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  1. Member p_l's Avatar
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    The PSU (model no. l305p) on one of my hand-me-down Dells, an Optiplex 755, just gave out.

    First, is it or is it not ATX so I could just swap in a different PSU?

    If not and it's a relatively expensive and hard to find proprietary format, how about just changing out its big capacitors which are bulging/leaking?

    And now the most important question: If I open it up and try that, how long should I wait after unplugging it before I touch those big caps so that they have time to empty their charge and not zap me seriously? I've read those big caps can kill.
    Last edited by p_l; 23rd Feb 2014 at 08:11.
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  2. I'm a Super Moderator johns0's Avatar
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    You can put in a regular atx psu from what i read,just make sure it's decent brand,as to replacing the the caps it's not worth it.The original psu is only 275w.Just make sure the original psu isnt one designed for that model only such as a proprietary power supply.
    Last edited by johns0; 22nd Feb 2014 at 00:31.
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  3. First replacing the caps might not fix it; you don't know if the bad caps didn't just cause some other component to die. If you're talking about a desktop 755 you're not going to fit a standard ATX supply in there. A 755 tower will take one, but you'll have to get creative to get it mounted; Dell likes to do things its own way. You can verify that the connector is the same, just compare the color of the wires and their position.

    You can easily get a Dell replacement from the many off-lease resellers all over town, check this guy and there's like a half dozen shops on either sides of the street.

    Those little 220F caps wouldn't kill you, you'd get a nasty jolt; some CRT TV caps on the other hand, the ones in the flyback circuit store thousands of Volts. If you want to be safe just short-out the 2 leads with a screwdriver, but the charge should be depleted after 30 minutes.
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  4. Member p_l's Avatar
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    Hey, thanks for the tips and the local link. It's not far from one of my work locations so I'm looking forward to checking it out.

    Yes, from what I gather, sometimes trying to use standard form factor parts in a Dell is like putting the proverbial square peg into a round hole. I was thinking of replacing just the caps more as a technical experience than a practical or cost-saving one, so if I can easily find a proper replacement at one of those liquidation places in VSL, I'll do that, plus they seem to have a lot of other goodies.
    Last edited by p_l; 22nd Feb 2014 at 13:31.
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  5. Member turk690's Avatar
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    It's true Dell likes to use its own form factors in many models so replacing with standard ATX can be difficult if not impossible. But in my experience, a purchased replacement PSU for a long-ago discontinued model will likely have come from a cannibalized off-lease same model, which, because of the same age (and the same probably heavy usage), have about the same probability of failing again. I mean, I open up a replacement PSU just to check it before mounting it to find out the same capacitors have also bulged and/or popped open, like as in the PSU I intended to replace. It almost always boils down to replacing the capacitors either way.
    Another thing to check are the electrolytic capacitors on the main board. They bulge and/or pop open the same way.
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  6. Usually high voltage caps in PSU are not root cause for failure - they rarely suffer from high ESR due of fact that they work with 60/50Hz - my advice is to change PSU for a new one, if any ATX fit then non Dell with sufficient wattage.
    Just measure size of Dell PSU and search for similar, i assume that bolt pattern is fixed.
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  7. It's my understanding that that model uses an ATX power supply, but the size of the power supply is specific to the Dell tower it goes into.
    That doesn't mean you won't find another brand that will fit, it just means you may need to do some searching.

    --dES
    Last edited by Des; 23rd Feb 2014 at 08:08. Reason: too many typos
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  8. Originally Posted by Des View Post
    It's my understanding that that model uses an ATX power supply, but the size of the power supply is specific to the Dell tower it goes into.
    That doesn't mean you won't find another brand that will fit, it just means you may need to do some searching.

    --dES
    It is not clear some of Dell equipment i know use industry standard and normal ATX PSU can be used. Perhaps this is possible - not sure this is my advice - verify this.


    If you are desperate you can consider moving PCB from one PSU to another (Dell) but i would not advise this.
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  9. It is not clear some of Dell equipment i know use industry standard and normal ATX PSU can be used. Perhaps this is possible - not sure this is my advice - verify this.
    Very true.

    I just checked at the Dell community forum, they note the Optiplex 755 Tower as standard ATX.
    http://en.community.dell.com/support-forums/desktop/f/3514/t/19523542.aspx

    It's my experience that all Dells after 2003 use standard ATX supplies.

    You can always buy a replacement PSU from Dell, but may be priced above market value.

    --dES
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  10. Member p_l's Avatar
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    I lucked out and got a free identical PSU from a 755 at work they were about to send off to the great computer farm upstate, so I'll try that first for the time being.
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  11. Member p_l's Avatar
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    Originally Posted by turk690 View Post
    But in my experience, a purchased replacement PSU for a long-ago discontinued model will likely have come from a cannibalized off-lease same model, which, because of the same age (and the same probably heavy usage), have about the same probability of failing again. I mean, I open up a replacement PSU just to check it before mounting it to find out the same capacitors have also bulged and/or popped open, like as in the PSU I intended to replace. It almost always boils down to replacing the capacitors either way.
    Another thing to check are the electrolytic capacitors on the main board. They bulge and/or pop open the same way.
    The free replacement PSU from work (where the computer itself is a hand-me-down from) didn't work, probably for the reasons mentioned above.

