VT100 PSU smelling.

Tothwolf tothwolf at concentric.net
Sat Jan 23 20:53:09 CST 2016

On Sat, 23 Jan 2016, Mattis Lind wrote:

>>> I don't have a ESR meter or similar. It is not shorted at least and 
>>> the resistance is in the several mega ohm range when measuring in 
>>> circuit. Are they likely to go bad in a non catastrophic way?
>> They can. The translucent yellow Rifa (now owned by Kemet) class X/Y 
>> safety capacitors in particular have a 100% failure rate and are on my 
>> replace-on-sight list. They usually begin to show visible signs of 
>> cracking in their outer casing before they finally go out with a bang. 
>> It seems to be due to chemical decomposition of the paper insulator 
>> which swells over time and causes the case to crack. I usually replace 
>> them with a film capacitor from Epcos of the same value and safety 
>> class.
> This is a 0.0033 uF 1600VDC SPARAGUE capacitor. It looks nice and orange 
> and there are no signs of cracks whatsoever.
> What is the likelihood of this being bad?

I've seen similar Panasonic and Sprague film capacitors fail in snubbers 
used in flyback circuits, so it is certainly possible. As inexpensive and 
readily available as these parts are, when in doubt, I just replace them. 
I would probably also replace the series resistor is there is any doubt 
that it might not be reliable (I often do this for carbon comp and carbon 
film resistors used in series with the Rifa safety capacitors I replace).

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