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Old 3rd March 2010, 10:27 AM   #1
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Default Broken toroid?

Hi. I have a Art tube preamp that has started to blow the fuse everytime I try to turn it on and because the warranty has expired I was thinking of fixing it myself. At first I just tried to see if anything looked strange or broken but I really don't have a trained eye for that My second try was disconnecting the tube to see if the problem would disappear, it didn't. At last I tried disconnecting the wires from the toroid to the PCB (19,2 V - 0 - 19,2V) and the fuse still blew. Does that mean that the toroid might be broken cause there's nothing left in the circuit except the powerswitch when the pcb is disconnected? Is it possible to replace the toroid with a similar one, I just can't seem to find any with 19,2 V - just 18V. Hope you can solve my problem
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Old 4th March 2010, 05:41 PM   #2
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Seems like your toroid has a shorted primary. 18 volts is not that much less. Try it.
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Old 4th March 2010, 06:24 PM   #3
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Could be a shorted capacitor if there is one across the input.

Or a short in the switch if it is dual pole.

Otherwise a duff transformer.

Might even be a short in the mains plug.
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Old 4th March 2010, 06:41 PM   #4
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Hi there, i'd follow the advice by nigelwright for starters to see what's going on.

On a different note 19.2/0/19.2 seems to be en extraordinariliy low voltage for any kind of tube amp, though i could be wrong I'd make sure it's not 192V per secondary before buying an 18V centre tapped transformer.

By the way, if you are desperate for the correct voltage & the transformer you purchase isn't centre potted or completely potted up it's relatively simple to add a turn or whatever to each secondary to bring it up to spec.

Best of luck
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Old 4th March 2010, 08:33 PM   #5
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Fuses blowing at switch-on and LV secondaries - check/replace any bridge rectifiers. When they fail, they fail 'short' - and are cheap.

In comparison IME it's unlikely a reasonably-rated transformer has died.
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Old 4th March 2010, 09:13 PM   #6
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oh ffs - do some SYSTEMATIC testing. You can't fix anything by guessing what is wrong.

Follow the power supply chain from the output end backwards piece by piece. Isolate and test each component until you find the failed component. Use a current limit device rather than a series of fuses. Its safer and cheaper. A 100W lightbulb in series will do it.

If you don't know how to do this, you shouldn't be messing around inside a mains powered device. Get help.
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