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Thread: How to open a GSI Lumonics G120DT laser scanner?

  1. #1
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    Default How to open a GSI Lumonics G120DT laser scanner?

    Hello Guys,

    I have recently become an owner of an spectra physics 171 Argon/Krypton laser.
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    I'm very happy with it and with every day the love for old lasers and the old skool goodness still increases.
    Gathering the parts to repair it is coming along well and with the help of some PL members I will surely get it back up and running if I listen with care and act with care.
    I'll post a thread when more progress has made with plenty of pictures for all you guys.

    However included was a pair of GSI Lumonics G120DT scanners with amps made by hb-laser.
    The unit didn't work at first but I fixed it as I measured there was an unproper power connection and some soldering and heat shrinking made it all work.

    replaced the broken mirror on one and it works fine.
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    However the other one seems to have trouble, it can move but squeels *at low speeds for visual testing* and the scanner board shows with an indicator it has problems maintaining its position without getting forced back by the spring mechanism.

    It might be toast but I want to give it a shot by opening it for a check-up.

    Now the main question is: Is it possible to open these scanners without breaking the unit?
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    Last edited by masterpj; 10-21-2014 at 16:36. Reason: added quick pics of laser

  2. #2
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    Yes it is but putting it back together takes an oscope and some patience. I'll let Dr. Roberts tell you more. Hell, Steve probably has a few laying around that he would probably sell you.
    Those who fail to grasp art are the ones who criticize it.

  3. #3
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    I cant type the procedure right now. I'm jammed up writing a business proposal.

    Do you have access to a nice 10 Mhz oscilloscope with a millivolt scale? (not one of those 80$ USB Ebay toys)

    I'm not so sure its the G120, often this is the amp was adjusted too critically. Some times the rotor just vibrates past the sensor zero when the set screw gets barely loose.

    Which model of amplifier is it connected to? That matters.

    You'll also need a English/Inch/Non-metric Allen key to loosen the base set screw. Do not loosen the torsion bar set screw until told to do so, or you will be getting another G120 from my basement.

    If you open the scanner, the circuit board leads are ultra short and made from #30 Teflon wire. If they break, your done. Also there is about 100- 200V AC peak in there. It will shock you if you power it up when its open.

    Fortunately I probably still have a dead one to demonstrate the procedure.

    This also requires pulling the baseplate off the galvo. This usually is a cosmetic disaster as they need to be glued back on.

    You'll also need a very good magnifier lens to solder under , three 2n2222 transistors,(prefer matched 2n3904 npn) and one heck of a lot of patience. One also wishes for a very fine tipped, temperature controlled soldering iron. Unless its a very new one, the parts are not surface mount. They are literally rammed into the galvo case.

    For that reason, lets check for broken torsion bars and check the amplifier circuit before we open a galvo.

    It looks like you can just clamp the torsion bar back into place, but the rotor neutral position needs to be set to milliradian accuracy. This is doable, actually its easy, but you need to hear how to do it, else the galvo will just slam to one side. If not set correctly you end up with a very confused, out of phase, position sensor.

    One way or another, a rebuild is a FrankenGalvo... They work fine, but your always wondering when it will decide to go to Silicon Heaven. On a good day I have a 60% success rate.

    See patents: US3959673 and US4135119

    The position sensor is this one:

    US4694235

    You do have the 171 field manual and schematic right?

    Ask HB for a amp manual, or at least where the position sensor test point is.

    DO NOT grab the mirror and twist, you'll possibly snap the torsion bar.



    Steve
    Last edited by mixedgas; 10-21-2014 at 19:24.
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    I have a Rigol DS1052E digital oscilloscope that is rated 50mhz but its firmware modded to work at 100mhz and it's dual channel.

    Swapping the x and Y keeps the issue with the scanner and it worked fine yesterday
    Yesterday: http://laserpon3.tumblr.com/post/100...anners-made-by

    Here a video of the issue now: https://www.youtube.com/watch?v=_2As...ature=youtu.be
    *note the squeeling of the galvo at the end*
    Here a picture of the amp:
    The board is made by HB-laser and is marked: SCANAMP HB-Laser 08/00
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    DZ suspects its a broken torsion bar

  5. #5
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    See if HB will tell you the position and velocity test points. Your scope is excellent.

