After replacing bad capacitors, still nothing

[gkirby]

I'm new to replacing capacitors, but the purchase of 30 Dell GX270 minitowers has made it necessary for me to try to learn. I have a few that have bad capacitors on the motherboard, but I have a bigger problem with the Power Supplies. I have opened up 12 of them that would not power up, and they all had swollen capacitors. After purchasing the correct capacitors I replaced all the swollen capacitors in the boards. They still will not power up. I paid a pretty good price for these towers and if I have to buy new power supplies I will come out in the hole. I know I did a good job replacing the capacitors, but now I know that there was probably another problem besides the capacitors. Is there an obvious place to start looking?? Other than an AC/DC electricity class at college 35 years ago, I am for the most part self-taught. Any help would be appreciated.

[davmax]

Re: After replacing bad capacitors, still nothing

Re power supplies. Start with the fuse, if it has one. Usually a cylindrical glass or ceramic device. Test with ohmmeter it should read zero ohms.

Next step is more complex. Remove the circuit assembly from the case and replace any swollen or leaking caps.

After replacing or checking caps carefully test the supply on the bench. Yes with it all laid out. A REAL HAZARD SO BE CAREFUL. The supply will not start untin thegrenn and any black wire on the motherboard connector are shorted.

If the supply does not start.

With power on measure the DC voltage on the large input caps. DANGEROUS VOLTAGE. Typically these are rated at 200V and a normal voltage is 155Volts or above.

If the input cap volts are Ok and the supply will still not start with good caps you havea more in depth problem.

[davmax]

Re: After replacing bad capacitors, still nothing

It is quite common for a short on a motherboard to prevent a power supply from starting so it is first best to check if the power supply is being affected this way.

Disconnect the motherboard connector and short out the green and black wire connections witha piece of wire. If the power supply starts (fan running) it is likely that the motherboard has a nasty problem. Whilst the power supply is running check the 5 volt red to black and 12 volt yellow to black on a hard drive connector.

[gkirby]

Re: After replacing bad capacitors, still nothing

I took a jumper wire and jumped the green and a black terminal on all of the power supplies and they all powered up. (I also attached a harddrive for a load). I put one of them in a tower and tried to power up the computer. The fans started slowly and I could hear the floppy drive clicking, but no boot. I checked the 5v/12v power, and it was only 8.6 volts/2.6volts. Not enough to power up the tower. After letting it run about 45 seconds, I heard a click and then the tower started booting up. When checking the power, I had 5v/12v. It was like something finally made contact and supplied the full power. It has done this same thing a couple of times this afternoon. Any ideas what this could be??
thanks for the help,
Gene

[willawake]

Re: After replacing bad capacitors, still nothing

which capacitors did you use?

[gkirby]

Re: After replacing bad capacitors, still nothing

Do you mean what brand, or what size?
1-4700uf @10v
1-3300uf@10v
1-2200uf@6.3v\
thanks,

[gkirby]

Re: After replacing bad capacitors, still nothing

Do you mean what brand caps, or what size.
The sizes are:
1-4700uf@10v
1-3300uf@10v
1-2200uf@6.3v

[willawake]

Re: After replacing bad capacitors, still nothing

brand / series

[gkirby]

Re: After replacing bad capacitors, still nothing

The 4700 uf is a Rubycon 4700uf@10v. PX and there is the number S0643
The 2200 is a Rubycon 2200uf@10v. MBZ T0543
The third one I got from Digi-Key. It is a 3300uf@10v the only thing I can find is: an M inside a square with rounded corners and CE NHG
I hope this helps,
Thanks,
Gene

[linuxguru]

Re: After replacing bad capacitors, still nothing

The 3300/10 is probably a Matsushita (Panasonic) NHG, which is a 105c GP/Audio cap. It's not low-ESR, high-ripple current or long-life; it's not a correct substitution in a PSU secondary. Is the sleeve brown in colour?

It's also important to replace several other caps in the PSU, including the ones on the +5Vsb standby rail, and *all* large caps on the secondary rails, even if some are not bulging or leaking.

[gkirby]

Re: After replacing bad capacitors, still nothing

The Rubycon's are black with a grey negative stripe. This 3300uf looks identical except that the stripe is silver.
Thanks,

[999999999]

Re: After replacing bad capacitors, still nothing

I took a jumper wire and jumped the green and a black terminal on all of the power supplies and they all powered up. (I also attached a harddrive for a load). I put one of them in a tower and tried to power up the computer. The fans started slowly and I could hear the floppy drive clicking, but no boot. I checked the 5v/12v power, and it was only 8.6 volts/2.6volts. Not enough to power up the tower. After letting it run about 45 seconds, I heard a click and then the tower started booting up. When checking the power, I had 5v/12v. It was like something finally made contact and supplied the full power. It has done this same thing a couple of times this afternoon. Any ideas what this could be??
thanks for the help,
Gene

This is strange, check the transistors and rectifiers but the PSU should not have been running at all like this, if it can't remain close to 5V and 12V it should not have continued trying to but rather shutting down within a few ms. Personally, I would not hook these PSU up to the systems anymore, first confirming proper function with a suitable sized dummy load, and only after achieving that then trying with one sacrificial system. Also try a new, known good PSU with the sacrificial system since you need to isolate variables by getting at least one board up and running.

If you don't mind me asking, where did you get all these systems with all blown parts? It is almost suspicious like someone took all their bad dead parts and threw them together to make it look like a whole system then sold it "as-is", playing like they didn't know the full extent of the damage? Granded a flaw in an entire series will cause same failure over and over but even then, not all systems have same exact roles so there are normally a few that survive.

