Mesa Boogie solo humming badly

[JC@]

I am working on an amp that hums quite badly, but I am almost sure about it is the PT itself what causes the hum, because disconnecting it completely from the circuit board it makes exactly the same noise.
I mean to say if I plug the transformer to the mains, with nothing attached to it's secondary windings, you can hear a 'pretty loud' snuffy humming noise coming out from the transformer.

Could there be a way to reduce that hum a little bit, before I have to tell the bad news to the owner?

JC

[Ptron]

I'd make sure the bolts holding the laminations together are tight.

[JC@]

I'd make sure the bolts holding the laminations together are tight.

Yes, I will try that too, thank you for reminding about this.

[Enzo]

Connect an ammeter in series with the primary and see if it draws any current. Do this with the secondaries all disconnected. We want to see if the tranny draws current just sitting there. Other than a little spike right when you power up, the plain transformer ought to draw just about zero. If it draws half an amp or something, there is trouble inside the thing. And when a transformer works harder, it will be more noisy.

[JC@]

It's drawing 1 Amp

[Enzo]

If your power transformer draws 1 amp through its primary with NOTHING connetecd to ANY of the secondaries, then the transformer is shorted internally and that explains the racket.

You can't cheat, disconnected all the wires from the secondaries. Only two wires connected to the PT - the mains.

[JC@]

There are three cables on the primary.
The secondary cables are all disconnected (easy, as they go in one same plug to the board)
The schematic for the solo is the same one as for the rectoverb

[Enzo]

Ther may be three, but I bet only two of them are connected across the 120VAC mains. In any case, if all the secondaries are floating and it still draws current, it has a problem.