    Plan B will now be to buy an ATX PSU probably at one of the local places suggested by nic2k4, see if it fits and then if it works. Capacitors in the PSU might be plan C. (Caps on the motherboard pass visual inspection.)
    Last edited by p_l; 28th Feb 2014 at 22:33.
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  12. I'm a Super Moderator johns0's Avatar
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    Are you sure it's the psu?Did you open it to see if the caps were leaking?
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  13. Member p_l's Avatar
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    Good point. Actually, no, I'm not 100% sure, and that was going to be my next move, actually opening up the PSU and checking if what widespread reports say is true, that Dell PSUs suffer from bad caps.
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  14. Member p_l's Avatar
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    So I've got four or five that look more like the one on the right than the one one the left: not burst or leaking, but a bit domed or bulging.

    Last edited by p_l; 1st Mar 2014 at 01:02.
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  15. I'm a Super Moderator johns0's Avatar
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    Bulging,leaking caps are usually worse than that,hard to say if its faulty,i replaced caps like that before and no success.Cap like that are cheap,just replace them to see if it works.
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  16. Might be worth borrowing or investing in an ATX power supply tester.
    This is one type: http://www.newegg.com/Product/Product.aspx?Item=N82E16899109003
    But there are other good and less expensive ones out there. I believe I paid about $10 for one similar to the one in the link.

    --dES
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  17. Originally Posted by p_l View Post
    The free replacement PSU from work (where the computer itself is a hand-me-down from) didn't work, probably for the reasons mentioned above.
    If that computer was being retired because it wasn't functional I guess you found the defective part. The only difficulty in using a standard ATX PSU in a Dell is getting the thing to stay in place. Look at the back of the Dell PSU, are there mounting holes, do they match with a standard PSU?
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  18. Member turk690's Avatar
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    Originally Posted by p_l View Post
    So I've got four or five that look more like the one on the right than the one one the left: not burst or leaking, but a bit domed or bulging....[/IMG]
    I'll run screaming to the soldering station as fast as my legs can carry me even if I see ones that just seem to be bulging, like the one on the left. Ok, seriously; any PC PSU that I expect to still connect in some PC that is >5 years old I open up, and replace all electrolytic capacitors in the output stages (+5Vmain, +5Vsb, +12V, +3.3V). In an earlier post I said electrolytic capacitors are the Achilles' heel of electronic equipment: all other components are good but just one bad electrolytic capacitor can junk an entire expensive motherboard, for example. Two important specs of electrolytic capacitors are capacitance and electrical series resistance (ESR, ideally zero). In typical cannibalized replacement PSUs for sale, the capacitors may look good and capacitance readings ok, but the ESRs are high (a new 2200μF, 10V, 105C should have just a few milliohms of ESR). ESR meters are the domain of specialist testing centers and is something I don't even have or use regularly, so I just chuck all output caps and replace them. Of course I have to point out: PSU and motherboard PCBs often have more than two copper layers, and will require some experience, attitude, and the right soldering/desoldering tools to replace components without damaging anything else on them.
    Electrolytic capacitors are wonderful components: so much capacitance has been packed in a tiny volume, all the better at smoothing out the B+ lines while occupying less and less cubic real estate. But one of the things inside the capacitor responsible for its awesomeness, the electrolyte, is also its weakest link: it gradually dries out. Recently, some manufacturers have been using solid electrolytic capacitors on their motherboards (with colorful advertising to match, see some Gigabyte boxes) and claiming we are all now beyond this problem. May or may not be, and solid electrolytic capacitors can be bought to be used for repair (more expensive), but as it takes years before problems appear, hard to tell for now.
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  19. Member p_l's Avatar
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    Maybe it's too good to be true, but I just couldn't pass up ordering one of these. So cheap, free shipping, no taxes.



    http://www.amazon.ca/PC-Power-supply-tester-LED/dp/B005CTCD6S/ref=cm_cr_pr_product_top
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  20. Member turk690's Avatar
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    Originally Posted by p_l View Post
    Maybe it's too good to be true, but I just couldn't pass up ordering one of these. So cheap, free shipping, no taxes.
    I have one of those. The problem, however, is that this device merely says that the output voltages are ok without load. Often, after connecting said PSU to PC, the whole shuts down after some time (a few minutes, an hour, etc). The bad electrolytic capacitors often eat most of the current, seriously loading the PSU and shutting it down.
    This device is good only for a cursory test and not much beyond that. Many years back I constructed a variable-load unit that can force an ATX PSU to its stated maximum current; many failed, e. g., a PSU a few years old would shut down when only about 10A of its stated 20A maximum capacity on the +5V line was being drawn by my load unit. I came to see this testing as cumbersome and have just developed this rule: a misbehaving ATX PSU normally used at more than 60% of its rated current(s) which is past 5 years old--all output electrolytic capacitors replaced.
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