, the price of being wrong could be very high. There should be no connection at all between any electrified component and the host, should there? The investigation continues as we speak, will update when I have more information.

Re: [MOSTLY FIXED] Repairing broken HeNe laser

First off put a silpad insulator between that Heatsink and the case connection. The back of the Tab of that mounting device is likely live.

Re: [MOSTLY FIXED] Repairing broken HeNe laser

Laser is dead again. The leads are too fragile and will not stay solid.

PISS POOR design, to connect such a sensitive and important component to the case in such a way that it will break off the board at the first drop or shock. Frustrating!

The tab does have an insulating washer between it and the heatsink. It's not the component itself that is shorting to the case, it's the entire ground bar on the board.

I don't think there is anything else I can do for it, I've tried resoldering the leads and the laser doesn't light up at all. Maybe I should send it to a member to see if they can do something with it, but I think there's not much else I can do with it.

ETA: Ergh, okay, laser NOT dead. Make up your mind, already, sheesh. The tube and HV stuff are just fine, it's those damn component leads that are the problem. Now if I could just find out where that voltage is coming from.

Seriously tempted to replace that heatsink with something else and then figure out a different way to hold the board in place. It really is a poor design that even a mild drop or shock will shear the leads right off, and it's a really nice laser, so might be worth it to find an "engineering fix" There is definitely a solution here, I just haven't figured it out yet.

Do you think it's dangerous to have that going to the case when there's 1800k ohms between the case and live?

Re: [MOSTLY FIXED] Repairing broken HeNe laser

Aww man, I think I just burned up that troublesome component, oops. I ran the laser without the heatsink in place to see if the part would get hot, and it sure does. Imagine that. It's a TIP50, an NPN power transistor 400 volts and 1 amp. Laser suddenly turned off and it turns out I popped the fuse, which was rated at 750mA. Leads me to believe the TIP50 got toasted.

If I did actually kill that TIP50 it might be just as well; it's no problem to replace it with a new one, it's a 50-cent part. The broken leads were a strong argument for replacing it anyway and the thought did cross my mind. With a new one I could let the leads be longer and therefore more resistant to shearing off if the unit gets bumped.

I wonder if I could replace it with an NPN transister rated at 400 volts but 5 or 10 amps. Hmm. Thoughts on that?

O well, looks like I will be off to Radio Shack tomorrow for some new fuses and maybe the TIP50 if they stock them. Been needing to go for a month now anyway, because I popped the 2A fuse in my multimeter by accidentally shorting an 18650 through the low current ammeter circuit.

Will update tomorrow based on what happens.

Update 2/28/14 @ 13:22 - Pretty sure that TIP50 is dead, replaced the fuse and it popped as soon as I turned on the power. Ordered some replacement TIP50's, which I will wire in such a way that it will be impossible to break the leads by jolting the laser case. I guess we'll just have to wait and see how it goes.

And since the tube is confirmed working, if nothing else I can just replace the whole board if need be, or use the tube in another project. So, regardless, this should end up OK.

Without LPF it never would have happened, I never would have had the confidence or know-how to diagnose and repair the laser. Thanks c0ldshadow and everyone else for making this possible.

Re: [MOSTLY FIXED] Repairing broken HeNe laser

There should have been a spacer in there to keep the board from moving around. Someone probably
opened it up and lost it. Just put something in there to get it to hold still and it should be fine.

Radio Shack does not stock the TIP50.

Re: [MOSTLY FIXED] Repairing broken HeNe laser

Well, having torn up my C6 beyond my ability I figured I needed *some* success in my day, so I opened up the Metrologic to see if I could find any.

The TIP50 was definitely internally shorted, no resistance at all between its leads and in fact the plastic was melted. I cut it off the board and desoldered the remaining leads and made sure none of the solder points were shorted. Powered up the unit and it didn't lase, but it didn't pop the fuse either. Same behavior as when its leads were broken. That is a good sign.

I have replacement parts in the mail as we speak and I have a feeling changing out that TIP50 will bring the unit back to life.

Also, I checked the resistance from case to mains leads again.

I have the DMM on the 2000k ohm setting. I connect one test probe to case, and the other to each mains lead in turn. When I do that, my meter reads 1500k to 1900k ohms for a split second, and then the resistance goes off scale. This happens with both hot and neutral leads. Very high resistance for a split second, then off-scale.

