RIFA capacitors
Old epoxy-enclosed X-class filter capacitors used in mains filter circuits have a habit of failing. The problem is not Tektronix specific, a great variety of equipment from the 1970s to the 1990s such as computers, printers, etc. are affected, typically those with switch-mode power supplies.
The most infamous brand of capacitor exhibiting this issue is RIFA's PME series.
These are epoxy-encapsulated metal paper capacitors. The usual problem is that the epoxy in 20+ year old examples develops cracks. When the equipment has been turned off for some time, humidity enters the cracks. When it is turned on again, the capacitor will fail, releasing foul-odoured smoke and sometimes even catching fire.
The issue has jokingly been called "RIFA madness". As RIFA AB was originally a Swedish manufacturer, one commentator has called them the electronic version of Surströmming.
It is best to preventatively replace this type of capacitor with a modern equivalent of the appropriate rating and classification.
NB: Class X and Class Y refers to safety certifications. Class X are for use where their failure would not present an electric shock risk but could result in a fire, such as a filter capacitor connected across the AC mains. Class Y are rated for use where their failure would present an electric shock risk, such as filter capacitors connected from the AC line to protective earth (PE). Various subclasses (X1, X2 ...) define peak service voltages. See e.g. https://electronics.stackexchange.com/questions/333029/what-is-an-xy-rated-safety-capacitor-exactly for more details.
Links
- RIFA Madness (Schaffner Repair) by EEVBlog @ YouTube / Forum thread
- RIFA Capacitors – Quick Look at why you should remove them @ YouTube
- Schaffner filter in a Solartron 7150+ DMM @ YouTube
- Why do RIFA Capacitors Fail? by Jenny List @ hackaday.com