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PC power supply

Discussion in 'Troubleshooting and Repair' started by johnfin, Mar 20, 2014.

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  1. johnfin

    johnfin

    18
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    Mar 3, 2014
    Anyone know alot about PC power supplies. I have a 24 pin dell power supply that died. P/N L375P-00 DEL p/n P8401. I jumped the green wire to get it to turn on but nothing happened. I am not one for sending stuff to the dump if the problem is a 10 cent diode or such.
     
  2. Jagtech

    Jagtech

    43
    1
    Feb 22, 2014
    Usually electrolytic capacitors that have gone bad. But many times when the caps go bad, there are also some FETs and/or fast recovery diodes that are taken out, too.
    Sadly, its likely better to just replace the P/S.
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    Hello John and welcome to the Electronics Point forums :)

    Problems in PC power supplies are often caused by dry joints, power surges, or electrolytics. The usual result is that the switching device(s) (transistors or MOSFETs) have their voltage or current ratings exceeded, and the magic smoke leaks out of them. Then the mains input fuse blows.

    Have a look for the mains input fuse. It will be the one with a shiny black coating on the inside of the glass, where the wire inside vaporised. If it's not blown, there's a chance it could be something small, such as the startup bias resistor(s). Let us know in that case. And be very careful; there is probably around 350V DC on the input electrolytic(s), which will give you a nasty surprise if you touch it, and in some cirumstances can be fatal.

    If the fuse is blown, look on the biggest heatsink and tell us the component types of all the parts screwed onto it, and whether any of them have chunks missing.

    Also, check the underside of the board for dry joints (Google it if you don't know what these are) and inspect all electrolytics for signs of bulging at the top or bottom, and leakage of the internal chemicals (electrolyte).

    Yes it's true, it's cheaper to replace the thing. But some of them have specific size or shape requirements, and a replacement may not be available. Also, if you want to learn about them, this would be a good opportunity. Google some relevant keywords - PC switching power supply transformer transistor MOSFET diode, etc. There's a lot of information about them on the web.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    A "feature" of many PC power supplies is that they are crammed into a very small space.

    Once you open them up you'll realise that they are hard to work on -- they're not built to be repaired.

    I opened up a 750W PC power supply the other day to replace a fan. I was really glad I didn't have to touch the electronics. (and as it was, the fan had an unexpected interface so I just lubricated the bearing and left it to carry on for a bit longer while I find a replacement)
     
  5. johnfin

    johnfin

    18
    1
    Mar 3, 2014
    Resistor

    Fuse looks ok. Where is that resistor located?
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    The resistor (sometimes there are two of them) are standard cylindrical resistors with colour bands, a bit larger than the small resistors. I don't think they're ever SMT. They're normally in the range 100~470 kilohms. Electrically, they're located between the main input electrolytic(s) and the controller or the switching device.

    Upload a few pictures of the main board taken from above, and one of the underside. I may be able to spot them. But be very careful. If the startup resistor is open, there may be no discharge path for the input electrolytic(s) and they could stay charged to 350V for days or weeks.

    Edit: If possible, take the photos outside on an overcast day, so the board is lit from all angles. Don't have direct sunlight on it. Don't use a flash. Take a photo looking straight down on the board, and two more looking down at slight angles. Most components with three or more leads, especially ones that are attached to heatsinks, are semiconductors; please make sure their markings are readable - photograph them separately if necessary, or mark them on the picture using an image editor.
     
    Last edited: Mar 22, 2014
  7. johnfin

    johnfin

    18
    1
    Mar 3, 2014
    pics

    [​IMG]

    [​IMG]

    [​IMG]
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    OK, good. In the first picture, look in the area at the top left, with the two input electrolytics. I can see a couple of vertically mounted resistors peeking out from below the heatsink at the top, near the corner of the board. What value are they?

    So you've followed the path from the AC input to the bridge rectifier on the little heatsink. The output of the bridge will go to the two input electrolytics. You can check VERY CAREFULLY the DC voltage across each of these electrolytics with the power ON, if you're confident you can do so safely. If my suspicions are right, there will be about 170V DC across each one. (They're connected in series.)

    Also can you have a look on the left side of the big heatsink next to those electrolytics. There should be at least one component attached to it, probably on the electrolytic side. Post the markings on it/them.
     
  9. duke37

    duke37

    5,360
    765
    Jan 9, 2011
    Are the tops on the big electrolytics flat or domed?
     
