Ubiquiti US-48-500W – No LAN Port LEDs, Possible 12V Issue?

[Robin21]

Hi everyone,

I’ve got a Ubiquiti switch (model US-48-500W, motherboard: USW48P_BCM) on my bench. The switch powers on, but none of the LEDs next to the LAN ports light up.

I’ve measured voltages across the board and suspect a problem with the 12V rail. At the coils where I’d expect 12V, I’m seeing around 6.3V. One coil shows a stable 6.378V, while the other fluctuates between 6.270V and 6.310V.

All other voltages seem fine — 1V, 3.3V, and 5V are present and match the expected values.

I've replaces both buck regulators already (AOZ1212) and tested all MOSFET transistors.

Has anyone worked with this model before and can confirm whether12V should be present immediately after powering on? Any insights would be appreciated!

Thanks in advance.


I couldn't try the firmware downloaded from ubiquity support. They are different in size and when I've compared them in HxD they are different as well.



Here's the original firmware dump:

[CapLeaker]

Interesting. This is a POE+ managed switch. What’s the resistance on the 12V output to GND? I just find it strange that two 12V rails go bad and do the same thing at the same time. What is the voltage input at the buck converter? Check the PSU. I think it is a dual voltage PSU with a 24V and a 48V rail. The 48V is For POE, but maybe something is up with the 24V one?

What is the status of the enable pin of the buck regulators?

[Robin21]

Hi,

The resistance on those two coils is quite high. It starts at around 20kΩ when I begin the measurement and gradually increases to 140kΩ, after which the resistance continues to rise very slowly.

Looking at the datasheet for the AOZ1212, on page 7 under Output Voltage Programming, the formula is:

V = 0.8 × (1 + R1/R2)

I removed both resistors from each buck regulator and measured them. The readings are consistent across both:
R1 = 140kΩ and R2 = 20kΩ
So:
0.8 × (1 + 140/20) = 6.4V — which is what I have measured.

It seems my 12V is coming from somewhere else. I haven’t found any coil showing 12V, although there are 12V labels on the board. Interestingly, every other voltage label matches the voltage on the coil next to it.

I will have to look at 12V rail closer and trace where it comes from.

[CapLeaker]

I think it takes the 24VDC directly from the PSU. Measure the 24v from the PSU.