VT100 PSU smelling.

[Tothwolf]

On Sat, 23 Jan 2016, Mattis Lind wrote:
> 2016-01-23 0:32 GMT+01:00 Peter Coghlan <cctalk at beyondthepale.ie>:
>
>>> R27 is part of the snubber network on the primary side of this 
>>> forward-type SMPS PSU. But why it it getting so hot. Is it normal? I 
>>> have completely forgotten how a VT100 smell when running...
>>
>> Snubber as in a resistor in series with a capacitor across the primary?
>
> Yes. It is a R, C, D network.
>
>> If so check the capacitor for leakage (electrical, not liquid) or being 
>> shorted.
>
> 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.