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Hello, I have a Loewe Mimo 28 (Q2500B chassis) which blows the main MOSFET in the switch mode power supply (Q624) after a couple of weeks of use. Can't see any other faults. Any ideas?

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  • philip988 Mar 25, 2011

    OK, that's general advice for switch mode power supplies, but I was looking for advice and experience specific to this model.

  • philip988 Mar 25, 2011

    The FET lasts for a couple of weeks when replaced before it blows again. Do you think a faulty electrolytic would show this kind of behaviour?

  • philip988 Mar 26, 2011

    Thank you. Not the specific experience I was hoping for, but good general advice.



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Look for bulging or leaking capacitors.
There's been a lot of capacitor failures in all kinds of devices lately.
Since you know what a MOSFET is, I'll assume that's really all the explanation you need to go looking for the damaged compunents.
Check most carefully on the power supply and inverter boards.

Posted on Mar 25, 2011

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  • Norman Skinner
    Norman Skinner Mar 25, 2011

    In general, all switching mode power supplies operate on the same principal, and power supplies in monitors (especially large flat screens) or all makes and models (most notably Samsung) as well as computers etc have been failing all over the place because someone got the electrolyte formula wrong, and several companies copied the formula - so it's not even capacitor manufacturer specific. Even amplifiers that don't have switching supplies are falling victim to the improperly made capacitors. Oddly these caps usually just outlive the warranty period. I'm sorry if I can't point you to a specific component or capacitor number, but it could be any number of them in the power supply, and often they go in groups.
    I apologize if the answer isn't specific to that brand and model, but it really is most likely the issue, and you'll have to look around the supply module, and possibly even the other circuitry for these things because it's not always the same ones that fail, even in specific makes/models of devices - some capacitors will simply fail before others.
    Again, I apologize if this information wasn't helpful to you.

  • Norman Skinner
    Norman Skinner Mar 25, 2011

    Not a completely faulty one, but a failing one.
    As the capacitor starts failing, frequencies will shift which can cause all sorts of faults including increased impedance and overloading or shorting outputs.
    When the load on the mosfet reaches it's tolerance point, it will last a while, but eventually fail.
    Failing capacitors can cause all sorts of issues, because their capacitance may vary with temperature too, so if say you use the TV about 2 hours a day, but much more on weekends, the chances of a weekend failure are much higher, meaning it might last 1 to 2 weeks, eventually only 1 week, and at some point it just won't turn on anymore.

    Anyway, you did say "any ideas" and that's my idea. It certainly won't hurt anything to go looking for bad caps. If I tell you to replace a specific component without being able to diagnose the board "hands-on" and you spend money on that part and the time to install it, and it doesn't work, I think you'd be much more upset than if I send you looking for what is most likely the issue - looking costs nothing but some time.
    Something somewhere is obviously operating at a degraded state, not a failed state. If something had failed completely, either the mosfet would blow immediately, or it'd blow a fuse or just not turn on at all.
    The only other possibility I could think of is if this mosfet is on a heatsink, the heatsink compound might need to be replaced and if there is a fan to cool the heatsink (or any other part of the unit) that the fan may have failed or is failing.

  • Norman Skinner
    Norman Skinner Mar 25, 2011

    At least that's all I can think of without being able to see the board since I don't have a schematic for it and can't put a meter to it myself.



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