atx supply will not start

[montanglo]

all of a sudden ,when you connect green to black wire on the motherboard plug.
What can be the cause of this?

[momaka]

Re: atx supply will not start

Can you be a bit more specific?
What is the brand and model of the power supply? Did the problem appear suddenly? If you have a MultiMeter, is there any voltage between the purple (5vsb) and black wires (should be close to 5v, +/- 5%)?
If your power supply is not under warranty anymore, pictures of the inside of the power supply would also be very helpful (but since there is high-voltage inside the power supply, make sure it's disconnected from the wall plug but connected to the motherboard before you open it).

[everell]

Re: atx supply will not start

Welcome to the Bad_Caps forum.

Are you asking for general causes of power supply failure, or are you seriously wanting to fix it? It would be nice to know a little about you. Are you a teenager or older gentleman? What country do you live in? Do you know much about electronics, especially electronic repairs? Do you have a voltmeter?

To better help you, we need to know what model power supply, and it would be most helpful to have some pictures of it. Most of the problems and power supply failures are caused by bad capacitors.

[montanglo]

Re: atx supply will not start

Rexpower brand, model 250 w I think,
I had enough electronics training to check resistors and diodes and seem ok, caps look ok and I doubt they are the problem because it stopped working so suddenly, probed red and green to black -both 0 volts

[montanglo]

Re: atx supply will not start

update
double checked purple and green both 5v,yellow~0.5v and the rest 0volts

[momaka]

Re: atx supply will not start

If there's 5v on the purple wire, it's probably fixable. Not sure if there should be 5v on the green wire, though.
Also, 0.5v on yellow? Should be 0v when computer is off and 12v when on. Red should be 5v when on.
When you connect green to ground, does the power supply fan briefly pusle once?
I think pictures of the inside might be in order here.
----
As far as the brand, I couldn't really find much info online on Rexpower PSUs. I did find a few images with Google, though. It seems as the power supply also goes by the name of "Maxpower" as recognized by the logo (a hand in a "thumbs-up" position behind the name). I also found 2 images of the inside (it was either a 350 watt or 400 watt model), and it just looks like the typical generic power supplies probably capable of no more than 200-250 watts. So I'm guessing yours is probably no better either. Not sure if it will be worth fixing it, but that depends on how much time you have. Should be good for learning experience if that's what you're looking for.

[ratdude747]

Re: atx supply will not start

sounds like a gutless wonder. i personally strip them for fans and and wire. mainly because i dont have time to "fix" them and since i learn the same skills on my robotics team, i see no use in fixing them if they are not really safe no matter what you do.

[montanglo]

Re: atx supply will not start

thanks but could not find the Rexpower/Maxpower schematics,wonder if there is a helpful generic schematics.
Also when you ground the green wire fan kind of starts spinning but then stops

[linuxguru]

Re: atx supply will not start

That looks like an overload or short, and the protection circuits kicking in and shutting it down.

[momaka]

Re: atx supply will not start

You don't really need schematics to troubleshoot the power supply.

Another reason for it to shut down is that maybe you're not providing a big enough load to start the power supply. You can use fans, a scrap motherboard, or any other devices that you don't mind sacrificing to load the power supply. If the power supply still does the same thing and refuses to start when you load it, then there is probably a short on the secondary side which is causing the power supply to shut down, as linuxguru suggested. Most of the time, it's a shorted diode (though it's also possible to have a shorted capacitor). Open the power supply and with a multimeter, check the schottky diodes on the secondary heatsink to see if they are shorted. Center pin is positive tap, while the two outer pins are connected to AC from the transformer. If you put the negative probe on the center pin, and the positive probe on either outer pins, you should get a few hundred ohms of resistance (but never 0 ohms / short circuit). Switching the probes around might not give you an open circuit as you might expect, though (especially if there are load resistors on that rail). But again, you should not get 0 ohms / short circuit. Do this for all 3 rails.

[everell]

Re: atx supply will not start

The last two power supplies that I fixed which had the same problem were shorted diodes on the output side. I agree - get that ohmeter out and look for shorted diodes.

[montanglo]

Re: atx supply will not start

thanks so this symptom is not caused by aged electrolitic caps but possibly heatsink mounted diodes
all with common anode?There are 3+2 components on 2 heatsinks, marking on 2 leggible is fr10036* and d130007k

[montanglo]

Re: atx supply will not start

Thanks for input so far
can you please link to rexpower pl350 solder side pics -I 'd like to check if what looks like a cut in the lead to the center pin of the switching element situated lowest in line of the primary heatsink, should actually be there
Though doubt this could be the reason of shutting PS output down and if high current made the cut why did the fast fuse stay ok

[momaka]

Re: atx supply will not start

I can't seem to find any pictures of the solder side/underside of this power supply on Google.

Did all of the diodes check out fine on the secondary heatsink? If there's a short on the secondary side, fuse will not blow, but if there is short on primary, fuse should blow immediatelly. Since it's not blown in your case, there must be something that is causing the short on the secondary side.

Also, what do you mean by "cut in the lead"? Is this on the switching transistor? Are there any burn marks on the transistor or on the PCB?

[lucky13]

Re: atx supply will not start

Montanglo,

Do you have a digital camera? If you do, post a few pictures so we can all "see" what you are seeing.

[montanglo]

Re: atx supply will not start

cell phone camera not good
outputs have no bigger resistances than existing resistors and secondary heatsink elements measure ~ok without desoldering them
By cut I meant possibly burned across-broken solder lead to center pin of Psomething primary heatsink element next to 2 di30007k
Since there is a sg6105 schematic (no pfc) might me similar to

but would sg6105 cause shutdown if there was a problem with operation of the mentioned switching element

[everell]

Re: atx supply will not start

The SG6105 chip is a combination PWM and Supervisory chip - a nice one. If there is a problem, the supervisory side of the SG6105 will shut down the power supply to keep it from self destructing.

The 13007 is a switching transistor. You will probably notice two of them side by side. If one of these transistors is bad, it will prevent the power supply from working. An ohmmeter check on the two transistors will show any shorted devices. Other components connected to the transistors will not allow you to determine if one of the transistors is leaky (resistive). The only way I have found to solve the problem is to unsolder and remove the transistors on the heat sink (and the heat sink - easier to remove all of it at once, then you don't have to remove the transistors from the heat sink). Once out of the circuit, measure the resistance of the transistors. I fixed a power supply by doing this - couldn't find the resistive part until I removed the transistors from the pc board. Then found one of them to be slightly resistive. This caused the slight spin on the fan, then shutdown, but didn't blow the fuse.