So apparently there is *some* connection between the mains and the case, but resistance between the two is above 2 megohms. I have no idea where it's coming from, but it sure isn't that TIP50, because the TIP50 isn't there anymore.

I tested the ground lead to the case. Dead short, no resistance. The ground lead is connected properly. I tested the ground lead to the mains leads and it's off scale. Doesn't show any connection to the mains at all.

All of this having been tested with the power switch on, and off. Fuse is confirmed good.

Is that OK, then? I tried googling for "how much resistance between mains and case is safe" and multiple variants of those search terms, but could find nothing beyond the fact that ~20mA through your body is enough to kill you, and that 0.2 to 1.0 mA can be perceived as a tingling on the fingers.

However by Ohm's Law, at 2 megohms and 120Vrms, the RMS current from mains to case to ground should be ~0.06mA. This is assuming a totally ungrounded outlet, and that the host (and person holding it) are connected to ground. For a grounded outlet, any current should be conducted straight to ground, since the grounding lead is connected properly.

Lastly, I have verified that all outlets in my house are, in fact, wired correctly; the hot and neutral are as they should be and they are not reversed. I did this via DMM, testing hot to ground, and then neutral to ground. For two-prong (ungrounded) outlets I tested using the copper water pipes as ground. All of this on 1000VAC DMM setting. Everything checks out.

When the laser was working, and even after pulling it from storage, I turned it on and held it in my hands and felt nothing at all. Never an issue with shocks or anything else. As a result, I had never tested it for a case short to mains. It was only after opening it up, fixing the damaged leads, and getting it to work again that I tested that - because I don't trust things I've worked on until I can prove they're safe. To quote Tyrion Lannister, "I like living!"

So that is the current update, pending arrival of my replacement parts. I really would appreciate any advice on the potentially live case issue though; how many megohms of resistance between case and mains are "safe" ?

Sorry for the rambling wall of text and seeming paranoia, but I really do care about safety and I really don't want to get toasted.

Re: [ONGOING] Repairing broken HeNe laser

There might be some class X or Y capacitors between ground (case) and mains which would register as a quickly climbing resistance that shows as open circuit once charged.


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Re: [ONGOING] Repairing broken HeNe laser

A climbing resistance indicates there is a capacitance somewhere. As long it is small enough, it won't
allow enough current to flow to pose an electrocution hazard. It may however cause a problem for the
transistor. I would put it on a mica insulator just to be sure. A sil pad is another option, but mica is
preferred. Then there will be no chance of electrocution or damage to the transistor. Then just put
something in there to keep the board from moving around before you close it up.

Re: [ONGOING] Repairing broken HeNe laser

The Lightning Stalker said:

A climbing resistance indicates there is a capacitance somewhere. As long it is small enough, it won't
allow enough current to flow to pose an electrocution hazard. It may however cause a problem for the
transistor. I would put it on a mica insulator just to be sure. A sil pad is another option, but mica is
preferred. Then there will be no chance of electrocution or damage to the transistor. Then just put
something in there to keep the board from moving around before you close it up.

Click to expand...

I keep forgetting to mention that the transister does have... a sil pad, is it? on it. And a little plastic tube thingy surrounding the screw. I think Radio Shack has a TO-220 mounting kit which includes a mica insulator, maybe I will see about that.

Re: [ONGOING] Repairing broken HeNe laser

Seeing all the recent mistakes, I'll save you from making another one. Be careful with the mica insulator.
Don't flex it or else it will "crease". Sometimes they even come "pre-creased" from the factory. So watch
out for that. Put some new thermal compound in there as well. Don't try and reuse the dirty old stuff.

Re: [ONGOING] Repairing broken HeNe laser

Yep, one of the reasons I prefer high quality sil-pads over mica.

Re: [ONGOING] Repairing broken HeNe laser

Many devices have low value capacitors to ground
for noise suppression / isolation. A DC meter will
read what you see. Once you get it running, try
measuring current from case to ground.
My old ham transmitter had a cap to ground on
on both hot and neut.
HMike

Re: [ONGOING] Repairing broken HeNe laser

well, it's a bust.

Got my parts in, soldered the replacement onto the board and no dice.

Another laser killed by Marco Polo.

Don't ever hire me to fix your stuff because all you'll get back is a pile of worthless junk for the metal recycling man.