Introduction
This guide outlines steps needed to repair an Ubiquiti Unifi AP AC Lite (model UAP-AC-LITE) access point which does not power on, and is identified by the PoE injector having a power fault (blinking white on the PoE injector).
In some cases, affected units' center LEDs may flash dimly when the reset button is held pressed in with the POE power in place. This may only be visible if the board is out of it's chassis.
The UAP-AC-LITE contains an over-voltage protection circuit driven by a unidirectional TVS diode. While this diode is not a fuse, it's expected failure mode is to short, and divert power away from the rest of the board. This short is also detected by the PoE injector, which causes it to not deliver power.
In this guide will show how to:
- Disassemble the UAP-AC-LITE
- Test if the over voltage protection TVS diode has failed
- Remove and replace the broken diode
Parts
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The front cover is secured by 5 tabs.
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Work the Jimmy into the space near a clip, and create a big enough gap to insert a prying tool. Moderate pressure from a prying tool should force the clips open.
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There are a number of alignment tabs that can get in the way. It is best to try and work the Jimmy towards the center of the front cover, hugging its inside surface.
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The PCB is held down by 3 screws. It is not necessary to remove the board for this repair.
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The TVS diode is located between the antenna and a large IC on the RJ45 port side of the board. It's PCB marking is D6.
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It is a LittleFuse 24V uni-directional TVS diode, with J leads (SMBJ24A). Note the location of the white bar on the diode, which marks its orientation.
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Use a multimeter or ohmmeter to test that the diode has indeed shorted. A shorted diode will read less than 1 Ohm in both directions. A working diode will read significantly higher resistance in one direction relative to the other.
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Desolder the broken diode. Apply heat to one lead while lifting the diode with tweezers. Once the solder melts the part should lift up. Repeat on the other lead.
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Install the new diode.
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Apply flux to the pads, then add a thin layer of solder. If solder does not stick to the pads, you can add it to the underside of the diode pins instead.
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Hold the new diode in place using tweezers. While applying downward force, apply heat to one pin near the solder until the solder melts and makes a connection. Repeat with the other lead.
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Verify that the connections are good. There should be continuity between: one pin of D6 and a pin in VR3, and; the other pin of D6 and D7.
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To reassemble your device, reattach front cover.
18 comments
Do you think this problem has something to do with Ubiquiti dropping 24V devices altogether? I’ve read about the firmware problems that make 24V passive + POE switch setups a fire hazard, but do you think think this is a design choice or a fix for a flakey on-device power circuit?
Thanks for publishing this, I’ll give it a dead AP I have.
Mine broke in exactly the use case warned against here: https://community.ubnt.com/t5/UniFi-Wire.... This kind of failure definitely seems to be linked to the ‘accidental’ 48V activation issue. Unfortunately the device is OOW, but I’ll try to hit up Ubiquiti and see if I can get some good will.
Thank you! This fixed my Unifi AC Lite, I plugged it into a PoE switch that fried that diode.
effbees -
Hi, can some help identify the corresponding Diode from this Tutorial on the Unifi UAP-AC-HD? Its POE sections is a little bigger. But didn’t find shorted Diodes. POE LED on Switch is on, but AP not powering on. It boots when 5 V are supplied manually, so somethings is wrong on the poe section.
https://i.imgur.com/WvljJ4T.jpg
It gets warm around the silver elko / capacity next to the transformator. But that’s all what I was able to figure out.
Maya S. -
