Solid amber on the MB means it has power.  When the power button has blinking or solid amber, then it's a problem. From your photos it looks like you have the SFF model. Optiplex 780 Documentation  From the 780 Service Manual: Optiplex 780 Tech Guide.pdf Answer from bradthetechnut on dell.com
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Dell
dell.com › support › kbdoc › en-us › 000126021 › a-reference-guide-to-the-dell-optiplex-diagnostic-indicators
A Reference Guide to the Dell OptiPlex Diagnostic Indicators | Dell US
Diagnostic LEDs for the OptiPlex series (Prior to 2005) ... NOTE: If you do not see any LEDs, or only a Flashing Amber Power LED, your computer may be experiencing a power issue before the POST starts.
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Dell
dell.com › support › manuals › en-us › optiplex-7460-aio › optiplex_7460_service_manual › diagnostics
Dell OptiPlex 7460 All-in-One Service Manual | Dell US
For example, the power status light blinks amber two times followed by a pause, and then blinks white three times followed by a pause.
Discussions

Opitplex 7060 micro blinking amber light 2+1
Having an intermittent problem with a 7060 micro PC that does not boot sometimes. The power button flashes 2+1 amber and it does not do anything. If you turn the power off at the wall and then back... More on dell.com
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December 10, 2023
Amber power light of death flashing Dell optiplex gx620
Hello guys i hope someone can help me. My desktop stoped working and it showed me the amber power light. First i cleaned the inside of the desktop from dust. I checked the battery it was fine ( 3V ... More on dell.com
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August 22, 2017
Optiplex 755 Flashing Amber Light - Processor Power Connector
I recently got the flashing amber light on my 755. I unplugged everything and reconnected them one at a time pressing the test button on the power supply each time. Everything checks out except whe... More on dell.com
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October 6, 2013
amber light on dell optiplex 755
hello, i have an issue with my pc, whenever i try to boot, it just gives an amber light and wont become green again. is it the power supply? or is it some other component that i must get? specs: dell optiplex 755 2GB RAM Windows 8.1 (64-Bit) Intel GMA 3100 275w PSU Intel Core 2 Duo E4400 2.00GHz More on forums.tomshardware.com
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March 14, 2022
Top answer
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Hi twd1987, I’d suggest that you follow the steps mentioned below and check if the system switches ON. Switch off the computer. Disconnect the power cable from the wall. Press and hold down the power button for 15 seconds. Then connect back all the peripherals and check if the system switches ON. If the system is able to boot up after performing the above steps then I would suggest that you run the diagnostics on the computer by following the steps mentioned below and check if there is any issue with the hardware on your system. Turn on or restart the computer. Immediately at the Dell logo screen, starting tapping the key once a second until the One Time Boot menu appears (if the computer boots to Windows turn the computer off and try again). At the One Time Boot menu, press the arrow key to highlight Diagnostics, PSA+ or Enter ePSA, then press to begin the hardware Diagnostics. Write down any error codes listed. Reply to us with the error codes. Dell Optiplex 755 comes with diagnostic indicators. If the computer is not starting up you can refer to the diagnostic indicators. Blinking Amber on optiplex 755 indicates ‘The power supply is receiving power from the source but is unable to properly distribute it to the components’. Also, I’d like to know the status of the diagnostic LEDs (1 2 3 4) which are located on the front of the chassis between the headphone and USB connectors, depending on the chassis.These diagnostic indicators are only active and visible during the POST process. Once the operating system starts to load, they turn off and are no longer visible. As I see that you have replaced the system with a known good power supply then it should be an issue with the system board.
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I have dell optiplex 755 .i want to know the meanings of (1,2,3,4) while blinking amber on turning the computer on
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Tom's Hardware Forum
forums.tomshardware.com › home › power supplies
[SOLVED] - amber light on dell optiplex 755 | Tom's Hardware Forum
March 14, 2022 - If it the power button that is amber it will mean a bad PSU or motherboard. Not sure if those have proprietary PSU, more then likely you could probably pick up a used/refurb dell off ebay for about what you going to be paying for a new PSU for that computer. ... The on and off button is normally ...
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JustAnswer
justanswer.com › computer › fahk3-power-button-flashing-amber-green-lights.html
Troubleshooting Dell Optiplex Diagnostic Lights and Flashing Orange Light Issues
The power light on the tower is blinking. on the monitor screen, there is a message that says: digital & analog is in ... Disclaimer: The customer and expert messages presented herein may have been altered or modified to enhance readability and safeguard the privacy of individuals. These modifications do not change the intent or substance of the original messages. ... OptiPlex 780 may show amber power button flashing and fail to display video output on monitors.
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Dell
dell.com › dell community › desktops › optiplex desktops
Optiplex 780 blinking amber light of death | DELL Technologies
November 8, 2020 - As I said before, it does not match any of the codes listed in the manual. It is just a blinking power button. I do not know how to edit posts. ... Blinking Amber power button is Bad power supply.
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Dell
dell.com › dell community › desktops › optiplex desktops
Optiplex 7010 MT flashing amber light, won't turn on | DELL Technologies
January 8, 2019 - Weird, have no idea why this worked, but appreciate the good advice here. ... If the power light is blinking amber, the computer is receiving electrical power, but an internal power problem may exist.
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Dell
dell.com › dell community › desktops › optiplex desktops
Optiplex 3010 Blinking Amber Light on Motherboard | DELL Technologies
May 25, 2021 - Then plug everything back in one at time, pushing test button in between, to see if green light comes on. Once it doesn't, you know what's at fault. Blinking amber on an Optiplex does typically mean bad PSU, so you're right there.
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Dell
dell.com › dell community › desktops › locked topics – desktops general
Amber blinking light | DELL Technologies
October 5, 2013 - I just unpacked my Optiplex 330 from a move, and it was working right before the move, but now its giving me a blinking amber light. I looked it up and saw that it is probably a power supply problem, so I got a new power supply and the same thing happened. I opened it up and started testing the connection and found that when I unplug the 4 pin plug from the power supply, it stops the blinking light.
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Dell
dell.com › dell community › desktops › optiplex desktops
Power Button Blink Amber Light | DELL Technologies
June 20, 2019 - you can also hot wire the PSU, to force it on. on my 3020 pin 8 (highest) pin is green wire and is power on. if I jump it to pin 7 (of 8) ground pin to green 8 the PSU turns on, even on the bench. PSU benched. ... I guess you said 2nd PSU fails TOO, but is not clear in your text sorry. and in same way flashing amber.
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Dell
dell.com › dell community › desktops › optiplex desktops
optiplex 3060m amber light | DELL Technologies
April 27, 2020 - faulty power button that flashes amber and refuses to turn on system could be suspicious sign of system power supply failing. For now you can plug the power cord into a power strip w a switch.
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Reddit
reddit.com › r/dell › optiplex 790 sff flashing amber on power button, 1 and 3 icons, possibily motherboard issue?
r/Dell on Reddit: Optiplex 790 SFF flashing Amber on Power button, 1 and 3 icons, possibily motherboard issue?
May 1, 2022 -

