Unifi USW-24 not responsing/dead. Low voltage PSU issue? - Page 1
Forum section: electronics / repair

author: gck303
author_rank: Newbie
timestamp: December 24, 2024, 11:33:01 am
content: I have a USW-24 (not the POE version).Symptoms are:- The PSU is good and it is giving out 12V.- No lights at all. Not response. Reset does not do anything- Thermal camera is showing something very strange near the power input to the boardThe board has been glued to the enclosure by a large heat sink underneath. This means I cannot easily see what is on the underside. It have looked from the side and I cannot see any components in the area that is getting hot.- What are the components that at marked PC and PL?- There are a small array of tiny holes to left of the PC components. What are these?- Is this part a secondary PSU to drop the 12V to 5V?What do I do next? I do have a DSO, but not probed around yet.

author: u666sa
author_rank: Frequent Contributor
timestamp: December 24, 2024, 11:50:23 am
content: Bruh, you have a shorted cap. Apply voltage and add some freeze, see which one gets hot, flick it off and it should work.

author: gck303
author_rank: Newbie
timestamp: December 24, 2024, 02:05:56 pm
content: I assume that the capacitors are the tiny components marked PC7 to PC11?When it is turned on a number of them get too hot to touch with a bare finger.If I use a multimeter they all register as a short (<= 1 ohm). I imagine that this is because they are in a circuit.Are you proposing just removing the PC components from the circuit and then expecting it to suddenly come into operation?PS There is no voltage to the 3V pins on the motherboard's USB pins. I can't detect any voltage elsewhere.... so I am thinking that it is a fault on the part of the circuit that drops down the 12V to 3V/%v.

author: u666sa
author_rank: Frequent Contributor
timestamp: December 24, 2024, 02:33:08 pm
content: It's good that they are getting hot.1. Apply power so they get hot.2. Turn over upside down canned air and spray / or, apply some isopropyl.3. Identify capacitor which is getting hot.4. Remove capacitor which is getting hot.

author: gck303
author_rank: Newbie
timestamp: December 24, 2024, 02:43:37 pm
content: Okay.... and do you think the board will work without those capacitors in the circuit?A further thought.... from what I can tell the entire board used 3.3V.What would happen if I removed the 12V supply and just applied 3.3v from my bench power supply into the voltage USB pins? Would that be a stupid idea?

author: u666sa
author_rank: Frequent Contributor
timestamp: December 24, 2024, 02:48:13 pm
content: Realize PC3 4 5, 7, 8, 9, 10are all on the same rail. If one is shorted, they all will show zero continuity.You don't have to remove all of them. You have to find which one is shorted. Your thermal camera does not have enough resolution to show you which one is shorted, it shows you a hot area. You do further investigation by spraying freeze or alcohol.We don't apply 3.3V, we apply less than 1 volt, 0.5 - 0.8 volt won't damage anything, 1 - 2 amps. You'll do fine with 12V power from the supply though.

author: gck303
author_rank: Newbie
timestamp: December 31, 2024, 05:59:15 pm
content: Okay.... an update....I bought a can of freeze spray. It was C10 that is shorted. I could tell as it was the first cap' that melted the spray. After a bit of poking with a soldering iron I managed to remove it (not the neatest job). And the switch now boots up and runs!Wow.What did that capacitor do?Should I replace it?Does the value of the capacitor matter much?What is the worst that will happen if I never replace the capacitor?

author: Swake
author_rank: Frequent Contributor
timestamp: January 01, 2025, 02:54:22 pm
content: QuoteCapacitatoring (new word for '25)QuotePreferably. It is on the board for a reason. Eventually only for somewhat extreme conditions, but who is going to tell....QuoteEverything is relative, but yes, it does matter what size it is.It was in parallel with some others. I guess you do not have something to measure one of those (have to take it of the board)?QuoteThe other caps can't keep up with the capacitatoring and also break.