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Thread: Looking for help to repair an Nd:YAG flashlamp driver in ALC30

  1. #1
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    Default Looking for help to repair an Nd:YAG flashlamp driver in ALC30

    Hello, I'm looking for some help repairing a flashlamp power supply. It is part of a 1997-made 30W average, 4.4kW peak, 0.5-20ms pulse duration dental /jewellery spot welding laser marketed in Europe by the German companies Alphalaser and Siro Lasertec and in the US by Gramm Technology:

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    (picture from Gramm Technology website)

    I believe that it is originally made in China for/by Sintec Optronics, although some parts are also Russian (binocular microscope head). It's almost certainly also been marketed under other designations.

    The lamp looks clean, only 700000 pulses, so I'd expect to see some light at least from the trigger ignition pulse - or hear a crack, or something... but nada.

    The power supply (only marking is LPL10-1 on the logic board and LPL10-2 on the trigger board) runs off a single phase mains connection. It is a 20kg unit with 4 fibre optic connections for monitoring/control. The inverter/capacitor bank is working, the voltage is fairly close to the front panel setting, but there is no flash. It's a series trigger unit with a large, high current IGBT. The control CPU seems to be operational, the UV protection works at the appropriate moment and the fans come on when the CPU thinks it's triggered a pulse.

    I've checked all the electrolytic caps, 2 and 3-pin semicondutors on and around the board - IGBT, diodes, rectifiers, transistors and thyristor. No sign of a fault, no evidence of obvious arcing or high current damage to tracks or components.

    Unfortunately, some kind soul has taken the time to file off all the IC markings - and now I'm stumped. I don't know how to check the trigger circuit without blowing up either the test equipment or the circuit.

    I'm not even sure how to properly test whether the trigger signal is getting to the logic/trigger board.

    Does anyone have any ideas or experience to share? Perhaps even a known working board for sale?

    NB I am fully aware of the hazards of working on this type of equipment, I have been doing it for over 30 years.

    Thank you, fellow laserists!


    Pictures:

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    Pic 1: This is the trigger board, I suspect the fault is here somewhere... The component bolted to the board by the 4 electrolytics is, I believe, the ignition thyristor (BT151-800R).



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    Pic 2: Trigger board mounted in the power supply unit. At rear left, the large IGBT and the main rectifiers can be seen.
    In the foreground, the top of the logic board with the 4 monitor/control fibre optics.


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    Pic 3: (out of focus) close-up of logic board with 4 fibre optic connectors. NB all IC markings helpfully filed off.


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    Pic 4: View of power supply with boards mounted on top of the capacitor bank.
    Last edited by iggythebrush; 01-21-2018 at 02:47.

  2. #2
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    Lightbulb Update and correction

    After a few hours' spent reverse engineering (in the daylight this time, duh!), I found out several things...

    Firstly, the flashlamp is NOT series triggered. There is a conventional trigger transformer hidden in the perspex pump chamber with its EHT output lead bolted to the reflector.

    Secondly, the board in question performs several functions: plasma ignition, simmer voltage generation and IGBT pulse generation. I have worked out bits of the circuit and managed to test most of the individual sections of electronics by supplying external DC and using a DVM and scope. Basically, there are only two inputs, both coupled to the logic board via optocouplers. One is the ignition trigger, one is the main IGBT trigger.

    I did not find any faults, and reassembled the unit. Measurement of the simmer voltage (ca. 700V open circuit) and the trigger transformer primary voltage (ca. 350V o.c.) all appear correct, but the flashlamp would still not fire.

    So I dug around and found my old Tesla resonant coil vacuum leak tester from the 196os, disconnected the trigger transformer primary from the firing circuit (I could not easily disconnect the secondary as it passes through several o-rings and the cooling water to get to the reflector) and---

    Hey presto! The leak tester held against the connection bolt for the transformer secondary tickled the lamp into plasma and by pressing the pedal I could (sometimes) generate a flash, although the simmer circuit could not maintain a plasma in the flashlamp and I had to keep tickling it with the leak detector to make it flash again.

    Checking beyond the output coupler showed a focussed spot of coherent 1064nm flashing on a target - the beast has lased again today!

