Trying to repair a broken LIME2 Rev K board

[mundragoon]

Hi everyone,
First off I have to thank the people at Olimex for the amazing work they have done for open source hardware and free software. Thanks to them, I have several LIME2 boards deployed as lightweight servers which run completely free software and are even open source hardware.

I was recently given a LIME2 board which was broken. It seems to have a power related issue. When plugged in to a power supply, the red power LED flashes continuously, as described in this post: https://www.olimex.com/forum/index.php?topic=6837.0
This power supply works on other LIME2 boards, so I know its good. Furthermore, if I measure the voltage on the PWR jack, I can see it cycling between 0 and roughly 5V. It seems as if the board is trying to power on, but some protection mechanism is kicking in and cutting the voltage.

After visual inspection, I noticed the L16 inductor coil was broken, it was possible to see the copper wire inside. Using a multimeter, no continuity between the L16 pads was measured.

So I looked up the part number ordered some replacement parts from here https://www.digikey.com/en/products/detail/taiyo-yuden/NR3015T2R2M/958029
A very kind friend with more experience in soldering small components helped me desolder the old coil and solder a new one. There now seems to be continuity between the pads. However, the red power LED still flashes when trying to power the board, so I guess something is still wrong in the power circuitry.

I've found the schematic for the rev K board and am going through it. My idea is to try and check/measure each step going from the power jack to the rest of the board and identify the fault that way. This is quite new to me, so I'd be very grateful for any tips, or confirmation that I'm heading in the right direction. Any feedback welcome.

Thanks!

[olimex]

The RED LED will flash only in one case - over voltage abuse of the board.

It's written 5V only, but this do not stop people to attach 9-12V adapters to it then to wonder why board stop working.

Fortunately there is TVS on the input which takes the blast and burn, it's on bottom layer just below the PWR jack. Please close check it will have signs of burning.

The TVS makes solid SHORT circuit and this way protects rest of the board from the over voltage, but this process is not reversable, once burned it stay short forever.

The RED LED indicates power supply is attached. it blinks as when the power supply adapter is attached you short circuit the adapter output and the internal protection triggers and disconnect the adapter, then the protection cool off and adapter again send power to the board and get shorted again.

This causes the red LED to flash.

If you remove the TVS the board will work again, but if you again apply more than 5V on the power jack without TVS this time all ICs on the board will burn as there will be nothing to protect them from overvoltage.

[mundragoon]

Thank you very much for getting back to me, and the highly useful information.

From looking at the schematic for the board, does D3 correspond to the TVS that you mention?

I can find D4 on the underside of the board, which is pretty close to the 3 through-pins of the powerjack. But I can't physically locate D3. I don't see anything on the underside of the board close to the powerjack that seems to be burnt or damaged.

Thanks