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Thread: Omnichrome 532/150T

  1. #11
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    Hello Steve,
    Let me compress these messages into one reply, your comments are in blue.

    MixedGas: “You need an isolated scope (isolated from POWER HOT, NEUTRAL, AND GROUND) for reading the test points. Which is dangerous as can be. There is a startup adjustment procedure post repair.”

    The search begins for an isolation transformer. There’s going to be a bit of iron in that transformer, for 5 kW’s. While that goes on, I can familiarize myself with the power supply and component location, looking for anything obviously burned up. Such as checking the picofuses.

    MixedGas: “You need to look at c35, D20, Q8, Q9, c11, c12, c13, the gate drivers / gate clamping on both sides of the gate transformer and D20 as a minimum on the power section.
    C35 / D20 failures happened often.
    Run in current if you can, till repaired. Oscillation was easy to obtain if your not careful with the adjustments.”


    D20 (MUR1560) and C35 (220 µF/250V) are straightforward, so I’ll get a few of those and Q8 & Q9 if I can find the part number.

    The remote connections will be setup to run in current mode being mindful of the possibility of oscillation.

    MixedGas: “There is a beast called a "noise potentiometer" and loop gain potentiometers that I remember needing careful adjustment.”

    At this early stage, I’m staying far way from any loop/PID adjustments. If they need to be adjusted that can happen when the PSU is performing more or less typically.

    MixedGas: “Also it never seems to amaze me about the number of bad / overloaded breakers out there. False tripping from a reactive load like this is not uncommon. Toss in a GFCI etc if you have one as a known issue with these. I know the 170 series seems to be a redesign including a power factor correction module and a lot of extra line filters.
    GFCI / AFCI do not like Oudin/tesla coils and switching laser PSUs.”


    The system is running in the shed, with few other things on those outlets. The breakers aren’t required to be the GFCI/AFCI here and I’m glad actually. The GFCI’s are very sensitive like you say to RF and switching PSU’s so I’d rather keep them out. What still bothers me is that after power down by the breaker going out, the laser system was powered back up and reignited fine.

    MixedGas: “Hark! There is hope. Check that the circuits match, but here are most of the waveforms I would have had to scan...

    https://www.skywise711.com/lasers/Om...ual/index.html


    Edit: Check all the picofuses... They look like 1/8 or 1/4th watt resistors but ARE fuses, before you do anything else.

    I quote his opening paragraph and agree:


    IMPORTANT SAFETY NOTE: Make sure the test equipment is AC line isolated before working on laser. Use an isolation transformer for the laser power supply. A rating of 5 kW is adequate. The isolation transformer may be removed for final noise measurements, after the scope is removed from the circuit.”


    Excellent, thank you for the links! Indeed, I’ll heed the warnings with these power supplies. Although, relatively comfortable working with lasers, high voltage, and power systems the safety aspects can never be over emphasized..

    I’ll take a good look at the fuses before going any further.

    Mark

  2. #12
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    1750 Watts calculated.... You might get away with a much smaller isolator.

    Also if I remember right, check the anode pins in the cable. If "cooked" they tend to pull away from the CPC connector on the umbilical.

    Series 4/5 CPC tools for pin change are out there.

    Steve
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  3. #13
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    Just a comment. Be careful and make sure that tube is lighting down the main central bore and not a gas return path. When the tube light down the main bore the ceramic will glow a unform blue. If it lights down a gas return path the blue will appear to glow on the outer side of the ceramic depending on which of the 6 paths it took.

    The inside of the tube viewed by a cross-section has a central 0.040" diameter main bore surrounded by 6ea 0.020" diameter gas return path bores. It is common for older tubes not run much to light down a gas return path and you will see a higher voltage due to the higher current density. If is does light down one of the 6 return path turn it off. if it runs too long it creates a carbon path and then prefers to light down that path all the time.

    Due to ion pumping, the Ar gas is pushed to one end of the tube when it is running. The 6 gas return path allows the gas to circulate.

    Cross section of the ceramic tube


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  4. #14
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    Was there any cure other than anecdotal for "Down the return Bore" ? I know the one guy has a patent on using a Helium, Krypton, Argon mix (8,2,1) for active cleaning of dirty bores during rebuild. I had one or two clean up over the years with a soft discharge at a 20 mA or so, but it was never repeatable.

    Steve
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  5. #15
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    Yes, the addition of another nobel gas was one option, but could reduce laser power or change the tube voltage. Our primary solution was to insert a 0.5" long piece of nickel wire into each end of the gas returns. This significantly helped reduce the likelihood of Ignition down a return. It's a balancing game to create a path of least resistance and maintain proper gas flow. If the returns are too small the tube will not sustain a discharge or extinguish
    at higher tube currents which increases gas pumping. Diameter of the bore to the diameter of the gas returns, the number of gas returns to get proper gas balancing, the distance of the gas returns in relation to the main bore, all of these together play a part in minimizing Ignition down a return path and sustaining a stable discharge.

  6. #16
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    The laser head ignites and stays stable at between 4-10Amps, tube voltage is around 111 VDC.

    To get there a few things happened.

    Bought and assembled an isolation transformer to test the waveforms.

    Bought a second powersupply (150) and head from the auction site we know and love.

