Electronics > Repair

Unifi USW-24 not responsing/dead. Low voltage PSU issue?

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gck303:
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 board

The 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.



u666sa:
Bruh, you have a shorted cap. Apply voltage and add some freeze, see which one gets hot, flick it off and it should work.

gck303:
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.

u666sa:
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.

gck303:
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?

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