Help needed to start troubleshooting board - Page 1
Forum section: electronics / projects

author: ilium007
author_rank: Frequent Contributor
timestamp: February 26, 2021, 10:57:07 pm
content: The board pictured is a 4G router out of Chyna - its a Comfast CF-E5. It was initially working fine, powered by 24V PoE. The board stopped working in front of my eyes when I removed the SIM card whilst it was powered on. Now when PoE is connected the power LED on the PoE brick dims down low and then goes brought repeatedly as if there is a short. I have checked the RJ45 socket pins, nothing shorted. I have checked each of the 4 on board bridge rectifiers with a multimeter (my trusty blue eeevblog meter), nothing on the board looks burnt or damaged. Not sure what to look at next. I thought there may be an issue in the ethernet transformers behind the RJ45 sockets but I'm not sure how to troubleshoot those. I have tried a known good PoE brick (Ubiquiti one) and the same happens to its power LED.I am not sure why there are four bridge rectifiers on the board either when the PoE power is 24VDC.

author: ajb
author_rank: Super Contributor
timestamp: February 26, 2021, 11:34:26 pm
content: Perhaps there are four bridge rectifiers because it can be powered from either jack? I don't see an isolated converter for the PoE, so it seems maybe the ethernet links are not isolated from each other and from the supplied circuitry as they should be (especially with exposed antenna and a USB connectors). I guess it's possible that removing the SIM card temporarily shorted the supply pins in the socket and that has caused one of the switching converters on the board to fail in such a way that its input is shorted and therefore causing the overcurrent protection in the PSE adapter to trip. This would be an unusual failure, but given they skimped on power isolation who knows what they've done.I see three switching converters, two of them are the little 5/6 pin chips on the right edge of the PCB with the inductors labelled "2R2", and the third looks to be that 8-pin SOIC next to the inductor labeled "4R7" towards the bottom. I would guess that the larger converter on the bottom converts the PoE supply down to 5V, and the other two provide the IO and core voltages required by the main IC from either the PoE converter or the USB connection. Does it work when powered by USB? If so then that would indicate that the main converter is dead. If not, or if the board isn't actually designed for that, then you could try removing the switching converter ICs and see if the PSE adapter seems to be happy with the board plugged in. Removing only one or two of them is risky, because supplying only one power supply to a chip that requires multiple rails could result in damage. It's possible but not guaranteed that holding the IC in reset will prevent bad things from happening. If removing the switching converters eliminates the immediate power fault, then replacing them with new ones (if you can identify a suitable replacement from the markings/circuitry) might get it up and running again. But there could certainly be other things that have failed. It's hard to say without a more detailed analysis of the board.

author: ilium007
author_rank: Frequent Contributor
timestamp: February 26, 2021, 11:42:03 pm
content: Thanks for the reply. I had tried to power via the USB connection but no lights come on either. I guess I'll start by removing the larger of the three switching DC-DC converter IC's.

author: ilium007
author_rank: Frequent Contributor
timestamp: February 26, 2021, 11:43:41 pm
content: Quote from: ajb on February 26, 2021, 11:34:26 pmI thought those MOC devices behind the RJ45 sockets were magnetic isolation transformers.

author: ajb
author_rank: Super Contributor
timestamp: February 27, 2021, 12:03:27 am
content: Quote from: ilium007 on February 26, 2021, 11:43:41 pmYes, but those are for signal isolation, not power isolation. Typically, on a properly designed PoE device, you'd expect to see the normal ethernet isolation transformers like those (or their higher pin count gigabit cousins) PLUS an isolated DC-DC converter with its own power transformer. It's that latter converter/transformer that is not in evidence on this board.

author: ilium007
author_rank: Frequent Contributor
timestamp: February 27, 2021, 12:05:12 am
content: Quote from: ajb on February 27, 2021, 12:03:27 amUnderstood. I removed the larger of the three DC-DC switching converter IC's and now the PoE adpator LED doesn't dim / pulse but again, no power when testing using USB cable. I think this one is for the bin

author: Renate
author_rank: Super Contributor
timestamp: February 28, 2021, 01:53:16 pm
content: Quote from: ilium007 on February 26, 2021, 11:42:03 pmIt looks like they didn't populate that diode that takes USB power.Solder a diode in there and give it a whack.Then you don't need the high voltage DC/DC.