Cisco AP won't come on randomly

[sebr]

I have a Cisco AP1252 that randomly won't come on and all three led's are solid red. I noticed that when I am heating up (hairdryer) the area where the 4 caps are located the AP has a much better success rate to start up (and working normally afterwards).
Even if I let it cool down for like half an hour it will still start sometimes.
The caps tested well (in circuit) with ESR meter - they are in parallel tho. As well no bulging or leaking.
So I am really not sure how to find the fault here. Bad solder joint? Caps?

[stj]

Re: Cisco AP won't come on randomly

is that liquid between the pair on the right?

put your iron on the connections and see if it smells.

[sebr]

Re: Cisco AP won't come on randomly

Hehe sorry for confusing you. I already put a bit of flux there since i planned to remove them but then , after thinking over, I might experience some cold solder joints.
BTW: let it totally cool down now and dead again (only three solid leds). What would you suggest?

[stj]

Re: Cisco AP won't come on randomly

change them, lifespan on smd is rubish anyway.
use fullsize caps at a 90' angle.

[sebr]

Re: Cisco AP won't come on randomly

Just finished replacing those 4 but nothing changed :-(
There are 2 other caps (100V / 10uF) that are obviously just for smoothing the PoE. They tested OK but I replaced them as well. Still no luck

[stj]

Re: Cisco AP won't come on randomly

all i can think of is reflowing stuff then.

[sebr]

Re: Cisco AP won't come on randomly

Well, unfortunately I don't habe a reflow oven, only a hotair station and heatgun. Maybe I give it a try with the heatgun. What temp / time is best practice for preheating / reflowing a board? Normal oven in kitchen won't get hot enough i guess.

[Agent24]

Re: Cisco AP won't come on randomly

If you have freezer spray (or an up-side down can of air duster) you may be able to heat\freeze and narrow down the temperature sensitive part.

[stj]

Re: Cisco AP won't come on randomly

i would use the hot-air station to just reflow parts individually rather than baking the board.

maybe you can use the station to just heat specific areas to pinpoint the problem.

[sebr]

Re: Cisco AP won't come on randomly

I removed some shieldings and there are lots of BGA IC's. So next I will try to heat these up as I could not find which one cause the problems. But i highly suspect them all because the get very hot during normal operation and have thermal pads which they touch the housing.
Before I ruin it completely: which nozzle, airflow and temps are proper for this kind of job? NoClean flux is available.

[stj]

Re: Cisco AP won't come on randomly

small 3-4mm nozzle, low airflow, temp - enough to melt solder

[japlytic]

Re: Cisco AP won't come on randomly

The electrolytic capacitors in the picutre are Nichicon UUD Series (low-ESR), with their "oval" insignia of Nichicon: