The center pin is ground, most certainly needs to be connected, and probably is connected. The center pin is connected to the heat sink surface on the top of the chip that appears to be soldered. If in doubt, check it with an ohmmeter. That short pin is likely zero ohms to the pad it isn't soldered to.
Thank you for your reply but most of that went over my head. Just to clarify, are you saying the resistance between the 2 points marked in red should read zero ohms and I should go ahead and connect them?
Or is the short leg just an extension of the long piece across the top of the chip which is soldered to the board?
The 78M05 chip is listed both with and without the centre leg:
The big tab on the back is probably connected to that pin, and appears to be soldered down. You could double check with the device unplugged by checking ohms between the disconnected pin and tab will probably be 0 or real close.
The PoE power circuit should be higher voltage than that 5v regulator could provide. However, with the device plugged in you should CAREFULLY confirm you have that 5v line as that will be one of the important “power rails”. If not present then the problem is upstream, if present then problem is elsewhere.
Silly question, is it powered from a “wall wart” power supply that’s external? I would check that is outputting the specified voltage (usually slightly higher when not plugged into the load). Also check near where the device plugs in for a fuse which would be an easy check also.
Thank you for your reply but most of that went over my head. Just to clarify, are you saying the resistance between the 2 points marked in red should read zero ohms and I should go ahead and connect them?
I meant to say the resistance between the 2 points should read zero, and there's no need to connect them because they (if zero ohms verified) are already connected. Good idea too to verify the voltage on the regulator input is higher than 5, and at the regulator output is 5. If that all checks out, the problem is elsewhere.
