On Sat, 23 Jan 2016, Mattis Lind wrote:
2016-01-23 0:32 GMT+01:00 Peter Coghlan <cct...@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.
