Needed help with repairing HeNe Laser system - Page 1
Forum section: electronics / repair

author: bborisov567
author_rank: Regular Contributor
timestamp: October 21, 2022, 05:34:36 pm
content: I got an old HeNe Laser system used for laser acupuncture. Unfortunately it is not working. I don't have schematic and finding one is impossible. The two left switches are labeled "Power" and "Laser preparation", the one in the middle - "Modulation". The writings on the knobs are - "Modulation" , "Time" and "Tone". There is also a weird wire coming on the far right of the device (the red banana plug, the other end is just a wire). When powered on nothing really happens - if the "Laser preparation" switch is off the led is flashing with the modulation frequncy. When the "Laser preparation" switch is on the led is solid red. The current indicator is not moving at all. I believe there is a problem in the high voltage part. From what i can see there is 3055 transistor, a transformer and the a bunch of diodes and capacitors. My question is how can i safely measure the output voltage, probably around 3-4 kV? I want to know if there is a problem with the tube or the driving circuitry.

author: james_s
author_rank: Super Contributor
timestamp: October 21, 2022, 05:38:39 pm
content: That's a fascinating system, I've never seen one quite like it. I bet Sam Goldwasser would be interested in seeing photos of that, he'd add it to his FAQ which is a wealth of information on lasers.https://www.repairfaq.org/sam/lasersam.htmYou can measure the voltage with a HV probe. Typically there will be a transformer supplying around 1-2kV followed by a voltage doubler or tripler which is followed by another multiplier to generate a high enough voltage to strike the tube. I can see the diodes and capacitors of the multiplier on one of those boards. The other boards are probably related to the modulation features. See if you can measure voltage on the primary of the HV transformer, that looks like a mains frequency iron transformer type.

author: bborisov567
author_rank: Regular Contributor
timestamp: November 01, 2022, 09:33:01 pm
content: I wanted to ask if there is a way to test the tube without power supply? Also there are 3 wires (low voltage, from the modulation board) going into the tube assembly alogside the HV cable. What might be their purpouse?

author: jonpaul
author_rank: Super Contributor
timestamp: November 01, 2022, 09:59:06 pm
content: tube testuse HV induction coil from neon sign or vacuum labpower supplyusually dead HV transformers or bad HV multiplier diode or caplifetime of the HeNe is perhaps 5..10 years due to He leakage thru seals.BEWARE OF LETHAL HIGH VOLTAGEbon courageJon

author: Gyro
author_rank: Super Contributor
timestamp: November 01, 2022, 10:16:23 pm
content: QuoteNot directly into the tube - HeNe laser tubeshatereverse polarity or AC. It will sputter the end window and kill it.QuoteI have one that is 35+ years old and still fine.

author: james_s
author_rank: Super Contributor
timestamp: November 01, 2022, 10:31:38 pm
content: Quote from: Gyro on November 01, 2022, 10:16:23 pmHard sealed tubes have an indefinite shelf life. Soft seal (epoxy holding the mirrors on) are much less reliable. Most HeNe tubes made in the last 30 years are hard seal.

author: bborisov567
author_rank: Regular Contributor
timestamp: November 01, 2022, 11:14:14 pm
content: Unfortunately i dont have a HV probe to measure the output. Here are a couple of pics of the HV board. Two wires go into it(green/white and brown). Green/white is ground. I have 25v here. Checked the diodes with multimeter - they are fine. Checked the two 3055s without desoldering, they seem to be good. Resistors are also good. Checked the transformer windings, the look good too. Now by looking at it the transformer has 6 wires going in(the black ones) and the yellow ones are HV output. Why the transformer has 4 primary windings? How it is called this type of inverter? I found similar schematic on Sam's website. I apologize if i sound stupid it is my first time working with this kind of device and i don't want to blow something or get shocked.

author: Stray Electron
author_rank: Super Contributor
timestamp: November 02, 2022, 12:07:59 am
content: I can't help you in Bulgaria but old HV meters and probes were commonly for color TV servicing in the US 30+ years ago and are still widely available in the used TE market. They usually read up to 30 kV. Take a look on Ebay.The problem with He-Ne lasers is that the Helium diffuses through the seals and the glass much faster than the Neon does, so eventually the gas mix is too far out of balance to fluoresce properly. Most gas LASERS only lase at a very specific frequency due to the Brewster angles so if there is no florescence or if it is the wrong wavelength then no lasing.