Further, wasn't Dell replacing these motherboards with no questions asked? I mean, how is it the former owner decides to sell them failed instead of getting a free replacement part? What if these ARE the bad boards sent to Dell that didn't get destroyed? I realize that seems paranoid but I bought a whole box of dead Dell video cards a (couple?) years ago, somehow some of their damaged products aren't being properly destroyed.

With this in mind you might also try a different PSU from one system with a different system, mixing and matching maybe some parts are working.

[gkirby]

Re: After replacing bad capacitors, still nothing

These computers came from Austin-Peay University in Clarksville Tennessee. I bought them on Govdeal.net. I bought 1 lot of 5 GX260 towers, and 1 lot of 6 GX270 towers. When I went to pick them up the guy said that the gX260's were probably in good working order, but they would not have sold the GX270's unless there was a problem with them. I told him that they should have mentioned that on the auction. I'm not even sure that they know that they are covered by Dell, but I am going to tell them right now. I paid $130 each for these 270's, and I expected them to be working. Now I am just trying to get them working so I can maybe break even.

[999999999]

Re: After replacing bad capacitors, still nothing

If they sell an item that is known unworking but fail to mention it, that may be fraud. I would see what they're willing to do about it, tell them you want a refund or to have them (being original owner as recorded by Dell?) submit a claim to get replacement boards sent. That you have altered them is a potential problem if Dell wants the dead boards.

As mentioned previously, you need to isolate PSU from board problems by having a spare known working system and a spare known working/compatible PSU.

Also, don't overlook some other universal potential problems like a dead battery, since some boards won't POST without a good battery. If they'd sat around too long it may have drained them.

Also, while you might feel that buying new PSU would make you come out in the hole, look at it this way: PSU have finite lifespans and new PSU mean the system is ready to redeploy for longer.

[gkirby]

Re: After replacing bad capacitors, still nothing

Thanks for all the help. I'm glad I found this forum. I've already learned a lot just reading everyone's posts.
Thanks again,
gkirby

[linuxguru]

Re: After replacing bad capacitors, still nothing

1) Get a known high-quality PSU and sort the systems out as suggested earlier. Find out which motherboards are functional, and which are not.

2) If there are functional/flaky mobos that boot, fix them first by recapping *all* caps of the bad brands with new replacements that have been suggested on various threads on this site. These have to be ultra-low ESR, high ripple-current types.

3) Try getting Dell to replace the completely non-functional mobos under their replacement program.

4) After you've got some mobos working, open up the PSUs and recap them. All large secondary side caps of bad brands have to be replaced, regardless of their appearance.

5) The Panasonic NHG 3300/10 is rated only for 1000 mA ripple current. The recommended relacements are Nichicon PW or Panasonic FC or a number of others, with more than 2000 mA ripple current rating, ESR below 50 milliohms, and endurance of 4000 hours or greater.

6) It is imperative that the caps on the +12v and +5Vsb rails are also replaced, with similar series as above.

7) With some effort, persistence and luck, you should be able to repair ~50%-75% of the systems that you purchased. Don't expect to be able to repair every unit.

8) Even the unrepairable units will have subsystems/parts that are usable in other units.

[gkirby]

Re: After replacing bad capacitors, still nothing

O.K., I'm starting to pull my hair out.
I took a good power supply and tested it against two which would not power up the ps fan, computer, or processor fan.
I jumpered the green wire to a black ground to power up the ps's. All three power up. I tested all the pins and compared them, and they all had the same voltages on matching pins.
I then pulled the jumpers and check the green wire voltages. The good ps showed 3.3v on the green wire, the bad ps's showed 5v on this wire.
The ONLY difference I can find between all three power supplies is that the good ps shows 3.3v and the bad ps's showed 5v on the green wires. Is this enough to make these two power supplies not to work??? If so, where could my problem be????
thanks,
Gkirby

[davmax]

Re: After replacing bad capacitors, still nothing

The PS_on voltage on the green wire should be 5 volts, supplied by an internal 5 volt regulator that is independent of the main 5 volt supply to the motherboard. 3 volts is incorrect but will still enable the power supply to start. Keep this seperation in mind and focus on the the main 3.3v (orange wire), 5V(red) and 12V (yellow) supply voltages and the capacitors on those voltages. If you take a PS board out it is fairly easy to determine what caps are connected to each coloured wire. eg connect one lead of an ohmmeter (set to diode or continuity)to the wire of choice and with the other lead probe the caps. A reading of zero and audible sound will indicate that you have found the associated cap or caps.

[severach]

Re: After replacing bad capacitors, still nothing

http://www.google.com/products?q=coo...ply+tester+LCD

You'd be better off buying a power supply tester for those 30 bum computers rather than shorting pins with insufficient load and measuring voltages one by one? The LCD model is way better than the old ones with the LED.

>Further, wasn't Dell replacing these motherboards with no questions asked?

They require an ownership transfer now. (Dell, Support, Customer Service, Ownership Transfer.) You don't get good service if you buy them from junk dealers like I do and if the messages on the Dell forums are any hint, you don't get good service if you are the original owner either.

>I paid $130 each for these 270's

We get tested and outfitted GX270 for not much more. The discard price on DOA returns and bad cap systems is a lot less.

[davmax]

Re: After replacing bad capacitors, still nothing

It is a good idea to have a tester as suggested. However this does not solve the power supply loading problem. A small tester like this measures only the no load voltages. I suspect that the small package could not even handle a 50 watt load.