[757labs] 37" TV, power supply troubleshooting

[telmnstr at 757.org]

37" TV.

Has a switch mode power supply.

Outputs are +24, +9, +5, +3.3 all DC of course.

Symptoms: Plugged into wall standby blue LED comes on.

Hit power, the back light flashes for a split second, LED also turns red. 
Then backlight cuts off. LED goes blue. Then later it goes through the 
process over and over.

Troubleshooting:

First thought was, it's the backlight inverter board. Dug into it, looked 
fine. It kind of dawns on you that it's a complex board a little later. I 
did a proceedure to check to see if any of the back light transformers 
were open, but all were fine. Looks good.

Had the idea to isolate the board, unplugged it from power supply, same 
symptoms.

Did the same thing with the CPU board, nothing really eventful.

Busted out the oscope.

Turns out +3.3vdc line is stable all the time. Always. It's what is 
powering the standby.

All the other voltages, when the TV cuts on all voltages drop. The power 
supply is being pulled down.

So I unhooked everything, and crossed the +3.3vdc line to the "power on" 
signal wire on the supply. It retries over and over much quicker, but this 
could be normal since there isn't a load on it.

Gonna poke at it more tomorrow. Kind of an odd one, because it's difficult 
to nail down which actual board is causing the issue.