If I trace the 12V STB properly, I arrive on 2651MTC-ADJ regulator circled in red in the picture.
I tested all the capacitors around and i disconnected the diodes to isolate the input and output of the regulator.
after measurements between the input (Vin) and output (SW) I feel that the Internal MOSFET is dead but this remains to be confirmed.
Looking at the spec sheet for that chip, assuming the circuit here is similar.
a)
Power off.
Can you measure the resistance of Vin, AVin (the chip internal power supply), SW, PB and PGND with respect to the 12V STB ground pin?
Only the PGND should be a short.
b)
Power off.
Also from your comment I assume the two zeners nor the input and output caps (found in the "typical application" section in the spec sheet) are shorted.
c)
Power off.
Can you measure resistance between SW and FB as well as FB and PGND to find if the mosfets are shorted.
Power on.
Finally if these two mosfet are driven to turn on both at the same time and stay on it would also look like a short to load the line, but not with the power off. Any voltage on these pairs of pins?
d)
Could it be that the problem is on the output side of the 2651 and the chip is trying to drive a short elsewhere? Particularly if test a) and b) are OK should the output side look like a short? That could overdrive the 12V standby psu before it 2651 cutoff protection happens?
Hopefully that chip is still available if needed.
Unfortunately my unit is fully back together so I could not take a quick look at the circuit, unlike the last time. However, if you need it I am willing to dismantle the front module again.
OK it is past midnight here. I'll chime in tomorrow morning if I think of anything else.