Burman 501 combo blows fuse

[Willholmania]

The KT77 heater wires measure 0.2 ohms to ground. I assume that's bad...

[Enzo]

No, that is probably OK. The heaters are low voltage, so a small number of turns of heavy wire. They will have very low resistance. Unless you mean they have continuity to ground even with the wires disconnected.

[Willholmania]

Yes, that had occurred to me, Enzo. I must admit I'm not convinced I've fully sussed this problem yet. I'm still not convinced the transformer is bad. But I can't work out what else it could be.

[g1]

How bright is the bulb lit when all secondary wires are disconnected?

[Willholmania]

Okay. I tested it again. With all the secondaries disconnected the light still glows brightly.

The resistance of the primary is 5.6 ohms, which means it's drawing around 40 amps. So I guess that's conclusive.

[MarkusBass]

Okay. I tested it again. With all the secondaries disconnected the light still glows brightly.
The resistance of the primary is 5.6 ohms, which means it's drawing around 40 amps. So I guess that's conclusive.

So what's the conclusion? Remove the transformer from the amp and test is outside the amp (with the light bulb). Looking bad.

Mark

[Willholmania]

I think that's it, Mark. I need a new transformer.

[J M Fahey]

Okay. I tested it again. With all the secondaries disconnected the light still glows brightly.

The resistance of the primary is 5.6 ohms, which means it's drawing around 40 amps. So I guess that's conclusive.

Sorry but no.

We care about inductance, nor DC resistance here.

My doubt here is about what you call "bright" which is not a precise definition.

If the lamp shines as bright as when plugged straight into the wall, then the transformer primary is shorted, but not otherwise.

To put some numbers into it, plug the amplifier into the light bulb limiter (er .... I guess you used an old filament lamp there and not a modern CFL one), only transformer primaries connected to switch and fuse, all secondaries disconnected and taped.

Turn it obn .... measure AC voltage across the lamp and across the transformer primaries.
You might measure, say, 60VAC each or whatever.

Please post results here.

[Willholmania]

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.

[g1]

If there were some strange parallel path or carbon arc or something across the primary, you could still have the bulb lit with all the secondaries disconnected.
If you have the primary connected directly to limiter lamp (and nothing else), all secondaries disconnected, and the bulb is still as bright as it was with all secondaries connected, (and bright like a bulb plugged straight into the wall), then yes the transformer is bad.
The point being made is that a good transformer with all secondaries disconnected can still give some light with a limiter lamp.

[Willholmania]

Thanks for the clarification. The lamp's just as bright as ever, sadly. So I'm now certain it's goosed. I've been quoted 134 quid inkling delivery to have a clone made - even down toy the red varnish. Sounds pretty reasonable to me. What do you guys think?

[MarkusBass]

Have you checked how much it would cost to rewire existing transformer? This should be half of this price or even less.

Mark

[Willholmania]

Hi Mark. For some reason your post has got buried way back on page one of this thread somewhere.

Anyway - yes. I have checked out the price of getting the existing TX rewound. It's more expensive than getting one made from scratch.

[g1]

Hi Mark. For some treason your post has got buried way back on page one of this thread somewhere.

Somewhere in your forum settings there is a choice of view type. I believe the default is "hybrid", you want to change it to "linear", then you will see all the posts in order by date.

The only thing now is that the output transformer is not also bad, as that is what you were originally told. If so, that would add quite a bit to the expense. However, going by your resistance checks you originally made on the OT, it sounds ok.

[MarkusBass]

Hi Mark. For some reason your post has got buried way back on page one of this thread somewhere.

Yes, I've seen this. I selected "Reply to Thread" (general button) and in hybrid view my thread is displayed somewhere at the beginning. I assume I should select "Reply" to specific post. If you select "Display->Linear view" it should be displayed more accurate.

Can you make a photo of the transformer? Sometimes the primary winding is separated from the secondary and then you have to rewind only one winding.
What is actual value of the amp?
Here: http://www.burmanamps.fsnet.co.uk/reviews/pro501.html is some info about the amp. It seems also that the company still exists and they have original parts (including transformers). Originally, the PT was from Partridge - the same as in Hiwatt.

Mark