I found recently a GE ballast (with silver label) that it smokes when plugged, so useless. I'm wondering: is only a specific part filled with oil or the WHOLE ballast?

are you talking about a flourescent ballast.

yeah a fluorescent ballast

i tried disassembling a flourescent ballast,it was difficult because the components of the ballast were potted in tar.

Tar? What is this?

Older ballasts were encapsulated in bitumen ( the same stuff applied to roads, but without the stones) and some sand as filler. More modern ballasts are encapsulated in an epoxy resin. The reason is to reduce hum and noise, as well as providing moisture proofing and environmental protection. The encapsulation also aids with heat transfer from the internal components to the case, as well as preventing the thin internal wiring from breaking due to movement from magnetic fields.

As a bonus for the manufacturer, it makes the ballast unrepairable. To easily depot place the ballast overnight inside a plastic bag inside a freezer ( or outdoors in Canada in winter) so as to make the bitumen so hard that it shatters like glass when hit, enabling you to remove the casing without damaging it for retrofitting a more modern interior whilst retaining the old style looks.

Ok thanks. If I understood, when frozen, the bitumen are breakable and I can take the transformers for making preheat ballasts right?