Whenever buying or restoring an old piece of equipment, it is best to start with a good visual inspection. Burnt diodes or resistors, leaking capacitors, or melted-down tubes can mean that a component has failed, that the device was not operated as intended, or that it has been modified in an unsafe manner. But there are also pieces of equipment that are stock, in good condition, but which still have nasty surprises inside.
One such nasty surprise are the radio frequency interference (RFI) suppression capacitors made by RIFA circa the 1980’s. These capacitors may be working fine when powered on, but they are ticking time bombs. The problem is the plastic enclosure: it develops cracks over time that allow moisture into the capacitor. The capacitor fails short, exactly what an X rated capacitor is not supposed to do, and causes some fantastic smoke to pour out of the equipment. The video below shows a successful attempt to document such a failure.
I found one of these RIFA caps inside of a Philips oscilloscope that I have had, and used, for years. I had no idea it was there until I saw it mentioned in a forum post about this particular oscilloscope. Rather than being on a circuit board, it is soldered directly to the back of the mains voltage selection knob at the back of the unit and hidden under a cover. If I hadn’t been looking for information about replacing the capacitors in the power supply section, I might not have found it until it failed.
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