HeNe not working

[Ttocsttammit]

I apologize if I'm in the wrong area for this question. I'm very new to lasers and want sure. I recently purchased a helium neon laser at a yard sale for a couple dollars. I dont have the power supply for it but have tried one that is matching at 12 volts unsure of amperage. Nothing has made the laser function, however the led power indicator is illuminating. Is this thing worth spending any time on or should I just trash it?

 

[paul1598419]

You will need a HeNe power supply for it. And it will have to be for one of similar power too. I would need to know which tube you have to help you further. I'm guessing it is the red 632.8nm laser that became ubiquitous during the last half of the 20th century, but it could be one of a number of other wavelengths. I also can provide schematics for building your own power supply.

It looks like yours has a driver inside the clear box. It may actually be good, but don't try hooking up power supplies to it trying to get lucky as that could destroy it.

 

[Ttocsttammit]

How would I go about finding the right power supply? The tube has no numbers or letters on it that I can see. The previous owner told me it was blue but I am unsure really of the color. The transformer/ballast component on the bottom has some specifications. I attached another photo. Thank you for your willingness to help.

 

[Anthony P]

Like Paul said... power supply. Anytime a HeNe is not working always suspect the power supply first.
HeNe lasers are not particularly valuable, but they are fun to work on. If you have any electronics experience, a supply can be built without too much effort. Also, pre-made power supplies are readily available. I will watch this thread and help if I can.

 

[paul1598419]

That photo does help. It states that the input to the HeNe supply is +12 VDC. The run Voltage is 2150 VDC at 4.5 to 6.5 mA. This is likely a 1 mW 632.8nm red HeNe laser. If the supply to the tube is bad you can pick one up used on eBay, but you will have to search for awhile before you will find one cheap enough. You can always buy one new at Meredith Instruments, but it will likely cost you over $70.00. If you can figure out which are the positive and negative inputs to the supply, try a +12 VDC at 2 to 3 amps. Otherwise I can give you a schematic for one that uses an automobile ignition coil, but you will need some skill at building circuits. Might try Meredith. If you aren't sure about the hookup, send more photos and I will be able to get you there.

 

[Ttocsttammit]

I have a decent understanding of dc electricity as I'm an automotive technician. Never played with ac much but I'm sure I could figure out how to get along enough to build a power supply.

 

[Ttocsttammit]

Is there a way I can test either the tube itself, or the output of the ballast/transformer component with a multimeter? My apologies for the lack of technical terms.

 

[paul1598419]

Don't know what a ballast transformer is, but these do have a ballast resistor on the anode of the tube. It is likely ~100K at 2 or 3 watts. The tube needs about 8 to 9 KVDC to start it lasing, then it falls to 2150 VDC to run. You might get lucky and have an output with the enclosed supply, but there is no way to test the tube with a DMM to see if it is working. Unless it has many long hours of use and the tube is still intact with no breaks or cracks it will likely work. These tubes are work horses.

Edit: Your photos are not well focused or easy to see through the clear box. I would take it apart to access the whole of the system and to be able to follow where the anode and cathode of the tube are connected to the enclosed supply. I need to see much more if I am to guide you to repairing this one.

 

[Anthony P]

"Audio Modulated". If the HV is working I would try to bypass modulation circuit. I suspect the audio transformer is toast. With power off and caps discharged, read resistance across secondary of "T1". I'd bet it's open.
From the pictures of that pc board, I would guess that is all audio modulation. There are no multiplier components that would indicate it is part of main power. That suggests that the black box under the tube is a self contained HeNe supply. The HV output would feed anode through the secondary of T1. It could likely be bypassed to get the tube running.

This is all speculation. If I had it in my hands I could probably figure it out, but trying to troubleshoot from pictures is challenging. If you decide to tinker with it, remember that HV is no joke. Although a HeNe supply probably won't kill you, it will make you choke on your chewing gum.

EDIT:
I did some searching and found that the modulated HeNe student laser sell for $670.00 new. That PC board is in fact an audio amp circuit. Just plug in microphone and transmit. Receiver sells for 150.00. But you can easily build one by connecting solar cell, photodiode, or non-diffuse LED to an amplified speaker ( like on computers).

 

[Cyparagon]

The easiest way to test this that I can think of is buy or borrow a working HeNe PSU and tube. Swap the tubes, and see which set lights. Yes, the current rating may not be the same, but a few seconds for a test won't hurt anything.

I'd guess it's a simple analog modulation circuit, so I think the beam should still be active with no input signal from the modulator section.

 

[Ttocsttammit]

Ok so in terms of circuitry and a/c electricity im somewhat of a dumbass. I have a pretty good understanding of d/c electronics but some things im not familiar with. How would I go about discharging the capacitors? Also the guy I bought it from sold it to me for 2 dollars, however at his yard sale he plugged a laptop power supply of 19.2vdc into this HV psu. I'm currently using a 12vdc 2.5A to try and power the device. It looks as if a few of the solder points on the board are brownish. Is that possibly a sign of overpowering the psu?

 

[Anthony P]

Everything in that case is DC. The only AC is the wall plug for you 12VDC supply. For those little HeNe tubes 1A is usually good.

To discharge caps, unplug and disconnect 12v power completely. With insulated screwdriver etc, short the terminals on the tube to each other. Then find the HV out from the HeNe supply ( black box under tube) and short them for a few seconds. Now it is ready to do some probing with logic probe or DMM set to continuity. From hv out of the black box, check to T1 audio transformer. You should have cont to one pin on transformer and then to a second pin with maybe a bit of resistance. Check T1 by itself for cont through secondary or some resistance ( probably up to 100 ohm). The secondary is the side that goes from power supply to the big resistor on laser tube. If you read nothing, then T1 is fried. The laser may still be made to work without audio. There are other things to check if that doesn't work, but that is where to start.

Edit:
It would be very helpful to see the wiring on the bottom side out of black box to pc board.

 

[Ttocsttammit]

I tested cont from the terminal at the front of the tube to T1 and found no resistance on the first pin with some resistance on the other two pins on the side of T1 closest to the tube. OL from the terminal at the front of the tube to the 3 pins on the side of T1 opposite of the tube. And across the 75k resistor I read OL

 

[icecruncher]

Sam's Laser FAQ - HeNe Laser Testing, Adjustment, Repair

 

[paul1598419]

Sam Goldwasser is a wealth of information about all lasers, including HeNe lasers. It would serve you well to read this.

 

[Anthony P]

Agreed, Sam'FAQ is the bible of lasers. "The Laser Cookbook" by Gordon McComb has a very detailed section on testing and powering unknown tubes.
Veify that the ballast resistor is good with ohm meter set to properly read 75k. They can burn out. Everyone uses a high wattage resistor which is fine, but they are not really high voltage resistors.
If you a comfortable with testing under power there are a couple quick ones. Treat everything as if it were ignition coil on car under power. Verify that you do indeed have 12V making it directly to the input side of the black box power supply. If yes go on to next step.
Attach a resistor to (3k is fine or there abouts) to your insulated shorting tool ( I use chopsticks with jumper wire). You should easily be able to draw a small arc from the tube terminal to terminal. Only do this test for the shortest amount of time. If no arc, power supply is fried. If there is an arc, the tube is broke.

As I said before, power supplies are easy to find. To build one only takes a small step-up transformer and a few diodes and caps... maybe couple of resistors.