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Repairing a Dell 2407WFP LCD

Discussion in 'Repairing Electronics' started by halleffector, Oct 21, 2016.

  1. halleffector

    halleffector Member

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    Edit: For anyone new to the thread, circuit diagram is here (thanks to rmn_tech): https://www.scribd.com/doc/207782153/Dell-2407-WFP-Power-Supply-Rev-A11-Schematic-Diagram

    I have an LCD with what appears to be a failed internal power supply.

    I immediately identified a blown 0.22 ohm resistor. However, it was pointed out by ronsimpson in a different thread http://www.electro-tech-online.com/...-where-can-i-salvage-one.149286/#post-1276833 (I created a new thread because this is a repair topic and the previous thread was just regarding the type of the resistor and where to salvage another one) there might be other components that caused this, so simply replacing the resistor might do nothing except ruin the second resistor.

    Maybe the resistor is not needed just for testing? Maybe someone has some advice for how to bypass this part of the circuit to see if it will work without it? Or would that be foolish?

    There were two resistors, only one had failed, I removed both. In the pictures below they are both missing but I have marked the locations where they are supposed to be in one of the pictures.

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    Last edited: Jan 8, 2017
  2. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    It is hard to know
    I think the two resistors should have the same value.
    You should test the two transistors (MOSFETs). Do a quick ohm-meter test. Do they measure about the same?
     
  3. rmn_tech

    rmn_tech Member

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    They are fusible safety resistors, as indicated by the serrated box around the component designation on the board. There will be a reason for failure, check the circuit section the failed resistor was feeding. I have known them to fail just due to age or from a surge. However I do not recommend just shorting one out, (These resistors are required safety components) nor simply replacing one (use same type) without first doing a check on the fed circuit section. Google for a Circuit diagram of the PSU board, (the number on the board looks to be 4H L2K02 A01 ) I will look later to see if I can help. Use the diagram if you find it to trace what the resistor is protecting, and what may have caused it's demise.
     
    Last edited: Oct 21, 2016
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  4. dave

    Dave New Member

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  5. rmn_tech

    rmn_tech Member

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    The diagram is here if you need it. https://www.scribd.com/doc/207782153/Dell-2407-WFP-Power-Supply-Rev-A11-Schematic-Diagram The two resistors you refer to are protecting two MOSFETs. R670 on Q561, R615 on Q601. So depending on the identification of the failed resistor I would check its associated MOSFET and circuitry.

    Edit. REPLACE ONLY WITH THE SAME TYPE OF FUSIBLE RESISTOR AS THESE RESISTORS ARE REQUIRED SAFETY COMPONENTS. Failure to do this could lead to catastrophic failure or in the worst case fire.
     
    Last edited: Oct 21, 2016
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  6. halleffector

    halleffector Member

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    Great help. Thanks.

    I will try to test the MOSFET.
     
  7. halleffector

    halleffector Member

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    The bad resistor was the one nearest the mains input.

    I found that its neighbour K3502 MOSFET has a short between its drain and source. So this part is defective.

    K3502: http://www.datasheetspdf.com/datasheet/K3502-01MR.html

    Could I use any of the MOSFETs I have included pictures of as replacements? I have one 18N50 (18A, 500V) and one 30N06L (30A, 60V). I'm assuming I can forget about the 60V as the original part is rated for 600V?

    18N50: http://pdf1.alldatasheet.com/datasheet-pdf/view/413374/UTC/18N50.html
    30N06L: http://www.alldatasheet.com/view.jsp?Searchword=30N06L

    Am I reading that right? It should be 600V and 12A? Regardless, can I use the 18N50? Does it need to be able to handle 600V? Shouldn't 500 be enough (at 18A)?

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  8. spec

    spec Well-Known Member Most Helpful Member

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    Afraid not. Functionally The 18N50 will do the job, but only 500V drain/source voltage rules it out, especially as your mains voltage in Norway is 220V RMS to 240V RMS.

    In fact, I would recommend a 700V upwards NMOSFET.

    Of course, this is a very crude analysis and a much lower voltage part may suffice, but I would not risk it. MOSFETs in off-line switch mode power supplies have a hard time with our mains voltages- not so bad in the USA.:D

    spec

    PS: if you put your location next to 'Location' on your user page, it will show in the window at the left of your posts.
     
