Ubiquiti US-48-500w switch no boot. Main PSU works.

[Swake]

Thanks for the update, interesting.
If you buy chips, make sure they're the "real deal".

[drhex]

Do you have any SFPs in the switch? Had the same issue, no boot, fans run up. Took the whole thing apart, starts working. Put it together again, broken. Puzzled, removed the SFPs which I had put back in and it worked just fine. Had already ordered a replacement which is now sitting on the shelf.

[acedogblast]

Do you have any SFPs in the switch?

Nothing except for power and the console cable.

[acedogblast]

Another test that I could do that I have not yet done is to try to interface with the two JTAG headers. I have not done it yet because I don't have a device that can interface with JTAG. There are 2 JTAG headers on the board, J11 and J12 which most likely is for U1 and U2 respectively due to the headers proximity to the chips. I have just bought a FT232H reader device of amazon and it should arrive tomorrow.

This way I can test if U2 is working before I attempt to do any reflowing of U1 or U2.

Here is a picture of J11 it consists of a 2x10 row header with the top row mostly GND pins except for the top left pin. This matches the description of the "ARM 20 pin" JTAG header that I found online. Going to try to interface with it to see if U1 or U2 responds or not.

[asis]

Before proceeding to the cardinal steps, I advise you to carefully read the pdf on the MX25L25635F.
Particularly the sections relating to data protection (9-30 - 9-36).

[Swake]

Good idea. You could also monitor other lines of those chips for activity. Would give you some more confidence if they're alive or not.
If you switch that thing on without the heatsinks, make sure to monitor the temperature of the chips and avoid them running too hot. You should be fine as the switched does not work much but the heatsinks are on there for a reason.

Anyhow, this repair attempt will have learned us some new things. Even if the repair fails that is a win.

[acedogblast]

Ok I have some data on the JTAG headers. I assigned the pin numbers to read out as a dip IC(bottom left corner is pin 1, top left corner is pin 20). As said before it is a 2 row 20 pin header. Pins 11 - 19 are confirmed to be GND and pins 20 and 8 as NC.
I would put this data using the table but I don't know how to use it.

Pin   Voltage to GND   Suspected Use   Confirmed Use    Resistance to GND    Adjacent resistor
1     3.29                   VCC                  VCC                   442ohm                   0 ohm
2     0.27                   TCLK?               -                        4.67 Kohm               Pull Down
3     3.27                   TDI?                 -                        5.55 Kohm               Pull Up to pin 1
4     3.27                   TMS?                -                        5.47 Kohm               Pull Up to pin 1
5     3.00                   TRST?               -                        5.48 Kohm               Pull Up to pin 1
6     0                        ?                      -                        4.71 Kohm              Pull Down
7     3.27                   TDO?                -                        5.55 Kohm              RC to GND
8     0                        NC                   NC                     INF                         -
9     0                        ?                      -                        4.70 Kohm              Pull Down
10   0                        ?                      -                        4.69 Kohm              Pull Down
20   0                        NC                   NC                     INF                          -

From what I looked up from the 20 pin ARM header (note pin order is different from mine) it is not exactly the same match. This based on the info given here: https://developer.arm.com/documentation/101416/0100/Hardware-Description/Target-Interfaces/ARM-Standard-JTAG

[asis]

I think it will be helpful...