Burman 501 combo blows fuse

[Enzo]

To amplify the point Juan made...

It seems your transformer is bad. But the 5 ohms resistance reading is not the evidence of it.

[J M Fahey]

I've pulled it out now, Juan. Surely the fact that it lights up the bulb just as brightly with all the secondaries disconnected means it must be internally shorting? I can refit it if you think there's a chance it's not shot but I can't really see how it can't be.

What's nagging me is this:

from post #24

So I lifted the supply to the rectifier diodes. Still lights up the bulb although not quite as brightly -

2 different levels of brightness can't both be "maximum" which is the true indication of a fatal short.
Or to put it other way: a shorted transformer does not react to varying connections on its secondary(es).

it lights up the bulb just as brightly with all the secondaries disconnected

as brightly as ?????????
not being a Grammar Nazi or something, just clinging to the last hope

Didn't ask you to pull the transformer, just to keep it simple and quick, but of course you can still test anyway.

So to put numbers instead of opinions, , plug it one more time into the bulb limiter, read wall voltage and voltage across the transformer primary.

Which is live, so I'm certain you'll tape wires to the table or something to avoid slips when measuring or whatever.

In a nutshell, what I expect is something like this:

If with a 100W filament lamp in series you measure, say, 60VAC or less across primary ... now you have a nice doorstop for windy days

And if you measure, say, 150VAC upwards, then what's (partially) lighting your bulb is magnetization current, nothing to write home about.

OK, which will it be? (fingers crossed)

[Willholmania]

Okay. I'll have to do this in a couple of weeks (as I'm going away today). I think I said the 'bulb glows slightly less brightly - I think.' It was wishful thinking (or viewing) rather than any real difference. It glowed brightly with all the secondaries lifted yesterday. But I will do the voltage check before I buy a new transformer.

Thanks for all the help. I've learnt a lot doing this.

[pdf64]

I just thought to check the PT primary resistance of a 100W amp I've got in at the moment; it's just over 3 ohms.

[MarkusBass]

The simple (and final) test is to connect the transformer to the mains with a fuse (2-3A slow) on the primary windings (secondary windings disconnected). If the fuse fails, the transformer is shorted. I had such case - I got smoke out of the transformer. Luckily, I was able to rewind existing transformer.

Mark