Two month old router stopped dead. Ideas on this circuit? - Page 1
Forum section: electronics / repair

author: NaDobraNich
author_rank: Regular Contributor
timestamp: January 24, 2025, 12:33:23 am
content: I have a router, it stopped dead out of no where. No lights, no bootloader, nothing. 12VDC IN seems shorted to ground.I first pulled the two 470uF 25V electrolytics. On my Fluke 28, one reads 430uF and 5.6Mohm steady. Other shows no capacitance, and 9-11M ohm rising or falling slowly as probes are on it. Assuming one bad cap.The short remains.It looks to me (bad diagnosis incoming!!)...that the 12VDC IN goes to R7, and also goes into an LDO circuit with a 5K/1K adjust. I don't know what this is. Unless R7 is some complicated part, I don't really see what the LDO is doing. My guess is this is a circuit to limit for overvoltage adapters, and to test this repair, I could just remove a few of these parts and wire power up after R7.I'd love to to fix this thing, but really all I need is a temporary boot so I can get the config file from memory for a replacement.Any ideas what I can test next?Notes:Annoying high temp solder! Difficult to remove cleanly. Can not seem to ID many parts with the short. The gaps between positive cap and ground are small and look messy here, 99% sure those are not the short, cleaned up after photo.

author: wraper
author_rank: Unknown
timestamp: January 24, 2025, 12:50:33 am
content: Electrolytic capacitors almost never fail short, you torn them off for nothing and probably damaged one in the process. Likely a dead buck converter IC. Slight chance of shorted MLCC. Picture shows shorted A,B,C,D. What is actual resistance? There is what looks to be a MOSFET between power jack and capacitors, if everything is actually shorted to GND, likely it's dead too.QuoteLooks like reverse polarity protection, not LDO.

author: NaDobraNich
author_rank: Regular Contributor
timestamp: January 24, 2025, 12:58:47 am
content: One cap is definitely not like other, maybe that's during process, but it was the gentle twist and break off method to remove. But alright.Actual resistances, yes, probably a great point here:A to B: 15.5ohm (Input to SOT-3)A to C: .3ohm (through R7)A to D: 4.5ohm (Input to ground)If I could determine what the A2 side was doing, or R7, I could pull parts until the short went away, then power with a bench supply to get it online one time.EDIT: If R7 is reverse protection and the side with two are anodes to a single cathode, then I'm just as confused what A2-circuit is doing. As it looks to me like whatever comes in to the jack, will go into the diode and become the cap supply.

author: wraper
author_rank: Unknown
timestamp: January 24, 2025, 01:25:09 am
content: Then I assume R7 MOSFET got shorted due to overload after buck converter IC (not on the picture) failed first. 2A (it's not A2) probably is a zener diode. R7 likely is xx3407 (like MDD3407, ESJ3407, CJ3407A JMTJ3407) P-channel mosfet made by many manufacurershttps://www.lcsc.com/datasheet/lcsc_datasheet_2412261755_ElecSuper-ESJ3407_C42420759.pdfSome have normal SOT-23 package, some a bit wider like on this PCB.

author: Gyro
author_rank: Super Contributor
timestamp: January 24, 2025, 10:36:05 am
content: I'm a bit surprised that nobody has pointed out that if the router is only 2 months old, you should be returning it for replacement or refund. I don't know hoe feasible it is to 'invisibly mend' the parts you have removed in order to make it not look suspicious.

author: NaDobraNich
author_rank: Regular Contributor
timestamp: January 24, 2025, 03:06:55 pm
content: Quote from: Gyro on January 24, 2025, 10:36:05 amIt is far more important I get my config file off for the replacement.I have a new one coming, but that isn’t the goal here.

author: NaDobraNich
author_rank: Regular Contributor
timestamp: January 24, 2025, 03:11:17 pm
content: Quote from: wraper on January 24, 2025, 01:25:09 amOk, so.... You think the problem is down stream, OK, I could get on board with that.I would know if I pulled 2A and R7 and still was shorted.If 2A is a zener, what is the circuit here? Low voltage protection or high voltage protection? I assume the idea would be to turn the P-Ch off in the even of some bad condition, right? Unfortunately for me... I think this board has a few SMPS, I see an inductor and a few caps next to a 6 pin chip with MICW on it, but seems too few pins for an SMPS and I have little luck of identifying it, Microchip maybe but no logo.Well, not looking good so far.

author: NaDobraNich
author_rank: Regular Contributor
timestamp: January 24, 2025, 06:40:53 pm
content: Pulled the heatsink off.There are 9 or so SMPS-looking circuits on this, and 6 of them are unique parts.MICWTS5eo1TDDEHMDCDuDzSEDtSo..... Yea.... unlikely if one of these is bad that I would find the correct one and fix it without removing all of them one at a time which I guess is an option, but then I would also need to find the correct replacement part.

author: wraper
author_rank: Unknown
timestamp: January 24, 2025, 07:33:21 pm
content: QuoteAs I said reverse voltage protection. Zener is for MOSFET gate protection. Something like circuit below. There is a bit more parts on the board, maybe there is something more to it than just reverse voltage protection. PCB has blank pads for apparently variant with a diode for reverse voltage protection and TVS for overvoltage protection.

author: NaDobraNich
author_rank: Regular Contributor
timestamp: January 24, 2025, 09:48:30 pm
content: Ok, thanks for your help wraper!I think this is a lost cause though. Too many SMPS circuits, most important things under heat shields, problem could be anywhere, all unknown unreplaceable parts, really really high temp lead free solder. Nah. Calling it a defeat.