    The short of it is that the first power supplies (150T) Q8 mosfet is dead. After finally taking the advice to test D20 (MUR1560), and the MOSFETs found that one lead of the diode was disconnected. The even more interesting thing is that the second power supply acquired, had the same diode lead disconnected. The diode is hidden behind the C35 cap and almost impossible to see the leads without taking the board out, let alone reconnecting the lead. Although, this could be a failure mode for the power supply since the board has a lot of weight on and can flex it also might be some shenanigans are afoot. The black diode image is the 150T and the blue color diode image is the 150.

    After testing the mosfets and reconnecting the diode lead on the newly acquire powersupply (150), running in current mode the laser head maintained discharge without issue. No coherent photons in sight, so the next step is following the alignment procedure. Need to build the alignment jigs first but it's promising.

    The second powersupply came with another head, which although low hours is even older than the original, and has ~200 hours, 1991 vintage. It's likely unsalvageable, pressure through the roof. Attempting to start it with an Oudin coil, resulted in external flashover on the laser body. No hint of internal discharge at all. Though the filament lights and glows without issue. It was a long shot, but it'll be a good donor for the other head. MIght have a go with the tube out so it doesn't have anywhere to arc to.

    Thanks,
    Mark

    P.S. The 150T has IXTH24N50's in it, which aren't made any longer, or are being sold for criminal prices ($33/each!). A possible alternative I'm looking at is SIHG25N50E-GE3, though slightly different might work. It was mentioned in other discussions that the MOSFETs are matched (might be?). Each MOSFET I tested from the 150T had written in the back 4.38, assuming the Vgsth (gate source threshold), the 150's mosfets were lower values. So matching is a must.
    Any suggestions on MOSFET selection? How closely do they have to match? I'd rather avoid having to buy 100 to match them...
    Attached Thumbnails Attached Thumbnails 150diode.jpg  

    150Tdiode.jpg  

    laseroutput.jpg  


  7. #17
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    I "batched" them, always bought a couple of pairs in the same batch. Never matched them, assumed two off the line would be close. Did plenty of replacements. Did nearly always end up adjusting the noise gain etc for stability.

    Let me know if you need a plasma tube for head two, or an aligned head, or a sealed mirror tube for that matter. They need to go.


    Steve
    Last edited by mixedgas; 01-23-2025 at 10:22.
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  8. #18
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    That makes sense, the plan was to buy 4-6 of them and like you say they should be close to each other. I'm up for the challenge of adjusting the Noise pot, sure it'll be easier than the optical alignment.

    I'm interested in any plasma tubes/heads/sealed tubes. I've got an empty Cyonics 2214 head without a plasma tube.

    Thanks,
    Mark

  9. #19
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    Loosen the two mirror retaining screws on one mirror assembly (Outer two screws) till you have enough slop to slip a 0.020 or 0.030 " feeler under the rim. Twist, tilt, apply pressure, pull, wiggle that mount. Tear down the dead one so you understand how fragile a Brewster stem is, but try the wiggle for a flash. It doesn't have to be that loose. You may get a few hundred microwatts of 488 at some point. Note the direction you are pulling tugging pushing and adjust the outer two adjustment nuts accordingly. Leave the Pivot nut alone. That only gets touched under certain fine tuning circumstances. Usually the HR is a good place to start.

    Some times you get lucky.

    The O-rings get stiff, I usually loosen by flooding a little methanol onto the stems. That can be very touchy on an older unit, the stem covers, that is.

    Cooling is critical. Lets discuss your cooling system, shall we?

    What make / model fan do you have? Is it on a extension hose?

    I marked the two screws in question on the attached picture with orange circles.

    Steve
    Attached Thumbnails Attached Thumbnails ALC OMNI.png  

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  10. #20
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    It lases without issue now. Putting out maybe 20 mW by calibrated eye-o-meter.

    Best to ask first then mess with optics, is the lesson here.

    On the HR, instead of messing with the 3/8" alignment bolts straight away, I loosened up the two allen heads shown in your image, then rocked the mount. Damn thing flashed straight away and I could hold the mount to get continuous lasing (video). With the allen heads loose, the mount needed to be moved to the right horizontally. So going against advice, I turned the 3/8" pivot bolt 1/3 of a turn and all lasing was gone. Welp damn, so I tightened the allen heads and boom strong lasing started again. This is all luck, next one is going to be hours marathon. Based on the shipping box and 1 sheet of news paper it was shipped with I expected some major alignment.

    The power isn't peaked but it's putting out a strong beam. Sadly, only at 488nm (threw a grating in front) time to start the search for multiline optics. Replaced the light sensor box on the front, but the laser just goes full power over the course of a half second, then says pinned. That'll be the next fix. Though, I did check the sensor initially with 2 - 9 volt batteries and it seemed to respond properly.

    I've been using the fan that came with the head. It's a Papst 6028S, ~300 CFM (294 from the 500 m3/h). The fan bearings seem fine, spins smooth and takes awhile to stop. The fan is sat on a spacer to improve flow, but the two rubber mounts are broken to hold the fan down.

    I tore the bad tube down until I had to start messing with the fragile parts, and things that would throw alignment way out. The O-rings were a concern of mine too, I'll give them a little lite soak in methanol (BTW where have you gotten GC/Spec grade from?) The mirror mounts/copper baffles come off by removing the allen screws, then working them out applying as little lateral force as possible?

    Thanks again!

    Mark
    Attached Thumbnails Attached Thumbnails head.jpg  

    bright.jpg  

    brighter.jpg  

    fan.jpg  

    fan2.jpg  

    overalltube.jpg  

    fins1.jpg  

    closeup.jpg  

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