PC won't start ... Motherboard LED on

by Nice
(Earth)

power switch header

power switch header

HP dc5100. PSU is working and tested, motherboard led is on. Tried different cpu's, pressing the power switch power led goes on but nothing happens(power led goes immediately off). Tried pulling the bios battery out resetting the bios, makes no difference. Yet i noticed on the same pin header unit together with power switch there is a transistor connected (which i never saw in any previous pc model before). Seems its base and emitter are being shorted. Could it be transistor failure the reason pc is not starting, could i rid of it somehow or try replacing it? thank you

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PC won't start ... Motherboard LED is on
by: Remus

I am not sure why they are using that transistor as a diode. There is actually base and collector that are connected together.
You can replace that transistor with a diode that you have.

Did you check the memory.

There is also a buck converter that supplies CPU voltage. I have some info about it here:

https://www.the-computer-problems-guru.com/computer-power-supply-troubleshooting.html

https://www.the-computer-problems-guru.com/computer-power-supply-problems-explained.html

PC isn't starting ... mobo led is still on
by: Anonymous

Thank you for replying. Indeed, i forgot to mention i tried replacing the memory and yielded nothing. Replacing transistor with diode i didn't but since diode regulating current flow in only one direction, i thought and took the risk bridging transistor pins on mobo with jumper gave no results. Tried without 6pin atx (once stumbled upon one case where disconnecting additional atx power, mobo worked) but nada with this one. Pressed cmos button for more than 30s just in case, didn't worked either. Did the atx test suggested, seems cpu is good. Tried 3 different working cpu's hoping to have at least one spin of fan, nope. Have to say i did open the psu and found few capacitors bulged, changed them, roughly checked voltages were ok (no oscilloscope thing), still mobo shows no signs of life beside led being lit. CPU cooler (vertical 2 sided cooling) had a bit of dust layered even with other side of the cooler being clean and having exit fan taking heat away, so could it be the socket got hot and needs reballing or a hit with the heat gun as reminder
that is not about global cooling lol, dunno i'm kinda out of options for now, thinking of taking small vacation from it maybe some idea or solution pops up, who knows. Thank you for helping and links was much appreciated.

PC won't start ... Motherboard LED on
by: Remus

There is a good website about motherboard repair, but it is in Russian:
http://www.rom.by/book/Rukovodstvo_po_remontu_materinskih_plat_0

I am still looking for a good source of information in English. The one I have found is a Youtube channel by Louis Rossmann
https://www.youtube.com/channel/UCl2mFZoRqjw_ELax4Yisf6w


that's how pro do it
by: Anonymous

Thank you, i liked "know it all" guy. It was interesting and funny in same time watching him go with mobos, certainly he could manage to get it done, giving me some hope and inspiration while laughing hard at his underlaying commentary, i was like looool too funny. In one of his clips got an idea if could have been the hybernation mode this mobo went in, since the pc was set to go sleep and hybernating during non operating hours, so i thought could be that pc stopped working during those hours while 'wake up' information stored on hard drive. Tried my best to have a breakfast with it this morning but no luck yet, the mobo was too sleepy lol may be there are some other special ways to wake it up out of those states that i'm not aware of but will try more if something new lights my bulb up, thank you once again.

PC won't start ... Motherboard LED on
by: Remus

Yes, he is a pro and his comments are mild now. But if you check his earlier videos ... LOL

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