author: daisizhou
author_rank: Super Contributor
timestamp: November 02, 2022, 12:12:35 am
content: First of all, make sure that the 25V voltage is a real DC voltage. It is recommended to disconnect the load circuit of the rear stage and find a small light bulb of about 25V to test whether the small light bulb can be lit. If it can light up, it means that the 25V voltage is real. If it can't light up, it means the fault is in front.

author: james_s
author_rank: Super Contributor
timestamp: November 02, 2022, 02:25:23 am
content: Quote from: Stray Electron on November 02, 2022, 12:07:59 amIt isn't fluorescence, it's stimulated emission. The vast majority of HeNe lasers do not have Brewster windows, they have dichroic mirros that are wavelength specific.That power supply is very unusual in that it's an inverter type that is not potted in epoxy. At a glance it looks like it's probably a Royer oscillator, a self oscillating push-pull design using what are probably NPN transistors. A similar circuit is used in most compact fluorescent lamps.

author: DavidKo
author_rank: Frequent Contributor
timestamp: November 02, 2022, 06:38:05 am
content: Stimulated emission can work even without mirrors if you have long enough thin tube. Long gas tubes like HeNe lasers mostly use the mirrors to help with selecting the transversal laser mode - ideal is TEM00. The tunable resonator can be outside of the tube and the Brewster windows are for such cases. But I think that was a long time ago, when solid state lasers were more or less unobtanium and gas tubes were easy to produce - HeNe for red or N2 lasers for green. With laser diodes everything changed, and anyone can have small laser in the pocket with significantly better efficiency and even power than HeNe. The HeNe power is given by the tube length and amount of the gas inside. I'm not sure why to use HeNe nowadays except low TEM mode (smallest divergence). Definitely not this application.Such a medical devices were more or less LASER hype. The device only stimulates the cells with ared light. You can use any red light source available with enough power, but laser looks better- added placebo effect to red light.You can try to check if the power supply is working since it had build in meter. Build the serial resistor network to have voltage rating cca 10kV (starting voltage can be higher) and cca 100uA@4kV. Connect it instead of anode and cathode of the tube. I expect that the red wire will be some sort of safety to ensure that there is no dangerous voltage on the metal housing (to check that the grounding is OK).

author: james_s
author_rank: Super Contributor
timestamp: November 02, 2022, 07:16:00 am
content: N2 lasers produce UV, gas laser options for green were HeNe (much less efficient than red, so much harder to make) and argon ion. Argon often had multiline optics that would produce several green, blue and violet lines at once, I have a small air cooled argon laser, as well as green, yellow and red HeNe.

author: Psi
author_rank: Super Contributor
timestamp: November 02, 2022, 07:24:31 am
content: Post your questions onhttps://www.photonlexicon.com/forum.People there are extremely knowledgeable about all types of lasers and how they work, and common faults.It's where all the gray beard laser guys hang out.

author: DavidKo
author_rank: Frequent Contributor
timestamp: November 02, 2022, 07:40:16 am
content: Quote from: james_s on November 02, 2022, 07:16:00 amYou are right I have mixed that up. The reason why HeNe lasers are so common compared to other gas lasers in visible spectrum is that they work without any hassle. In principle it is straight Neon tube with additional gas and openings on the end.

author: Gyro
author_rank: Super Contributor
timestamp: November 02, 2022, 09:18:36 am
content: Quote from: bborisov567 on November 01, 2022, 11:14:14 pmHave you checked all of the diodes and capacitors on the secondary side of the transformer for shorts? A shorted part in the HV multiplier would screw up its output voltage.

author: james_s
author_rank: Super Contributor
timestamp: November 02, 2022, 04:40:50 pm
content: A trick you can use to test power supplies of this type is hold a neon indicator such as a NE2 lamp up against the transformer, if it is working the lamp will usually glow. You can also use a neon tube from a sign to check the output.

author: Mazo
author_rank: Regular Contributor
timestamp: November 02, 2022, 05:33:25 pm
content: Здравей,пробвай да пишеш на БАН(естествено на някой адекватен по темата институт),със сигурност ще се намери някой който да има схеми,акъл за даване и т.н.Ако не се оправиш,мога да опитам аз през 1-2ма души,които имат подобно изглеждащи лазери,точно оттам.Относно високоволтовата сонда,от Викиват можеш да си вземеш високоволтови,високоомни резистори и да си направиш сам делител,че да измериш високото с мултицет(внимавай все пак!).---------------------------------------------------------------------------------------English:Told the guy to ask around in our academy of sciences as the laser looks made in Bulgaria,and there will probably be someone who has the schematics and all the repair info/parts.Some instructions on where to buy suitable HV resistors in our country so he can build a HV probe if he feels confident enough.