    Last edited: Oct 23, 2016
  9. halleffector

    halleffector Member

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    I will order and use a K3502 then. Thanks.
     
  10. spec

    spec Well-Known Member Most Helpful Member

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    No probs halleffector, but I would be inclined to get a 700V or over NMOSFET, or you may have the same problem again.

    spec
     
  11. halleffector

    halleffector Member

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    I'm still waiting for parts.

    Would this type of resistor "0.22 ohm 0.22R 5Watt Metal Film Resistors 5W" be safe to use for the fusible resistor? I'm unsure what kind of resistor I should replace it with beyond that it should be 0.22 ohm.
     
  12. halleffector

    halleffector Member

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    I finally got the parts.

    The new resistor exploded immediately when connecting mains power.

    The transistor however appears to have survived.

    I noticed that there appears to be something leaking from a 450 volt capacitor. If this cap is bad, could it be the cause of the resistor failing? Or is that an indication of something else?

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  13. pfofit

    pfofit Member

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    Hi halleffector.
    Just looking thru this thread, sorry to butt in here but from your first post, your last pic shows this power factor chip L6561 on the back side of the board to have exploded and it controls that FET and is fed by that resistor. It either took out the fet or was collateral damage.

    need to be checking all components that are connected to it.
    added: like r669 from the .22 resistor and Q654 on the vcc pin
    cheers.
    123.jpg

    edit to add . That 120 uF cap is probably flaky. Can you check it.
     
    Last edited: Nov 18, 2016
  14. wibble

    wibble New Member

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    These psu have a common failure mode. The main filter cap dies, which causes a domino effect, taking out a mosfet and usually two ICs on the reverse side of the PCB, plus of course the odd resistor or zener diode.
     
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  15. GromTag

    GromTag Member

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    Snips of info, That resistor shows a high volt/speed destruction, as aforementioned the main filter caps poles could be fused at the base resulting in rectified mains power bypassing that cap and producing a reverse bias like effect on the affected parts, hence the burst IC, DC/DC driver and the fast pace fail on the new resistor. Not sure of the transformers status as the FET went and if had any cycle load effect on the winding long enough before the FET failed that would have heated the enamel to have cause any type of short within the transformer itself. Dropping back now, as I don't want to cause the affect of too many chefs stirring the pot.
     
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  16. halleffector

    halleffector Member

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    Thanks for all the input.

    I replaced the MOSFET, the main filter cap, the power factor chip (L6561) and the R670 fusible resistor.

    Giving it mains power immediately destroyed the R670 again. It seems the MOSFET is also broken again, but it seems the L6561 survived.

    How should I proceed to repair this? Do I need to desolder all of the diodes and resistors and test them by hand?
     
  17. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Sorry to not be very helpful, but you need to replace ALL faulty (damaged) components - or (as you have found) it just goes BOOM!! again.

    I've no ideas on this specific monitor, but 'back in the day' I created parts kits for common TV's, VCR's etc. with dead PSU's. They varied depending on the model, and what commonly died. Interestingly, where wibble mentioned the main filter (reservoir) capacitor failing, this only causes failure on some PSU's - others, even using the same IC, seem to survive quite happily - and even work 'fairly well' with a completely O/C reservoir.

    Common parts in my 'kits' were the IC, the FET (transistor), a resistor or two, a few diodes (including zeners), and a couple of capacitors.

    A less common failure was the actual transformer, either O/C, S/C between windings, or shorted turns.
     
  18. halleffector

    halleffector Member

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    Thanks for all the help. I need to order new SMD parts to replace the ones that are presumably bad. Sorry for the silly question but how do I identify the package size for the SMD parts? I'm not familiar with working with them. I can identify the ohm rating for resistors, but what is the package size? Should I try to measure the physical dimensions and compare them to some size chart? What do people working with SMD usually do in a case like this?
     
  19. Les Jones

    Les Jones Well-Known Member

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    I use digital calipers to measure them then look at the manufacturers data sheets to find one about the same size.

    Les.
     

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