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Thread: The quest to fix a broken argon switchmode PSU

  1. #1
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    Default The quest to fix a broken argon switchmode PSU

    I posted this on LPF first, but thought I'd put it here too just incase anyone had idea's and not on LPF

    OK well, a while ago I "bought" a free argon laser from Dave. He offered them without power supplies or any information. Thats fine, I got one anyway.

    I originally intended to build a simple linear PSU, however my mains voltage being 240V, I can't just rectify mains and chuck it into a passbank, I'd require about $200 worth of transformers!

    I saw that timelablasers had a complete, but broken argon laser for sale, so I asked him if he'd be willing to sell me the PSU. And he did

    First obvious things were a FET that was cracked, and a burnt up resistor. He had already replaced a fuse next to the blown FET.

    I bought some new FET's and replaced the cracked one, hid behind my kitchen window, and powered the PSU up. It didn't explode, but it didn't do much either.

    There is a 200 ohm resistor on the neutral input line. Beside it is a relay. I have come to the conclusion that this resistor is to limit inrush current, and is bypassed by the relay once the PSU has powered up. However, this relay is clearly not doing anything, and the resistor just gets hotter and hotter. No lights or anything show on the board either, just 1 hot resistor.

    So, I unplugged it, and being a switchmode PSU, I left it for a while, and tested the tank caps. They had no charge. This could be because they were already drained by other components, or they didn't get any power at all.

    The only components between the mains input and those capacitors is the rectifier, and 2 big inductors. I have removed all the boards from the PSU, but have no idea where to start testing for dead components.

    I would check the rectifier first, but can I do that while it is still on the board, and with a regular DMM? If the rectifier was dead, that'd explain nothing getting power, but it wouldn't really explain a dead FET, unless the rectifier was killed after the FET (Tube wires shorted or something maybe?) I don't know.

    Here are some pics:











    Considering the component count on this beast, I have absolutely no idea where to start. The fact nothing seems to get power points to a PSU issue though, so I guess I can eliminate pretty much all the logic circuitry.



    Any help or pointers would be great If you need any more information, please do ask, I'd like to give this thing a 2nd life instead of having it sitting out in the garage for the rest of it's days

    Cheers,
    Dan

  2. #2
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    Default

    You are right about the relay bridged resistor ! Empty capacitors would charge really fast without this and kick the relay contacts into oblivion fairly quickly.

    Do the diodes test properly with your DMM ?

    Pulse width modulation is used to establish operating B+ voltage range (often through an opto-isolator) . So FETs won't do much till that voltage drops and at the moment you don't seem to have a voltage to drop yet.

    Real pig !

    Bit tricky fixing that without an ISOLATED O-scope.

    Have you got a circuit diagram for the head and PSU ?

    Are you trying to test the tube or do you assume that works OK ?

    If so what voltage do you expect at low tube current?

    Lots of other questions before anyone can help much.

    Good Luck

    Cheers

  3. #3
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
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    E= IR
    E=90V Vtube
    I = 10A max

    90/10 = 9 ohms,

    Your test load must be low inductance, 10 ohms or greater, and 1500-2000 watts.

    It needs to be on a contactor so it does not short the bus until time in.

    Tungsten lamps come to mind.

    Otherwise you do not get past the startup circuit.

    Steve
    Qui habet Christos, habet Vitam!
    I should have rented the space under my name for advertising.
    When I still could have...

  4. #4
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    Quote Originally Posted by mixedgas View Post
    E= IR
    E=90V Vtube
    I = 10A max

    90/10 = 9 ohms,

    Your test load must be low inductance, 10 ohms or greater, and 1500-2000 watts.

    It needs to be on a contactor so it does not short the bus until time in.

    Tungsten lamps come to mind.

    Otherwise you do not get past the startup circuit.

    Steve
    Blimey!... you have a lot of lights at your place Steve !

    I think maybe an electric fire might be easier.. or a kettle (if you like tea).

    Cheers

  5. #5
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
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    Quote Originally Posted by catalanjo View Post
    Blimey!... you have a lot of lights at your place Steve !

    I think maybe an electric fire might be easier.. or a kettle (if you like tea).

    Cheers
    Old photocopier fuser lamps, quartz heater lamps, and electric water heater elements. But the heater elements have many fine turns of wire on a iron core and thus have huge kickback spikes.

    Steve
    Qui habet Christos, habet Vitam!
    I should have rented the space under my name for advertising.
    When I still could have...

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