• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

magnetized power transformer!

resident

Disabled Account
2004-09-10 4:57 pm
Earth
I just bought an old Philips Philicorda organ and for some reason it burnt the thermal fuse inside the power tranny. The transformer is ok and as I was searching what's going on and what caused the problem I realized that the power transformer is magnetized.
How can I demagnetize it? Is there any problem if it's magnetized?
Does it play any role if the core is a little bit rusted?

When I short the fuse the organ plays without any problem and the tranny doesn't become overheat. But what causes the thermal fuse to burnt? All voltages seems ok....
By the way, all tubes and caps are stocked... I'm planning to replace them....

It's a pretty cool circuit, all tube + rectifier (EZ81) and tube diodes (ZZ1001) at the oscillators and tube regulated PSU! All tubes are PHILIPS...
 
Fuses do not last forever. In the normal course of operation which includes many turn on and off cycles, a fuse can let go for several reasons. A power spike or momentaty serge, or the inrush every time the organ is power on. Especially if the fuse is close to the normal operating current. For tube circuits a fuse should be a slow-blow type. If things work well as you say, then it was probably just a tired fuse. Is it a true thermal fuse that only works from heat and not excessive current? I ask because current fuses are basically thermal for the most part.

Some surface rust on the outside of the laminations is not a problem. Only when the inside surfaces of the core become rusted can you have problems. If the core is not swollen and is tight, you're ok.

As far as a magnetized transformer goes, this is weird. Since a transformer works on AC, it should not become magnetized. An AC field is what would be used to de-magnetize it. If you had a severe imbalance in the power supply rectifiers that put DC through the winding, I've see that cause strange things to happen with the transformer during operation, but never to permanently magnetize it. In normal operation it de-magnetizes itself.

Victor
 

resident

Disabled Account
2004-09-10 4:57 pm
Earth
Hi Victor,
Really thanks for the help...
HollowState said:
Is it a true thermal fuse that only works from heat and not excessive current? I ask because current fuses are basically thermal for the most part.
The fuse is inside the primary windings. I think that these kind of fuses are thermal, aren't they? I can post a pic if you like but you can see only the leads and a little from its bottom.

The core is really tight and well build. Hope it'll demagnetize by itself.