Short in amp: house circuit breaker pops, amp fuse does not.

[slidincharlie (Carlo P)]

I hope the post subject is clear enough...
I did overall maintenance, total cap job and a few tone stack mods in my 1482 two years ago. The amp has worked normally ever since.
Now there seems to be a short somewhere: as soon as I insert the mains plug into the wall socket (or a few seconds later) the house circuit breaker pops BUT the amp fuse does not blow.
What should I check? Is there a step-by-step troubleshooting to chase a possible short?
I have subbed in a new rectifier to no avail.
Amp schematic is in attachment, as well as a picture of the chassis. The power supply section is a little messy because I had to use single filter caps instead of an expensive cap can. But again, the amp has been working fine until now.
The power transformer is a replacement with 230V primary.
TIA,
Carlo

[Steve Conner]

Is your house circuit breaker the earth leakage type? GFI, RCD, ELCB, whatever it's called in your country, you can recognize it by the presence of a test button that trips the breaker when pressed.

If it's one of these, then it's probably tripping because current is leaking to earth. It only takes a few mA of leakage to trip them, which won't pop any fuses.

This implies that you've used a faulty component somewhere on the primary side of your PT (maybe even the PT itself has bad insulation?) or made a wiring mistake, most likely getting neutral and earth mixed up, or the neutral wire is touching the chassis somewhere.

[52 Bill]

Could you have a problem with the ac cord itself? Maybe a frayed wire at the plug end?

Just a thought.

[Don Moose]

neutral wire is touching the chassis somewhere.

I'm thinking Hot and Neut reversed or Hot touching Chassis - it appears the amp doesn't even have to be ON.

[slidincharlie (Carlo P)]

I'm thinking Hot and Neut reversed or Hot touching Chassis - it appears the amp doesn't even have to be ON.

Right, the amp doesn't even have to be ON to make the house breaker pops.
And yes, the house circuit breaker has a test button. Is there a way to detect current leakage to the chassis?
Re: faulty parts or bad insulation on PT: the amp has worked alright in the last two years. Maybe a component (filter cap? power lamp?) has gone south? I'll check the power supply area, connections and cord.

[izzybell2810]

I'm with Steve and bill on this one. I'd put money on it being the power cord causing a fault current to earth. Anything over 30ma will or should trip an RDC as 30ma at 230v is enough t kill you in the right conditions. At least you know your RCD is working!

[slidincharlie (Carlo P)]

I'm with Steve and bill on this one. I'd put money on it being the power cord causing a fault current to earth.

The plug is a sealed "schuko" (a.k.a. as "German") plug. I can't open it to check. Anyway I'll install a new cord and see what happens.
Thanks,

[Enzo]

Your schematic shows a cap from mains to chassis, is that still there? Get rid of it.

[slidincharlie (Carlo P)]

Your schematic shows a cap from mains to chassis, is that still there? Get rid of it.

No, I tossed it long ago. Now that end of the fuse goes directly to ground and to the PT. Is that the correct configuration?

[loudthud]

I noticed that there is no strain relief where the line cord goes thrrough a hole in the chassis. Is there any damage to the cord there? If you ohm from either side of the line plug to the chassis do you get anything less than over-range?

[slidincharlie (Carlo P)]

loudthud,
this is just what I was thinking about...
I'm going to bed now... Tomorrow I'll post the results of my job.

[Albert Kreuzer]

No, I tossed it long ago. Now that end of the fuse goes directly to ground and to the PT. Is that the correct configuration?

No. Just leave the cap out. The PT primary should not have a direct connection to ground.

Cheers,
Albert

[slidincharlie (Carlo P)]

I have just installed a new power cord (with the ground wire lifted for now). The result: neither the house breaker pops NOR the amp turns on! The mains AC is on the power switch but the amp stays totally off. No tension anywhere. It's like the PT does not receive the mains AC or it does not deliver it to the rectifier...
Maybe the PT is dead?

No. Just leave the cap out. The PT primary should not have a direct connection to ground.

Albert, I have simply tossed away the cap from mains to the chassis and left the wire that goes from there to the PT (see red arrow in the photo below). But there is continuity between that pole of the fuse holder and the chassis (i.e. between the PT primary and the chassis).

[Albert Kreuzer]

I have just installed a new power cord (with the ground wire lifted for now). The result: neither the house breaker pops NOR the amp turns on! The mains AC is on the power switch but the amp stays totally off. No tension anywhere. It's like the PT does not receive the mains AC or it does not deliver it to the rectifier...
Maybe the PT is dead?

Or maybe this time the fuse blew before the house breaker...

Albert, I have simply tossed away the cap from mains to the chassis and left the wire that goes from there to the PT (see red arrow in the photo below). But there is continuity between that pole of the fuse holder and the chassis (i.e. between the PT primary and the chassis).

Yes, I can see it on the picture - you did it right. If there is still continuity between primary and secondary, you may have a shorted PT. Disconnect all PT wires and measure between primaries and secondaries.
Just had one like this last week. Exactly the same symptoms.

Cheers,
Albert

[slidincharlie (Carlo P)]

If there is still continuity between primary and secondary, you may have a shorted PT. Disconnect all PT wires and measure between primaries and secondaries.

With all PT wires disconnected (including the central taps) there IS continuity between one primary wire (the one coming from the fuse holder) and each of the two high tension secondaries a well as their central tap.
There is NO continuity when the other primary wire (the one coming from the power switch) is checked. Also there is NO continuity between the two primary wires!
Is there a short? How did it happen? Could a faulty power cord cause such short in the PT?