i am working on a PEAVY guitar head amp, the fuse for the power supply had blown so i disconected the secondaries to the actuall board and replaced the fuse. (to make sure the problem wasn't on the board) and it still blew. what i am wondering is if the secondaies on the transformer(step up) shorted would this cause teh fuse to blow?

 

Indeed, it could be the cause, as greater current flowing inside the secondaries can induce more current to flow inside the primary of the transformer.

 

It is quite possible that the power transformer is defective, but it is just as likely that there are defective components between the fuse and that transformer. Input filters, smoothing inductors, MOV's for overvoltage supresssion, etc. could also cause the blown fuses. Check everything from the transformer back to the fuse before you settle on a bad power transformer.

 

The chance of a bad rectifier and smoothing capacitor being faulty is many times that of a faulty transformer.

Check those first.

 

But with the secondary line disconnected, the fault has to be in the transformer or line filtering (if any).

Try an ohmmeter on the transformer. Any reading to the core from a primary lead means total failure. If it's not shorted, you can place a 15 watt bulb in series with one primary lead and see what the secondary outputs read. With no load, they should be very close to the rated voltage.

 

But with the secondary line disconnected, the fault has to be in the transformer or line filtering (if any).

You're right, beenthere. I have the wrong impression that he has removed the DC input to the board.

 

I have also just discovered that the input Filter cap is shorted. idk if this would cause the problem or if it is a result of the transformer.

 

Most likely cause of the problem. Do check diodes/bridge for failures.