Burman 501 blows fuses

ICBM

Yes. To be absolutely certain, disconnect the bias feed and then the filament loom, testing each time. My guess is that even with nothing connected on the secondary side at all, it still lights the bulb.

Could someone have been messing about with the voltage selector, possibly?

Fatfingers

They could, yes. Who knows? Although I doubt my nephew would. He has no technical ability at all and I can't imagine him even looking at the back panel, let alone doing anything with it. Maybe it just gave up through old age. 

I'll lift the bias feed and the filament feeds and report back.

Fatfingers

Yup. The only way to put out the bulb is to disconnect the PT's primary - resistance across it is five ohms.

Do you know where I can get out rewound?

Fatfingers

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

ICBM

No, not unless that's with all the valves (preamp as well) out and the PT disconnected. The valve filaments have very low resistance when cold. So does the PT winding.

Fatfingers

Yes. I've been pondering that. The heater winding is very low voltage so will obviously have very few turns, hence the low resistance.

Following your advice I fitted a set of preamp (and PI) valves and all my tests today have been with them in. I'm not totally convinced this power transformer is goosed. I think I'm missing something, somewhere. I just don't know where!

ICBM

If you disconnect *all* the secondary windings and it still lights the bulb or blows the fuse, it's goosed. (Making sure that none of the disconnected wires can touch each other or anything else, of course.)

With no load on the transformer it should draw a tiny current, not enough to light the bulb.

I go through this exhaustive/wishful thinking process every time I get an amp with a blown transformer. It's really a good idea to eliminate every other possibility completely before finally coming to that conclusion, but if it's conclusive then you have to accept it.

Burman were making reissues a few years ago, it may be worth trying to contact them.

Fatfingers

I need to do that check again. I might have missed some secondaries.

One of the things that's niggling me is that when I disconnected the rectifier the bulb lit more dimly - I think.

But yeah - it's probably shagged!

Fatfingers

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.

jpfamps

Fatfingers said:

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.

You can't tell anything from the DC resistance of the primary. The DC resistance of the primary of any transformer is a few ohms. It needs to be otherwise the transformer will overheat due to I squared R heating.

The primary inductance of the transformer is high though, and this stops excessive current being drawn by the transformer.

Remember the mains is AC.

What is conclusive is the fact the light glows brightly when with the transformer unloaded.

Fatfingers

Thanks. Any idea where I can get a suitable replacement? I know I could fit something from a Marshall etc but it would be nice to fit something the same spec and appearance as the original Partridge. I've been quoted 134 quid to have one made (inc VAT and delivery). Does that seem reasonable?

ICBM

Fatfingers said:

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.

Yes.

The short is in the secondary HT winding, in fact - that's why you got a false reading from the rectifier diodes.

Fatfingers

If the short's in the secondary HT winding, why does the light still illuminate with all the secondaries disconnected. Do you mean the secondary is shorting to the primary?

ICBM

Fatfingers said:

If the short's in the secondary HT winding, why does the light still illuminate with all the secondaries disconnected. Do you mean the secondary is shorting to the primary?

No, the secondary is shorted inside itself. That produces an effective load (a very heavy one) on the primary.

Fatfingers

Ah. Right. Thanks. Been a good learning experience, this.