I do not have a clue, but I would offer a guess that T is for transistor. They seem to be using C for capacitor and R for resistor. That sounds like a fun project. Good luck!

If you are still on it or if it even helps, but when not blown up the component is labeled "AP E7"

@ScottB.ca 

Hello, I managed to kill mine by feeding it 48V passive PoE. Exact same thing (T9) showed signs of death.
But, mine reads "K7", on burned part of it a "P" can be seen, so I guess "AP K7" would be the full marking.

Yanking it out, revealed a secret.


I'm guessing zener diode.
Rectifier bellow seems fine, measures 640ohm from AC to (+) with negative probe and to (-) with positive probe.

@ScottB.ca 

Hi, I've the exact same problem.

First off I have one EAP225-Outdoor that I've never managed to get to work, neither with the passive PoE adapter included  with the product nor with any of the "active" PoE switches I tried it on. It was mounted high on a pole that I cannot reach.

I exchanged it with another EAP and it didn't work either. I changed the cable and still it won't work. I finally bought a cable tester and managed to measure the cabe length at 75m, which is past the 60m passive PoE capability of EAP devices but not past active PoE. Now I got a proper TP-Link, omada supported, PoE-Switch. Still it won't work, but it shows a power drain of 1W.

Fast forward to a few weeks ago. I installed another EAP225-Outdoor high on a pole in a different location. I had preconfigured 2 devices that worked well with active and passive PoEs at home. None worked with either. I shortened the cable, tried a different cable, still none would work.

I brought one of them home and connected it locally. Now it won't work at home with previously used cables and active/passive PoE, but I did notice the classic electronic component burning smell.

The same component is burned, as in your picture. I'm guessing the 2 remaining EAPs still mounted on poles have the same problem.

active PoE standard allows up to 57V to reach the device. I mesaured all my PoE switches output and they are around 53.5V, so I very much doubt that a passive PoE of 48V could damage this devices.

My only guess is that it might have to do with shielding and wether the active PoE switches connects the shielding to the - or + terminals. On all the pole-mounted EAPs I used shielded cable and a convienient DIN rail mounted Dahua PoE switch first. That might have been the problem. At home I used unshielded cables for configuration.

Can you confirm if you used shielded cables and the type of power source for your burned EAP?

Edit: As for the burned component itself, T might stand for TVS or transient voltage supressing diode, as it has only 2 terminals and transistors are usually designated with the leter Q. This would also indicate that there was a problem with a too-high voltage at some point.

FB24/FB25 are ferrite beads, basically shortcircuts that filter high frequency noise

C1026 and C605, having the designator 'C'  means they were originally designed as capacitors, but the marking '000' means they were replaced with 0 ohm jumpers. This means that the TVS is directly connected to the shielding of the RJ45 socket, which reinforces my shielding problem theory.

Also, the big size of C1026/C605 indicates they were probably high voltage capacitors. If they had been high-voltage caps, a DC voltage on the shielding (with respect to PoE) would have created a small inrush current which the TVs could have handled, but since they were replaced with 0-ohm jumpers, the DC voltage remained constant thus burning the TVS and probably some other component, since removing the TVS doesn't allow the device to start.

@ScottB.ca I did use unshielded cables, but indoors only.

It worked with 24V passive PoE from the bundled injector and from Mikrotik RB960PGS (forced-on mode) with 24V power source.
It also worked with active PoE (48V, 802.3af) from D-Link 1100-05PD, which was powered from the Mikrotik above with 48V power source, which did not power the EAP, I guess Routerboard is not very good (read very bad) at detecting PoE loads as It took several tries with long wait to power the D-Link switch.

Then I decided to try "forced on" in the Mikrotik with 48V, the switch powered on, the EAP fired "short circuit" errors in the Mikrotik, then it did not power from the switch and the injector and had the familiar burning smell....

I assumed since the EAP is 802.3af capable it must be able to handle 48V, yes, but also no.
Somewhere I read that active PoE begins with low voltage to detect load, then if yes, turns on the 48V.
So, the EAP probably uses that initial low voltage to turn on its 48V=>24V power circuit (a simple voltage regulator would produce a lot of heat), otherwise it expects 24V.

802.3af and all active Poe is a negiotiated power source. Passive just applies voltage to the line. This can for sure cause problems when the amperage is to high at 48v. I will attach the readings from my Cisco SG-300 Poe switch to show what my still operating outdoor ap is pulling.

the question is what is your passive POE adapter putting on the wires.

if we look at the numbers on the Active POE.

the we calculate the Negiotiated amperage at 48v

my switch shows 3700mW   so thats 3.7Watts

so the AP is pulling 3.7 Watts at 48 Volts from the Switch.  That is a total amperage of 0.077 amps.  

if you are connecting a 48v 3 amp passive injector you will probably blow it apart.

please let me know if my math is wrong.

scottb.ca

@ScottB.ca I still think it's not high current that killed mine, but feeding it 48V directly (passive) from the Mikrotik with forced-on mode, skipping the negotiation phase.
The switch (D-Link DGS 1100-05PD) did not die because it was not designed with 24V passive PoE in mind.

The questions are:
Did that diode blow up as a result of something else?
Can that something else and the diode be found in a local store (or alternatives) and replaced by a person with soldering skills enough for that T9 diode, but not for something smaller?

EDIT: Actually, current probably is the killer. I'm not too deep into electronics, but I know some things :)

Let's assume:
The AP requires max 12W (for easier math).
The board is designed to be powered by 24V (after passive/active step down and whatever happens in the PD part).
PD (power delivery) stage implements very simple current limiting through a stupid resistor.

Then, 12W/24V would be 500mA by 48ohm resistor, If we power that with 48V, the current will be 1A, which might be too much for what is next on the circuit.
So, when the EAP negotiated active mode it must step down the voltage from 48V to 24V, and feeding it 48V directly bypasses that and the rest of the board pulls more that it can eat and dies.

The questions remain, what died and can it be replaced?

@ScottB.ca After those RV1-4, we have one rectifier D20 on two of the pins, and another D22 on the other two, then the blown T9 parallel to D20 and a bigger one T8 to D22.
What happens next and what died after T9 is above my paygrade :)