Engl Raider 100 PT - learning the hard way

[Mick Bailey]

Never come across one of these before.

An output tube shorted and now the PT reads open circuit on the primary. Why didn't the 500mA fuse on the PCB blow? because it's the fan fuse and not the HT.

I contact the distributor, who's very helpful and puts me in touch with Engl, who are also very helpful. I was intending to get a replacment PT. "Just one thing", says Engl, "our technicians say the fuse inside the transformer should protect it from a fault condition". Except there's no evidence of a fuse, the primary leads dissappear under the pretty standard looking brown plastic terminal shroud which is firmly stuck with silicone. After cutting this away and prying off the cover there's a 1.6A fuse lurking beneath. How many people knew that?

Now all I can hear inside my head is my mother shouting "bad boy, dirty boy, now go to your bed".

For all those who may travel this way.

[The Dude]

That's what I was referring to in THIS thread. I did not elaborate because this is a public forum and it's not something a novice should be messing with. Repairing this incorrectly can be hazardous- electrocution, fire, etc.

[Mick Bailey]

I'm not a novice - I've been repairing and building tube amps since 1978. The label on the transformer gives no indication of an internal fuse and it's a stock 20mm glass fuse - T1.6A. There's the clue why it blew; a 100w amp with a 1.6A fuse in the primary and a shorted output tube. No other problem. I wanted to alert others to the presence of a fuse here as it's hidden and could save people time in diagnosing what looks like a failed PT. The MP active tube management didn't save the day by shutting off the failed tube in time, so it looked like a more serious fault than it actually was because of the close fusing. Even Enzo gets the run around with mystery fuses (Deville III heater fuse lurking under the PCB).

I've seen plenty of PTs with internal thermal fuses - usually wrapped up and sometimes a one-way ticket (Crown amps spring to mind). This is a different scenario; the main fuse fitted to the IEC socket is rated at T2A. The internal PT primary fuse is T1.6A. So, regardless of the fault (excepting a short across the mains switch) the user-accessible fuse would never blow.

If we suppress all of the potential here for "electrocution, fire, etc." the whole site would be pointless and non-informative. I'm for freedom of information, sharing and cooperation. Not one single person who posts here has all the answers, regardless of experience.

[Enzo]

Relax, he wasn;'t pointing at you, he was telling you why he didn;t publicise it in general.

[The Dude]

Mick,
I think you misunderstood my post. I was definitely not pointing at you as being a novice as Enzo said. I was referring to other "dabblers" who might check the site. My apologies if you took it any other way.

It's one thing for a trained tech with some experience to mess with transformers. In my view, it's an entirely different thing for someone unqualified to start ripping into a power transformer. I agree with you about freedom of information to a point. That said, there's also a degree of responsibility for the safety of others that I feel when posting here. If you or anyone else wants to detail this, that's your prerogative.

So we're clear, I'll say it again. I WAS NOT CALLING YOU A NOVICE AND I APOLOGIZE YOU TOOK IT THAT WAY.

[The Dude]

As an addendum: There was a tech from my city a few years ago who caused an apartment fire due to an "ill performed" transformer repair. Again, I'm not going into details on what exactly he did, but he was sued for damages. (No, it was not me) Hence my reluctance to discuss this in an open forum. Luckily no one was killed.