author: bborisov567
author_rank: Regular Contributor
timestamp: November 02, 2022, 06:29:44 pm
content: Няма да се наложи, проблемът явно е в тръбата. Високо напрежение има - изгорялата 7106 в мултимера ще го потвърди. Явно 15 мегаома делител не са достатъчни.Та, ще си остане да напомня какви неща сме правили. Мерси все пак, не се бях сещал да се свържа с БАН.============================Well, there is high voltage at the output. The blown ic7106 in my DMM can confirm it. 15Mohm divider wasn't enough i guess. I checked the HV cable and the ballast resistors at the tube. So everything seems to be okay but the tube.

author: james_s
author_rank: Super Contributor
timestamp: November 02, 2022, 06:31:51 pm
content: It's possible you have a bad capacitor in the multiplier and are not getting the full starting voltage. Most of the time even a bad tube will still light up, unless it has completely lost vacuum. Is there a getter flash on the glass?

author: bborisov567
author_rank: Regular Contributor
timestamp: November 02, 2022, 06:51:05 pm
content: Unfortunately i can't get the tube out of the cylinder because it is glued, i can only see from the back, here are a couple of pics. I was looking at the tube in the dark and there is no sign of light. Is there a way to check the caps from the multiplier without desoldering?

author: james_s
author_rank: Super Contributor
timestamp: November 02, 2022, 07:09:03 pm
content: Can you find another similar HeNe tube for testing? Or a regular neon tube? HeNe tubes used to be fairly cheap but I haven't looked lately.

author: bborisov567
author_rank: Regular Contributor
timestamp: November 02, 2022, 07:55:32 pm
content: Finding another tube here in Bulgaria is going to be a challenge.

author: DavidKo
author_rank: Frequent Contributor
timestamp: November 03, 2022, 11:32:11 am
content: Quote from: bborisov567 on November 02, 2022, 06:29:44 pmWhat you mean with 15MOhm divider? You had not written the input resistance of your meter. Had you built a divider or the resistor was only used in series with the multimeter. The 15MOhm:1.5MOhm divider will be probably influenced by the internal resistance of the meter.

author: bborisov567
author_rank: Regular Contributor
timestamp: November 03, 2022, 08:41:27 pm
content: I used the resistors in series with the multimeter. I don't know the exact input resistance, nor the exact voltage i am measuring. I assumed 1Mohm for the input resistance. Which should allowed me to measure around 15kV on the 1000 V range.

author: james_s
author_rank: Super Contributor
timestamp: November 04, 2022, 01:50:35 am
content: Quote from: bborisov567 on November 02, 2022, 07:55:32 pmWhat about a neon tube like from a sign? Are neon signs common over there? Is there a sign shop around? I know nothing about conditions in Bulgaria but here it's fairly common for sign shops to have bits and pieces of neon left over from broken or decommissioned signs. You need something that would operate on a transformer of about 3kV.

author: LesWright
author_rank: Newbie
timestamp: January 07, 2023, 02:41:46 pm
content: For a quick and dirty test of the tube, disconnect it from the PSU, and hook it up to a piezo multi-spark cooker igniter similar to the one in the picture at the end of this post.These generate about 10-15kV at current low enough not to damage the tube, but high enough that it will flicker and Lase (albeit at very low power, microwatts per pulse).If the tube is good then its worth looking at the PSU.

author: jonpaul
author_rank: Super Contributor
timestamp: January 07, 2023, 04:23:43 pm
content: These were popular in the 1970s..1990s.Superseded by diode lasers.The HeNe laser tubes need many kV to start nd run at 500..3000 V DC perhaps 2...50 mA.Thus the PSU is LETHAL and dangerous to test equipment.Usually the HV transformer insulation or the HeNe tube fails after decades.Not fixable.Jon

author: james_s
author_rank: Super Contributor
timestamp: January 07, 2023, 07:33:41 pm
content: Quote from: jonpaul on January 07, 2023, 04:23:43 pmHeNe lasers are still used and manufactured, it's true that diode lasers have replaced them in most applications but not all. The beam quality of a gas laser is far higher than that of any direct emission diode laser and HeNe operates one one of a few very specific and precise wavelengths which are not affected by the temperature of the tube.I have never had a transformer fail, although I have had other failures in the power supplies which are mostly potted bricks, I have fixed quite a few of them by de-potting the affected area. If the tube has failed they can still be found. The power supplies can be dangerous but they are not likely to be lethal, the current is very low, typically 3-6mA depending on the tube.