    So (please do correct me if I'm wrong here) everything points to a dying flash lamp:

    1) Trigger circuit cannot generate high enough voltage for ionisation and plasma;
    2) Simmer circuit cannot maintain plasma even after external triggering.
    3) Difficulty discharging at several hundred volts, even with external triggering.

    The only other possibility would be the trigger transformer, but this looks fine with a 28R primary and 2k secondary resistance. In addition, a damaged frigger transformer would not explain points (2) and (3) above.

    I had previously removed the flashlamp from the pump chamber to check it but saw only a bit of the usual white ablation on the glass around the cathode, no other obvious signs of damage, no darkening. The pulse counter indicates around 700,000 pulses - not that much, but I suppose if it was simmering all the time the laser was on there will be a fair bit of wear and tear going on.

    So it's off to the local flashlamp shop for me with my pocket money... Unless someone knows otherwise...


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    (picture from Gramm Technology website)

  3. #3
    mixedgas's Avatar
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    Start with the lamp, then insure the simmer current is modest. I have no data on that model, which is why I did not reply at first...
    Make sure the lamp is installed the right way, one of the easiest ways to make a hard starting lamp is to use an un-coated anode as a cathode.
    The cathodes are impregnated with an electron emission compound, the anodes are not...
    ~
    Simmer does wear the lamps down slowly over time, and if you do not fire the lamp periodically, the simmer wears away the emission layer.. Making the tube hard to start.
    Then sputtered material slowly buries the gas. On the other hand, no simmer makes for an unstable, hard to fire, laser. Usual sequence with pulsed yags is simmer on, charge the caps, dump the caps into the simmering lamp using either a Thyratron, SCR or an IGBT, wait around 256 uSec to charge the rod to its typical upper level storage time, and then fire q-switch if present.
    !
    Once its firing check the capacity of the storage cap, laser flashlamps do not like to be ran below the minimum number of design Joules..
    !

    ~Be careful with pulsed 1064 nm laser light. It is very dangerous...
    !
    Steve
    Last edited by mixedgas; 01-30-2018 at 13:17.
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  4. #4
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    Thank you very much Steve.

    I've taken the lamp out and had a closer look... The quartz is very slightly darkened, and there is clear damage around the cathode, see photos.

    I've ordered a new lamp, should be here by the end of the week.

    As for the laser light, I am aware of the hazard and only looking at diffuse reflection from card, held out of focus. In full sunlight, so irises fairly shut.

    Cheers, Ilya

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  5. #5
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
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    That lamp is SHOT, BAD, get a new one . To work on ND:YAG such as this, you need a pair of OD6 safety goggles, no exceptions.
    A mere card is deceptive with 1064 nm. PLEASE get a pair of known good Optical Density 6 or OD7
    glasses to save your eyes! You cannot assume the scatter from a card or viewing card is safe!

    Steve


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ID:	53556[/QUOTE]
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  6. #6
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    Laser Warning Fixed!

    Finally managed to get it up and running again, as the weather had been too cold for cooling water (the laser's in a barn).

    It turned out the problem wasn't the flashlamp, or at least not only the flashlamp, as it didn't work with the new flashlamp either. The problem was the energy storage capacitor in the thyristor trigger circuit (not the main electrolytic capacitor bank).

    It is the large red rectangular cap in the photo, a 470nF 603VDC metallised polyester film. It was showing low capacitance (240nF) and relatively high ESR (4R5) when tested out of circuit, presumably holes had been blown in a large proportion of the dielectric and so it was playing up under tension and/or did not have the storage necessary for a rapid enough collapse of the magnetic field in the trigger transformer.

    Anyway, now it, the thyristor and the flashlamp have been replaced and the laser is happily punching holes in 3mm steel, just as nature intended...

    NB The laser cavity and beam path are fully enclosed with UV and IR protection, as well as an automatic failsafe LCD welding protection screen. You can look down the stereo microscope as you weld with fully protected eyes without requiring laser goggles. The biggest risk is burning your skin if you don't take care where your fingers are when pressing the pedal.

    Thanks for your help,
    Ilya

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