Sat May 06, 2023 2:52 am
Ok, there is something wrong with PoE-Out on RB951Ui. The DC+ pair of ether5 is floating at ~6V (decreases under slight load) and not driven anywhere by router regardless of ether5 poe setting (off, force on).
First, I followed the overvoltage damage manual and checked:
- terminating diodes - all 75 ohm
- TR102 pins voltage drop - all around 0,32 ohm
- D2 - open loop; D4 - around 3V drop measured in circuit, open loop when soldered out; D1000 - open loop; all those have proper forward drop of ~0,31V; when device is powered, the port side of D4 does not any leak voltage though
Then I powered the device and took following measurements from both sides simultaneously:
- Q1001 - if that's a UT4421 p-mos, then Vgs = -0,385V regardless of PoE setting; bringing its drain to ground enables the port so I assume it's ok
- Q1000 - if that's a n-mos, seems ok, not driven
- Q1004 - if that's a n-mos, seems ok, not driven, can't find what's driving it - only culprit is Q1003, but I can do it forever this way blindly...
- Q1 - Vgs -9,88V, seems ok, powers source of Q1001.
Scanning around Q1004, I probed voltage on U1000's pin 1 and... the poe turned itself on despite I disabled the output in RoS. Further probing caused the voltage output to toggle between on and off, red diode coming back and forth accordingly. Something driven Q1004, but only by around 0,65V. When poe is working, voltage on U1000's pin 1 in reference to ground is around 0,16V, when off - 3,3V.
Long story short, even touching around U1000 makes PoE out (and its switching in ROS) working until next power cycle. IDK it's a glitch or a sign of, for example, damaged totem pole output.
Is my device really damaged? If so, is it beyond repair?
Last edited by
bderleta on Wed May 10, 2023 2:02 am, edited 1 time in total.