Trying to repair a roasted Mainboard - Page 1
Forum section: electronics / beginners

author: TheGenesis
author_rank: Contributor
timestamp: December 22, 2020, 09:56:10 pm
content: Hi there,I have a Mainboard wich starts (everything lit up, CPU gets warm, Fan spins etc.) but did not bring anything to the monitor.On closer inspecting I noticed a burned component near a Power-Stage. The part is totally burned away so I cannot read any markings. Fortunately I have a very similar Mainboard were the arrangement is nearly identical.Can someone tell me, what this component is?And should I also replace the capacitor below?The NIKOS below the capacitor is a "N- & P-Channel Transistor". Can I check this thing while in circuit or should I change it anyway?Thanks for any help!best wishesThomas

author: george.b
author_rank: Frequent Contributor
timestamp: December 23, 2020, 12:34:15 am
content: Quote from: TheGenesis on December 22, 2020, 09:56:10 pmIt's a 7A trip, 3.5A hold, resettable fuse, made by PPTC. Part number SMD1206P350SLRT by the looks of it.http://www.pttc.com.tw/DataSheets/Data%20Sheet%20for%20SMD%20SLR%20Series.pdf

author: Manul
author_rank: Super Contributor
timestamp: December 23, 2020, 12:48:08 am
content: I'm quite sure it is Polytronics PTC. You may try to search their catalog for exact match. Probably there is a failure down the line.If capacitor is not shorted and looks ok, just leave it. If there are heavy burnmarks and PCB is carbonized, then definitely clean that area, remove carbon and replace capacitor, because it might be damaged by heat.Ivestigate which power rail this PTC protects and look if anything is shorted on this rail. Yes, most often you can test mosfets and bjts in circuit.

author: TheGenesis
author_rank: Contributor
timestamp: January 09, 2021, 08:14:52 pm
content: Ok ... had to order a replacement (nanoSMD350LR-2) wich has arrived yesterday.I removed the fuse checked the cap, wich was fine, and soldered the new one in.The Board now starts again.But it won't pass post (stopping at 8A) when a PCI-e card is inserted.Tomorrow I will check voltages on the PCI-e connector. According to Wikipedia there should be a 12V and a 3.3V rail.Mhmm ... but the dead fuse protects the 5V rail ... could be more than one issue.Any tipps what i can check besides this pins?

author: SilverSolder
author_rank: Super Contributor
timestamp: January 09, 2021, 08:50:22 pm
content: If you are lucky, the fault is on the PCIe card itself? Have you got another one you can try?

author: TheGenesis
author_rank: Contributor
timestamp: January 09, 2021, 08:59:18 pm
content: Yes. I tried 3 different, known good, cards. When I unplug the card the system boots. Everything else is working except these damn Slots.

author: TheGenesis
author_rank: Contributor
timestamp: January 10, 2021, 10:07:23 pm
content: I have checked Voltages and they are Present:Pin 1+2+3 (top) = 12VPin 2+3 (bottom) = 12VPin 10 (top+bottom) = 3.3VWhen a card is inserted into one of the 4 PCIe Slots the Boot-Screen stops at the following Post-Codes:PCIe-16x Slot 1: A0PCIe-1x Slot 2: 99PCIe-1x Slot 3: 99PCIe-16x Slot 4: 99Curiously Ctrl+Alt+Del is still functional with Slot 1.I also noticed that the DSUB (VGA) is not working.I checked every fuse on the pcb with no luck and ran out of ideas.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 11, 2021, 11:21:03 pm
content: Sounds like something non-trivial is broken, maybe the chip that drives the slots?Might be an idea to look on eBay for a replacement motherboard?