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Thread: strange noise from lexel 85 head

  1. #1

    Default strange noise from lexel 85 head

    Hi there
    i post this message because i need some help about strange things.
    last week we break a brewster window of our lexel 85 head (we not know how ^ ^).
    to repair the resonator we change it with another one.
    this tube was from a box and "sleep" since 10 years.
    at the fisrt start the discharge won't start.

    after some times with cathode on the discharge go on and the tube current go to 16A.
    the passbank voltage is over 50v (out of the green scale) but not to high to kill them.

    but after 20 to 30 secondes a strange noise appear from the start transformer. and the discharge stops.

    we decide to let the cathode filament on for one hour to let the gas come from the reservoir.

    after this the laser work for more time (about 1 minute) and the noise appear again and stop the tube.

    now the tube voltage is near 124v @15A (i know this is too low) and when we try to increase the current to 17A the tube stop with same noise.

    someone know this phenomen? we made a video and i think to post it here ASAP.

    this is strange because the noise is similar of a start high voltage discharge and we kill two voltmeter (tube voltage) when the phenomen appear.

    but we not notice blink of the discharge when this noise appear.

    have you ever seen this thing on gas laser?

    here is the video

  2. #2
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
    Infinitus Excellentia Ion Laser Dominatus
    Join Date
    May 2007
    A lab with some dripping water on the floor.


    2 things, you have a new tube that was really a dead one at low pressure.

    Then your either 1 current meter reading was out of adjustment from bad passbank.
    You accidently over currented a low pressure tube, causing plasma oscillations.
    These cause high voltage spikes to flow through the power supply.

    or 2. Possibly also metal bypass capacitor in the laser head was bad , allowing ignitor energy spikes to damage ignitor and passbank.

    either way, you have experienced what is known as "Plasma Chirp" by the laser manufacturers, it is mainly seen on older glass spectra physics tubes at low pressure, but can occur on a lexel. I have seen it three times in 20 years of laser work, including once here at the university.
    It is not your fault, you could not have known it would happen.

    before using power suppy again, check the 2 ohm resistors in the transistor emitters for shorts or opens to ground, and check the many current summing resistors (usually 150 ohms) that feed back to the current card for shorts or burnout. If you have a adjustable 0-30V power supply you can check the current card out for function without a tube. I can tell you how to do that, Plus check all the passbank transistors for shorts.

    a extreme possible third option is if you were in light mode, the damping capacitor (on switch labeled fast, medium, slow,) on the light card was bad or set to fast.

    I know where there are some new tubes in boxes, go to He also has the spare ignitors.

    You'll need to tap the boost down more though, 50V is too much , the factory book says 22-24 @ 18 amps is best, the green range on the meter is shall we say, optimistic?

    rebuild the power supply and check the meters for correct function using the test points, you may be able to salvage one tube. correct tube V is 155 +/- 5V

    Sorry for your loss, Steve
    Last edited by mixedgas; 05-02-2008 at 09:01.

  3. #3


    thanks for your help

    i think the PSU is ok because she work fine just before changing the plasma tube (before we break the brewster window) and without oscillations but i will make some measurements.

    about the tap setting this is the first thing we try but even with the minimum voltage position the passbank voltage is too low.

    i will see your link for spare tubes but i paid this one 200 euros

    the PSU is used in current mode not in light mode we are sure.

    my friend give me a spare umbilical cable and i have one spare lexel OEM worhing head. i try to connect this umbilical to test the PSU with another head.

    i keep you informed thank you
    Last edited by Genesis; 05-02-2008 at 09:14.

  4. #4
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
    Infinitus Excellentia Ion Laser Dominatus
    Join Date
    May 2007
    A lab with some dripping water on the floor.


    use the lexel 88 tap chart we have posted in Sam's laser FAQ if you do not have a manual.
    Also the 88 schematics there are pretty much the same as a single phase 85, the difference is in the light card and front panel controls.
    good luck,


  5. #5


    the tap position is correct i'm in the minimum HV position (to keep the passbank in green scale) like in the manual (i have the original one from lexel with schematics )
    this is the original linear power supply.

    here is the head with his PSU
    Last edited by Genesis; 05-02-2008 at 10:20.

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