    Goal is to lift the output fuse so there is no drive to the coil, look at the posit signal on the scope and gently rotate the shaft about 5 degrees each way. This lets us know if the sensor is working. The dual color LED may be wired to be out at "0" position, and brightness proportional to position. DO NOT ATTEMPT to manually rotate the mirror with the drive from the amp enabled. The output fuse must be lifted during manual tests.

    Turning the mirror manually can snap the Torsion bar if overdone, there are no "stops" to prevent breakage.

    Or you could loosen the setscrew and see if the Tbar clamp nub drops out. If it drops, the bar is broken. But lets not do that yet. If the Tbar is still good, loosening the setscrew is bad.

    The position sensor outputs are microamp currents, riding on RF "hash" so the best way is to use the position demodulator in the scanner amp to read the sensor.

    see http://www.laserfx.com/Backstage.Las.../Pinouts7.html

    Disconnect the galvo, reach for an ohm-meter.

    Series coil is about 7 ohms, parallel coil is about 3.2 ohms. ( selected coil config is choice of amp maker)

    Driving the coil with a AA or AAA battery will see if it rotates. Do not exceed 2.0 amps DC in the coil, ever! Just pulse the power on for a second, do not leave DC flowing in the coil at high currents. A variable bench power supply that can go down to a few tenths of a volt is a better deal, but the battery will slam the galvo to one side and limit current. Unlike a modern galvo, which will just stay where the battery put it, the torsion bar will return a G120 to mechanical zero. Because of the Tbar, position is K*coil input current where K is a proportional constant.

    (if you have a one watt or so audio amplifier, you can run a 20 Hz sine wave into the coil and see what happens, again just a Watt max)

    Wow, that is the most complex G120 amp I've ever seen... Even more so then a Turbotrack.

    Since your mirror is epoxied direct to the shaft, that makes repairs, well complicated.... (Never mind, edit, I watched your video and see you have the mirrors off....)

    Final edit:

    I viewed video two. Please NEVER run a tuned G120 amp without the mirror mass on the galvo, IT will most likely oscillate unstably..... this leads to broken torsion bars. OUCH!

    Its OK, you didn't know about the old ways...

    Steve
    Last edited by mixedgas; 10-21-2014 at 20:09.
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    Quote Originally Posted by mixedgas View Post
    One way or another, a rebuild is a FrankenGalvo... They work fine, but your always wondering when it will decide to go to Silicon Heaven. On a good day I have a 60% success rate.
    That's pretty darn good success rate Steve. I have been through a lot of these and was usually able to make one working "FrankenGalvo" out of about three bad ones. GSI warned me I couldn't repair them because if I pulled the armature out of the magnet, it would become demagnetized and ruined. I took that as a challenge. They were pretty expensive as I recall - I think about $600 1970's dollars each. It was the only game in town.

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    After seeing the video, I have no doubt that it is a broken torsion bar. The fact that you have to turn it to get it to work means that when there is no power to it, the torsion bar is not zeroing the armature. When you apply power and turn it, you are manually zeroing it enough so that the PD picks it up and the amp starts doing the work. I bet that the instant you disconnect power, the shaft turns 90 to magnetic 0.
    Those who fail to grasp art are the ones who criticize it.

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    The sure test for broken torsion spring - If you can spin the shaft with your fingers round and round like a motor - the torsion spring is broken.

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    Quote Originally Posted by Photonbeam View Post
    The sure test for broken torsion spring - If you can spin the shaft with your fingers round and round like a motor - the torsion spring is broken.
    There is most certainly force on the shaft and like a clock it has several stages the spring will force it into position.
    I will do what mixed gas did but yesterday it didn't squeel and make errors.
    It's weird that yesterday this problem wasn't there and I didn't start turning or throwing it, etc.
    I had them in their mount ready to glue mirrors on for the next day because I had no epoxy

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    Quote Originally Posted by masterpj View Post
    There is most certainly force on the shaft and like a clock it has several stages the spring will force it into position.
    Again, sounds like a broken torsion spring. The "stages" you are feeling are the magnetic zero. The galvo will not work in those positions which is why you have to turn the shaft before the galvo starts working. I gave two of my old g120s away. If I still had any left over, I would have given it to you for the cost of shipping.
    Those who fail to grasp art are the ones who criticize it.

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