Switch Mode Power Supply Repair....... Help!

[Robbie010]

Afternoon all, novice here, please be gentle........ I have been trying to bring an old amplifier back to life, it initially started up but there was obviously something not right, so after doing some initial inspections, minor tidying up inside and replacing a missing resistor, I powered her up and said missing resistor turned up...... shorting the output of the switch mode power supply 

I have pulled 80% of the components off the power supply board for testing and have yet to find anything major, I have also done a full recap of all the electrolytics. Now, I have tried powering it back up with a variac but the output from the power supply cuts out at around 30v with a little squeak from one the components. From my initial testing, input to the SMPS is fine, 240v input (AC), gives 320v (DC) in to the SMPS as per the schematic.

The only thing thus far that I have not attempted to remove and test is the transformer T603. Could anyone please take a look at the schematic and tell me whether it is likely that the transformer could have blown a winding?

Any other pointers more than welcome....

I have attached the schematic.

[TheMG]

Couple of things:

1) SMPS generally don't like being ramped up slowly on a variac. In fact, some SMPS won't start up at all if you do that especially if they are a self-oscillating design, and many have under-voltage lockout and won't operate below a certain voltage either.

2) SMPS transformers are rarely a point of failure. If you have continuity through the windings then it is very likely just fine. Nearly impossible to damage from a momentary overload or short. Primary switching transistors and mains fuse will pop long before any damage is done to transformer.

[inse]

Oh heck - miraculous discrete Sony circuit.
I wonder at which frequency this SMPS is running (or should be).
Is the transformer chassis mounted or on the PCB?
Nevertheless, transformer damage is rather unlikely.
Did you check the secondary diodes?
Is the power supply disconnected from the load?