Power transformer – voltage on shell

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Power transformer – voltage on shell

Postby Leif B on Thu Jul 21, 2016 9:18 am

Sick Ampeg V4 – keeps tripping power randomly. Sometimes the amp can be working fine, then it just trips power. Other times, it trips as soon as you power it on. Sometimes it trips as soon as you flick it off standby. Replaced D5 on the earth line.

I'm certain it's not valve related, and I've made it trip without any power caps in line. I think it's voltage going to ground, tripping either an RCD, or just shorting.

So I've taken the power TX out of the amp and applied voltage on its lonesome.

Using ground as ref, I'm getting up to 120-odd volts on the copper band and the leaves. Is that normal? Could a bad earth-bond from TX to chassis be my problem?

Another annoying thing is, my workshop shares power with two offices. If I trip my power, theirs goes down too. So I can't keep tripping, otherwise I'd try anything.

Any tips appreciated

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Re: Power transformer – voltage on shell

Postby Baron Von Machinenmann on Mon Jul 25, 2016 10:24 pm

If you are reading voltage on the Faraday shield, that transformer is probably shorted and a potential electrocution hazard. :shock:
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Re: Power transformer – voltage on shell

Postby Unit_1 on Tue Jul 26, 2016 2:10 pm

AYE CARUMBA!

that's pretty scary

:Zap:

be there done that, but at 385v. Nasty experience, could have died, but I was 19 and lived. 60 now. :thehorror:
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Re: Power transformer – voltage on shell

Postby pdf64 on Thu Jul 28, 2016 4:50 pm

Leif B wrote:my workshop shares power with two offices. If I trip my power, theirs goes down too. So I can't keep tripping, otherwise I'd try anything.
Any tips appreciated

A light bulb limiter would almost certainly be a useful addition to your shop. :thumbsup:

It's possible (though perhaps not likely) that the voltage you are measuring is just due to stray em coupling and would collapse if any current was drawn from that point.

Consider wiring a power resistor eg ~10k ~5 watts, in parallel with your meter; if the 124V collapses to a few mV then it's just stray, whereas if it stays up there, then the PT probably has a fatal short.
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