I recently purchased two listed-as-broken Ubiquiti Toughswitches off eBay for $65. They were listed as not working, unable to boot. They admitted very little troubleshooting had been done to the units, but I couldn’t pass up the price since I’d been looking for one of these switches, but they sell for more than I wanted to pay, and a friend also wanted one, so I figured I’d sell it to him if I got both working. Buying them, I figured there were two likely causes. One being a firmware corruption, and they just need to be re-flashed using their tftp mode, or two being the power supplies had failed (power surge, lightning strike, wear and tear, etc). I know from research that these switches have an internal AC/DC power supply that supplies the switch board with 48V, which I can replicate easily enough on my bench. It seems it’s not that uncommon for people to replace those internal PSUs with DC/DC converters for use in wireless installs that only have DC available, but that’s not what I’ll be dealing with here, hopefully.
The unit’s were delivered on Memorial day (oddly), and initial inspection showed no power up at all, and no ground continuity between the ports and the power ground. Huh. I opened both units, and found the failure modes to be completely different. On one unit, the power supply was dead. The fuse had blown (easy to replace), and the main SMPS chip, a TEA1755T was cracked. The failure was bad enough that it blew the trace off the ground connection. So I ran some 20AWG wire from the ground to the screw so there was a good ground again. I ordered and replaced the TEA1755T and the power supply started working again. Sadly, the switch board also had an issue, but more on that in a bit.
On the second unit, the PSU was good, but the LM5005 buck regulator that takes the 48V input from the PSU and converts to 24V was blown (complete with a hole in it). I ordered a replacement and replaced it, but man, TSSOP chips suck to replace. Once that was replaced…
One the first unit again, supplying power to it (even with a bench PSU), results in a hissing sound from the Buck Regulator section, but a clean 24V output from the regulator. But, none of the unit starts up, as I don’t see anything on the main oscillator, and the unit certainly doesn’t boot. Tracing down where the 5V/3.3V rail comes from was a bit of a challenge, but it originates from…
