Burman 501 combo blows fuse

[Willholmania]

Hi. I leant my Burman 501 combo to an impoverished but talented nephew and, a few years later, have been rewarded with a dead amp. He says it was working one minute, not the next. He took it to someone to get it fixed (whose abilities I know nothing of) who claims the output transformer is goosed (he has also nicked all the valves from it!).

It shows no signs of burning. The transformer certainly doesn't look like it's got hot and still looks fine in its red varnish. Measuring resistance across the primaries I get around 83ohms across the coil and 43 and 38 measuring from one end to the centre tap, which seems okay. Measuring from the primaries to ground shows open circuit.

However, measuring from the secondaries to ground I get 0.1, 0.2, 0.3 and 0.4ohms, depending on which tapping I connect too. All of the above are with the transformer still in circuit, no valves fitted and no speaker connected (although connecting one makes no odds).

When I power it up like this (with no valves) the onboard fuse blows after a few seconds (there's only one slow blow, 3amp fuse on it and no standby switch).

Any ideas on how to start tracking down the fault? The next time I attempt to power it up I'll use a light bulb current limiter.

[Enzo]

The primary of the OT will connect to B+ Disconnect that B+ there. NOW do we still blow fuses?

[Willholmania]

Hi Enzo. Thanks. I'll try this later – after I've made a light bulb current limiter. But resistance readings between the primaries of the OT and ground show as infinity.

[Enzo]

Maybe so, but that doesn;t tell us what happens when several hundred volts are applied. Your meter applies at most 2v.

[Willholmania]

Fair enough. All the more reason to get the light bulb current limiter made!

[MarkusBass]

The schematic if you need it.

Mark

burman_pro-50x_amp_sch.pdf

[Willholmania]

Thanks Mark.

[Willholmania]

I just went through the four rectifier diodes with a meter. Only two of them show infinity one way and around half an ohm the other. The other two are short circuited.

[52 Bill]

The other two are short circuited.

Well, that's what blowing the fuse. Now to get your tubes back.

Oops, your valves back.

[Willholmania]

That's unlikely, Bill. I don't even know who it was my nephew took it to and although I can probably find out, he lives 300 miles away and it was a couple of years ago, so I reckon it's a dead trail. He's clearly an unscrupulous type though.

[Jazz P Bass]

Will, if your post name is your actual email address, you really should change it.

Unless you like Spam, that is.

[g1]

Being corporate, the spam may not even get to his mailbox. They usually have pretty good firewalls/spam filters.

[Willholmania]

Yeah - I tried to earlier but couldn't see how to.

[52 Bill]

Unless you like Spam, that is.

I thought everybody in the UK loved Spam! SPAM, SPAM, SPAM, SPAM. SPAM, SPAM, SPAM, SPAM.

[Willholmania]

Actually I can't bear the stuff!

And I really would like to change my username but couldn't see how to. Will have another look.