cisco poe switch PSU issue

[madan1]

Hello everyone.
I'm troubleshooting a cisco switch, which seems to have an issue with the PSU.
The psu has the following power rails - +12v, -53.5V and +3.3V
When connected to power, the +12 and -53.5 seem to work fine, but the +3.3 starts at ~3v and then goes down to ~0 (I guess this is the cap discharging). This happens both when the cable is connected and not to the switch board.
Any ideas what might be causing this behavior?

[madan1]

So I started taking apart the psu, the caps measured ~290uF for the 330uF, ~410uF for the 470 and 45 for the 47, which I think is OKish.. at least for now.
Traced the 3.3V rail to the 33063 regulator. It gets 12v on Vcc (between pin4 and pin6), so I guess it has failed.

[madan1]

Well, I changed the IC and still have the same issue - on power on it outputs ~3v and immediately shuts down. On power off I see another surge of ~3v, so I guess something else in that circuit is wrong.

[stj]

33063 is a buck convertor - it needs an inductor and a fast diode followed by a low esr cap.
check the example and then check your board.
https://www.ti.com/lit/ds/symlink/mc33063a.pdf

[CapLeaker]

Sounds to me like a feedback or protection like OVP problem.

[harp]

What is resistance on 3.3v rail? If he managed to hold 3.0v it is relative high... after you disconect psu from mainboard, check all diodes and major transistors, you may want to inject some current into 3v3 rail to see where is problem.

Also, advice to clean pcb, check and resolder all THT pads, especialy over main transformer and power semiconductors, looks crack or cold joined.