Dc to Dc power supply shutting down

[rantwhy]

I have a Panasonic PTZ controller that stooped working last week.

I have taken it apart and found that the fuse has blown and 2 transistors and 2 diodes have failed.

There are 4 voltage rails 8,7,5 and 3.3

The 3.3 is fine

The 8 is going through a linear regulator and looks fine.

The 7 is the one with the failed transistor and diodes.

The 5 is at 4.5 volts. 

I have connected the 12 volt input of the controller to a bench-top power supply current limited at around half a amp and have bypassed the fuse for now.

The problem is if you give the circuit in the picture below more then half a amp the circuit will shut down.

If I force 5 volts on the 5 volt test point of the circuit to test if the PTZ controller works it will take about 1 amp on start up and settle down to half a amp. But for some reason the DC to DC circuit will not support more then half a amp.

Im assuming there is a over current shutdown built in somewhere that is doing this. There is no dead short and nothing is getting hot. Im stumped as to why it is shutting down. It would make seance if it was drawing 5 amps or something.

The circuit will stay running only if you current limit at half a amp but it will only output 4.5 volts but the input will be puled down to 9. something because of the limiting.

I down think having removed the failed transistors for the 7 volt rail would cause this.   

There are 2 IC's that look to be running the show but I cant seem to find the right data sheet the first main one is 3375 E10735 M76 and the other is 2903 5g1 JRC

Thanks for any help