I recently replaced a faulty D-Link 8 port Gigabit ethernet switch and was about to trash it, then I starting wondering what the actual problem was.. The switch was a DGS-1008D
The symptoms were: power LED on, complete bottom row of green LEDs flashing about once every second, and obviously it just didn’t work.
I assumed that, the power supply was OK (since the power LED was on) but after checking it out with an oscilloscope I realised that I was wrong.. There was a heap of ripple on the power supply line. If I bridged another 220uF capacitor in parallel with one of the existing ones in the power supply section it worked fine, it also worked with another power supply.
My conclusion is that the DC plug pack power supply which is transformer based (not switch mode), must have a filter capacitor that has died..