STB shutting off after a split second

[starLED]

I have a Set Top Box for TV (STB) that worked about two years fine, but now when I plug in adapter cable then power green light comes on for a split second then it shuts off, and no green on and red off light and LCD display is on.

I checked voltages randomly on board and there are 12 V, and adapter cable also measures 12 V.
STB model is NewGlee GL-c850AQ

What can be a problem?

 

[Nigel Goodwin]

Have you checked for ripple on the incoming 12V?, a common failure is high ESR electrolytics in the power supply.

 

[starLED]

I didn't opened the adapter.
There are 12 V constantly on the board.

 

[starLED]

STB box adapter is 12V 1A.
I tried two different adapters:
1) 12V 1.5A - same issue, green light power off after a split second
2) 12 0.85A - green light doesn't turn off, it flickers.

 

[Nigel Goodwin]

Have you got a scope to check for ripple on the 12V?. And are you using the original adaptor?.

 

[starLED]

No, I don't have a scope.
It's the original adapter.

 

[starLED]

I was checking caps for short and I was wondering if this is normal.
Two caps near the main chip (CPU ?) have 072 ohms, and I was wondering can that be shorted to ground?
Other caps that I have tested are all higher then 500 ohms.

 

[starLED]

I noticed there is a black discoloration on the bottom side of the PCB under the CPU (in red circle), it can't be seen good on the image.
Can that be the sign of a damaged CPU that overheated?
Also, caps in the circle also measure 072 ohms.

 

[Nigel Goodwin]

It's unlikely you can test the capacitors in-circuit, it's most probably reading through other parts of the circuit.

You could always remove one that you suspect?, and check it out of circuit - and also check across the PCB where it was.

Generally on STB's, it's either the power supply electrolytics, or it's scrap - there's generally no schematic, and no spares available. And quite often, if PSU electrolytics have failed, then the HT rail increases and causes further irreparable damage.