  10. johnfin

    johnfin

    18
    1
    Mar 3, 2014
    Caps

    I thought filter caps were always in parallel. Just a quick look at the bottom of the circuit board shows them to be in parallel. Would be great if I had a schematic, tracing circuits by eye is bad. The tops of the large caps in addition to the others are all flat.
     
  11. johnfin

    johnfin

    18
    1
    Mar 3, 2014
    Those resistors at the top are not banded, they are marked 100K ohms and are huge, at least 5 watts. There is a tiny resistor near the wave bridge rectifier. Its a 1/4 or 1/8 watt and its hard to see the bands but the first one is blue, 6 right. I put a meter on it in the circuit and it reads 265K, its getting resistance from something else.
     
  12. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    In the mains input circuits of switching power supplies, the input electrolytics are normally in series. Are you sure they're in parallel? What are their voltage ratings?

    It would be worthwhile removing those big resistors and measuring their resistance. If they're connected in series, you only need to remove one of them from the board, because that disconnects one end of the other one.

    Before you start, make sure the voltage across each electrolytic is less than 10V DC.

    I wouldn't worry about the resistor near the bridge rectifier. But it could be helpful if you post some more pictures showing the area from the electrolytics to the big heatsink next to them, taken from different angles.
     
  13. johnfin

    johnfin

    18
    1
    Mar 3, 2014
    pics

    The caps are 20uf 220v
    Here are some more pics:
    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]
     
  14. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Are you sure those big electrolytics are 20 µF? I think they're probably 200 µF. And they definitely should be in series.

    That bulging electrolytic is on the output side, so it's probably not the cause of the problem, but it definitely needs to be replaced. Personally, I would replace all of the electrolytics of that same brand in the output section. These electrolytics have a capacitance value, a rated voltage, and a temperature specification, like all electrolytics, but they also have specifications for ESR (effective series resistance) and ripple current.

    The ESR specification and the ripple current rating aren't marked on the electrolytic, but they are given on the manufacturer's data sheet for the particular series of electrolytics. The series is a two-letter or three-letter code - something like KMF or LZF (I think I can read LZF in one of your pictures).

    So to recommend suitable replacement electrolytics, we need the following information for each one:

    - Capacitance in µF
    - Rated voltage
    - Rated temperature
    - Manufacturer (or a photo of their logo)
    - Series (e.g. LZF)
    - Outside diameter in mm
    - Pin spacing in mm

    Also please tell us where you're located. There's a field for this in your profile and it's a good idea to fill it in.

    Have you measured those two resistors?
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Kris, I have seen power supplies with input capacitors in parallel, but I agree, with that voltage rating, unless the power supply has a maximum input voltage of 110V it's not likely.

    A picture of the underside of the board would clear this up and assist for a number of other reasons.
     
  16. Jagtech

    Jagtech

    43
    1
    Feb 22, 2014
    The caps that are bulged certainly need to be replaced. As for the big input caps, I have only once ever seen a bad one, and that was due to an overvoltage condition. FWIW, I would just change out all of those smaller caps, for the small cost of the parts. If they are not bad now, they certainly don't have many hours left in their life. No point in just changing the ones that "look" bad.
    When replacing those caps, I would go with 105 degree, as opposed to the cheaper 85 degree. Last longer, less likely to fail in that application. And almost no difference in cost.
     
  17. johnfin

    johnfin

    18
    1
    Mar 3, 2014
    volts

    ok, my 8 key wasnt working, typo, they are 820uf and yes the max voltage is 110vac, I live in the USA. I too have never run into many large caps go bad. Most of the stuff I have repaired like TV's its usually a diode that is bad. I tested most of the diodes in this units , zeners too and they seem ok. There are a bunch of IC's and they worry me, havent the slightest what they are used for. One must be the brain for the power switch, how long it is depressed and such for the on/ off feature. Power transistors are going to be tough to check. Do they need to be removed to check them?
     
  18. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    I'm interested to know whether these caps are in series or parallel. 820 µF is already a pretty high value, which suggests they're in series, but their orientations are opposite, which suggests they're in parallel.

    Can you post the markings of the ICs, and any large semiconductors.

    Have you measured the two 100k resistors yet?
     
  19. johnfin

    johnfin

    18
    1
    Mar 3, 2014
    Parts

    I measured the resistors "in circuit" and they both rangout at 33K ohms, should be 100k but they are in the circuit so who knows. The caps are opposite orientation so they appear to be in series, + to -. Most of the switching transistors are too hard to get at so if the problem is with those components its to junk pile. One of the IC's is PS224
     
  20. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    I think it's worth checking those resistors. That's why I suggested it.
     
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