Dell Optiplex 780 SFF PC Motherboard Repair? - Page 1
Forum section: electronics / repair

author: Chris56000
author_rank: Frequent Contributor
timestamp: December 02, 2018, 12:43:39 pm
content: Hi!I bought a used Dell Optiplex 780 SFF Desktop for use as my Design Workstation, installing Design Spark PCB, sPlan, Sprint Layout, Real View, W7 Pro (Dell supplied OS Disk) and Office 2003, and come to turn it on this morning it won't go!Symptoms are:-a) Power Button LED solid yellow and remains like that;b) Green LED on back of power supply lit;c) Briefly lights up bottom diag. LEDs '1' & '3' then '1' and '2' start flashing together;d) NO video to monitor or beep-codes;e) Fan running high speed continuously.There isn't a lot of help from Google searches, except a non-specific general M/B failure - has anyone come across this issue and been able to pin it down?I've tried removing the CMOS battery and leaving it half an hour - it makes no difference!I've got access to decent SMD reworking-tools both at home and at work, so if any Members know of this issue any help would be greatly appreciated!I don't suppose any Dell M/B schematics have leaked their way on to the Great Wide Open, have they?Chris WilliamsPS!When it was working up to midnight (UK time) last nite it was running quite sprightly with no sign whatever of impending doom, no lockups, graphical glitches, etc!Some Dell S.M.'s state it's a "hardware resource conflict" and others say "System Fatal Failure - Contact Dell Technical Support" - which?This does seem to be a well known recurring failure, so if the cause is well known I can get a repair attempted rather than simply buy another M/B that may show a similar failure after purchase!

author: GreyWoolfe
author_rank: Unknown
timestamp: December 02, 2018, 01:09:04 pm
content: Have you tried removing components one at a time to see if it will boot? Hard/optical drives first, then memory, then processor, trying to power it up after each removal. Maybe something else failed and is keeping the MB from booting. That's typically what I do.

author: neil
author_rank: Contributor
timestamp: December 03, 2018, 12:00:40 am
content: Old Dell's can give weird problems from dying electrolytic caps on the motherboard. Take a good look at them, and look for any bulging tops. They're through-hole radial mounts, so not too hard to replace. Good luck with the problem.

author: orefat
author_rank: Newbie
timestamp: December 03, 2018, 08:12:20 am
content: First disconnect all of the components,leave just front panel connector and MB power supply cables, and turn PC on, if you hear clicking noise then you have 80% faulty PSU, you can disconnect fans, just to make it quiet enough to hear.Try removing your CPU heat sink and turn on the MB and see if CPU gets hot, don't let it be powered for too long.If it's cold then maybe you have issue with VRM or your PSU. I've seen also that some OEM motherboards won't pass the POST if the CR2032 battery is on low voltage/faulty.After that you could also measure the voltage on BIOS chip (on CS and VCC), flash memory, and post it here so we can see if it's sufficient for SPI flash to initialize.Broken, short circuited USB port can also create this problem.

author: Chris56000
author_rank: Frequent Contributor
timestamp: December 03, 2018, 10:35:58 am
content: Hi!Member Orefat, thank you for your tips – I'll fit a new '2032 battery and try your suggestions and report back afterwards!Chris Williams

author: GBowes
author_rank: Contributor
timestamp: December 03, 2018, 05:28:52 pm
content: I have a Dell service manual here (different model) with the following LED Diag explanations:1 & 3 = Video controller failed to initialize or respond.1 & 2 = Memory failed to be sized or enabled.I recently had a Dell 760 Optiplex SFF die unexpectedly and without warning. After spending way too much time looking for complicated problems with power supply and motherboard, I found that one of the memory modules had died. If you have more than one, I would remove them and try the PC with one at a time.Graham

author: Wan Huang Luo
author_rank: Regular Contributor
timestamp: December 03, 2018, 05:49:42 pm
content: Quote from: GreyWoolfe on December 02, 2018, 01:09:04 pmThat's exactly what I would do. Reseat everything, try one or the other memory modules, try known good components, maybe a known-good PCIe video card; disconnect everything except the essentials needed to see a POST screen.