Restoring power and then no system start

[davmax]

I have read about the problem of disconnecting power and when reconnecting the power supply will not start.

Well it happened to me. The problem turned out to be caused by a short circuit of the 5VSB supply in a USB socket, drawing over 3 amps from 5VSB. A small metal earthing tab had been bent by plug insertion so that it connected with the 5 volt supply. This was intermittent and varied according to the stress cause by an adjacent plug, there being no plug in the offending socket.

I still have a problem and have to resolve whether the 5Volt regulator chip has suffered or there is a poor connection at the motherboard connector( it seems to work better will a test prod inserted there). Will report further.

[davmax]

Re: Restoring power and then no system start

Further results.

5VSB is now not a problem. However if power is removed and then restored whilst the system is hot it cannot be not restart. The Power On voltage does not get pulled down by keyboard initiation or the front panel button. I have reflashed the bios to verify no problem there.

So somewhere there is a heat sensitive circuit. I have not been able to find any changed voltages for either the hot or cool condition. Starts when cool.

Any ideas?????

[linuxguru]

Re: Restoring power and then no system start

1) Recap the pi-filter caps on +5Vsb, even if the voltage is within spec and the caps look OK.
2) Look for small caps near the Southbridge - these can go bad occasionally.
3) Check for oxidation on the ATX connector, especially on +5Vsb and PSON.

[PeteS in CA]

Re: Restoring power and then no system start

If you have a spare P/S, you could substitute it and determine whether the problem is the P/S or the MB.

[gonzo0815]

Re: Restoring power and then no system start

Most PSu`s have very od badcaps at the 5vSb supply. Theyr usually bulging without any stress. That said, i would certainely recap those two caps, as most psu`s develop a no start until replug behaivior with non working VSb.

[davmax]

Re: Restoring power and then no system start

Thanks guys. I recapped power supply to hopefully eliminate that possibility. Gonzo. Re 5VSB caps in PS, I have only replaced the output cap in this circuit are there others I have missed. I understand that this circuit runs from a small transformer winding, rectified then a 5V linear regulator. There should be a cap in rectifier circuit.

Thanks Linuxguru I will check these items. All very helpful.

[davmax]

Re: Restoring power and then no system start

Guys the MAJOR clue is that when the problem exists 5VSB is OK but PS ON line does not pull down when the power button is pressed. This indicates motherboard to me. Remember bios has been re-flashed and if the computer is allowed to stand for a while to either cool or discharge the problem goes away. Suspect it is a heat problem.

[gonzo0815]

Re: Restoring power and then no system start

Hm well, i assume, that the PS_on signal is not affeckted by bios. I think it is hardwired. That said, follow the PW_sw on the board and may be you can locate the badguy. May be a bad smd transitor or bad pull down resistor.

[davmax]

Re: Restoring power and then no system start

Thanks Gonzo. I will have to take the board out again. It does have the characteristic of a dry joint or bad connection. The concern is that the PS ON signal is controlled by a toggling circuit in a chip somewhere, it certainly is linked into the op system ie Shutdown is a toggle. I just hope this is not the problem

[davmax]

Re: Restoring power and then no system start

Before taking the computer apart I checked the current drain on the 5VSB supply whilst the system is shutdown.
Normal ISB without unplugging is 0.42A
After unplugging and replugging power the current is 0.43A whilst hot and 0.38A cool. There is no start if unplugged hot and replugged.
After unplugging and replugging when cool the current rises to 0.72A and the system will start, returning to 0.42A at the next shutdown.
ISB is negligible whilst the system is running.
So for some reason the motherboard cannot initiate a PS ON without replugging when cool and passing an additional 0.3A !!!!!
It sounds so strange I am tempted to live with it. It can be managed.

[davmax]

Re: Restoring power and then no system start

Oh also 5VSB is a steady 5.04Volts and as long as power is not lost eg unplugged whilst hot it will always start.

[linuxguru]

Re: Restoring power and then no system start

Just guesses - could be a blown cap or polyfuse/thermal fuse on +5vsb on the motherboard that blows and then heals on cooling.

[gonzo0815]

Re: Restoring power and then no system start

Or some dry solderjoints under a BGA chip this would be no great discovery. Is this an RoHS conform board?

[davmax]

Re: Restoring power and then no system start

linuxguru. That is a good prospect. It would line up with the short in the USB socket that I cleared. May not be able to investigate for week now. Will post if and when the problem is uncovered. Thanks also to Gonzo0815