Dell Optiplex 790 SFF has started power-cycling as soon as you plug the power cord, the power button "1" and "3" icons flash amber, I know that this should be a RAM issue but I don't think that's the case.
I've disconnected the internal PSU as it was emitting a loud whining noise and connected an external known-working PSU and the PC still turns on by itself with the 3 amber lights but the power-cycling disapperead, now the PC stays on as soon as you plug it in or flip the PSU switch.
I've unplugged power and data to all disks, removed GPU, removed CMOS battery, tested all ram slots with all ram sticks individually and nothing has changed.
I've then removed the CPU heatsink and when the system turns on by itself I can't feel any heat on the CPU heat spreader, reseating the CPU also did nothing, from a visual inspection there are no bent pins or burned components on the motherboard, I'm thinking that It's a motherboard and possibly PSU issue, maybe the PSU went bad and took the motherboard with it.

Any suggestions?

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Dell
dell.com › dell community › desktops › optiplex desktops
Optiplex 9010, flashing amber light on on switch | DELL Technologies
April 29, 2020 - I had a similar situation, but I ran into an undocumented cause of an amber blink.
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Experts Exchange
experts-exchange.com › questions › 29223176 › Dell-OptiPlex-3020-Power-light-blinks-Amber.html
Solved: Dell OptiPlex 3020 Power light blinks Amber | Experts Exchange
August 27, 2021 - It may have a module on the motherboard for the AC adapter whereas the SFF has a power supply built-in. I'm not sure if they are truly using the same codes ... Sorry, think we were all fooled by the mention of 3020 throughout your question.
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Dell
dell.com › dell community › desktops › optiplex desktops
Optiplex 3080, CMOS battery, power light flashes white 1, amber 3 times | DELL Technologies
May 14, 2024 - The diagnostic code of 1 white 3 amber or in reverse order 3 amber 1 white was an indication of CMOS battery failure. See the last error code in screenshot of partial diagnostic light codes below. When a coin cell battery failed or when AC power was disconnected for a long period of time, the BIOS lose its settings.