H786 troubleshooting

[Wai-Sun Chia]

Jumped into my first serious PSU repair.. :-)
It's a dead H786 (from a BA11-N), tripped my house's CB initially..
Traced it to a shorted bridge rectifier...promptly replaced that (with a 
higher spec 35A/1000V, original was 30A/600V).

Put it a dummy load (a 12V/1A fan), fan turns.. YAY!
But to my dismay, 12V o/p measured to just 3.5V while th 5V o/p measured 
around 100mV (!).

Throughout all these, a high-frequency whining/buzzing is being heard..

Any clues to go forward?
Do I start checking from the output to input or do I move forward from 
the choppers?

Thanks.
/wai-sun