Scope smell

This afternoon, I tried to instil some A Level physics knowledge into Sophia by giving her a practical electronics lesson. Unfortunately, part way through, my trusty oscilloscope made a nasty crackling noise and emitted an unpleasant smell. It did however keep working, so as we were near the end of the lesson we pressed on but switched the scope off whenever it wasn’t needed.

My scope is a Phillips PM3217 which I bought more than 30 years ago as unused Navy surplus and it has never given me any trouble before.

A quick search of the web showed several other people with similar problems with this model and the very similar 25 MHz version. The most common cause was listed as a burnt out mains filter capacitor. There was also a handy video showing how to remove the back.

I opened up the relevant part of the power supply inside the back cover and sure enough the filter capacitor was a complete mess. Apparently, these “RIFA” capacitors were commonly used in equipment of my scope’s vintage and are notorious for failing after 20 or 30 years as the plastic case breaks down and lets moisture in.

Fortunately, the failed device was easy to remove and unsurprisingly I had a replacement capacitor (from a different manufacturer) in my extensive component stock. The oscilloscope is now working OK and with less of the nasty smell.

  • Old filter capacitor - Note damage visible underneath