Carvin Legacy help needed

[JC@]

I've got one of these amps over here. It blows the mains fuse even with all the tubes out of their sockets.
With all the boards disconnected it doesn't. As soon as I connect the power supply / preamp board, it makes the fuse to jump, so the problem is in this board. There are no shorts in the diodes, electrolytic caps, or the transistors I believe.

My ammeter conected in series with the amp jumps to it's limit, 5 amps, before it clicks the amp's 3 amp circuit braker.

any help please?

[R.G.]

I've got one of these amps over here. It blows the mains fuse even with all the tubes out of their sockets.
With all the boards disconnected it doesn't. As soon as I connect the power supply / preamp board, it makes the fuse to jump, so the problem is in this board. There are no shorts in the diodes, electrolytic caps, or the transistors I believe.

My ammeter conected in series with the amp jumps to it's limit, 5 amps, before it clicks the amp's 3 amp circuit braker.

any help please?

Make yourself a light bulb limiter to limit AC power line current so you stop blowing fuses. It's almost impossible to debug a problem in the 50milliseconds between turning power on and fuse blowing 8-)

It's quite difficult to state with any certainty that there are no shorted diodes, electro caps, or transistors when what happens is that a fuse blows. Obviously something is indeed shorted. Could be a trace or a wire, but if the amp ever worked, the parts are more suspect.

If the problem is indeed on the single board as you say, then you must do a divide and conquer. Follow the power path, and look for two kinds of components: components in shunt to ground, and components in series with the power flow. Locate a series component somewhere about half way through the circuit. Remove it, or unsolder one lead. Do you still have overcurrent? (Note that with a limiter, you can tell whether or not you have the problem without wasting one fuse per test.) If not, the fault is past the component. If yes, it's before.

Now pick another series component, about halfway through the section which contains the fault, from the first test. Raise a lead and look for overcurrent. When you run out of series parts to divide up the circuit, start raising the lead of one shunt component to ground at a time. At some point, the overcurrent will vanish... IF IT'S A COMPONENT!

If it happens to be a solder problem, wire short, or maybe a solder ball caught between two pads and shorting, you'll run out of components. While that sounds bad, it is actually quite instructive. At that point you really can say it's not one of the components.

Notice that the typical power supply choke is both a series and a shunt to ground component; it can be shorted to the frame which is tied to chassis.