PSU won´t power with fan connected

[rogfanther]

One interesting one :

Fot this CoolerMaster RS-460-PCAR from a friend. He said it stopped when he transferred it from one computer to another.

We had already gutted it from its fan to improve one of his computer´s airflow, and I had it laying in the workshop. Then, yeterday I decided to try fixing it, or at least blowing it up enough to put in the trash without remorse.

Changed the caps for the UC 3843B, and tried it without a fan connected : voltages ok.

Got a new fan, and installed it. Measured all wires, checked and rechecked soldering , everything ok. Try to power it up, just a twitch in the fan / voltages.

As I had revised all my soldering, the problem had to be with something I added for last, so tried disconnecting the fan.

The results, tested a couple times, are that without the fan, the PSU seems to work. With the fan connected, not.

I still have to rig some old hard disks to use as a load, but if there was a 12V rectifier burnt shouldn´t the psu stop working even without the fan connected ?

Thanks for your ideas ..

[TELVM]

Re: PSU won´t power with fan connected

That happened to me once when I soldered the fan with the wires reversed (+ on - and - on +) . Rearranging the fan wires solved it.

[rogfanther]

Re: PSU won´t power with fan connected

Thanks, will take a look,

[rogfanther]

Re: PSU won´t power with fan connected

(Un)fortunately, the wires are in the correce position. Measure a good 12.07V from black to red. Also, with another , smaller fan, it gives just a twitch in the 12V line then shuts down. Tomorrow will try to remove the 12V rectifieres and measure them offline.l

[Pentium4]

Re: PSU won´t power with fan connected

How do the leads look on the bottom of the board? Once when I was working on a PSU and plugged in the fan connector, it took some force, and since the PSU originally died from overheating, it weakened the traces, so plugging the fan back in ripped the pads on the bottom off, causing a short, so the PSU wouldn't start. Might not be the case here, but it's worth checking.

[rogfanther]

Re: PSU won´t power with fan connected

The original fan had just the wires soldered to the board, and as the points were too far apart for a connector, I soldered two short wires with a connector , as my new fan has a connector in it. Will redo the solder, but as without the fan connected the pins in the new connector measure correctly the 12V, I would venture the solder are correct. But thanks for the insight , it is a good thing to keep im mind with these cheap plastic connectors/psus

[rogfanther]

Re: PSU won´t power with fan connected

Well, from the two 12V rectifiers, SBR20100CT, one gives +- 130mV drop, other gives 117. I want to believe one of these is bad. Will have to search through the scrap pile for two similar pieces to do the testing..

Edit : Well, no dice. Swapped the two with new ones, higher in voltage drop but I belive just for testing that should be ok ? Same thing : little twitch of the small fan, but measures voltage without fan connected.

My understanding until now has been that something is shorted/shorting , and that fan presents just enough load for the psu to "feel" shorted. Would you agree ?

[momaka]

Re: PSU won´t power with fan connected

My understanding until now has been that something is shorted/shorting , and that fan presents just enough load for the psu to "feel" shorted. Would you agree ?

Without good pictures, I can't say anything for sure.

First try to put a load on all of the rails. A hard drive or a 12V incandescent/halogen light bulb on each rail would do.

You can also straight-wire the fan to 7V (+fan wire to 12V and -fan wire to 5V) or 8.7V (+fan wire to 12V rail and -fan wire to 3.3V rail) and not have to worry about the fan controller. In fact, this might load-balance the PSU just enough for it to start if you don't have anything else to load it with.

Personally, I think the PSU is fine. Or at the most, there is something wrong with the fan controller. Should be an easy fix, though. The RS-460-PCAR is a decent PSU.

If the 12V rectifier was shorted, the PSU would have never started in the first place, nor would you have seen 12V. If a rectifier is shorted and a PSU fails to detect it and tries to run, the results is usually on the primary side.

[rogfanther]

Re: PSU won´t power with fan connected

After a long time, I got a camera to take some pics. Not the best camera ,though.

I´ve marked the places where I soldered the fan cable. It seems to derive it´s feed from the -12V line and the 3.3V line. The VCC to the LM 339 also comes from the -12V line. I could not get a good angle to show that transistor markings, but it is a B772.

[momaka]

Re: PSU won´t power with fan connected

You can check if there is a short circuit between E, B, and C terminals on that transistor. But appart from that, I don't think this is the problem. Like I said, try loading the PSU with something more substantial. Several bulbs - one on each rail, should do.

[budm]

Re: PSU won´t power with fan connected

Try heating up those two start up caps using hair dryer (or just replace them) with the fan still attached to see if it will spin up.
How are the other 2 caps?

[rogfanther]

Re: PSU won´t power with fan connected

Hi Budm,

Those you circled in the top are for the 5VSB, I think. The chip near them is a TNY.
I´ve already replaced the two to the right. That chip is a UC3843B.

Will have to check those two near the violet wire, though.