Mike Harrison wrote:
>
> SMPS startup problems are frequently caused by dried out electrolytics
> on the PRIMARY side - the first thing I do with these is replace all
> the low-voltage electrolytics on the primary side - there are usually
> only one or two,  using 105 deg.C types. This often fixes the problem.
> Also, the high-value (100K-ish) startup supply resistor going high is
> another possibility.
> Secondary-side problems seem rare with small SMPSUs.


Not exactly true. :o)
The caps that cause the problem are small electros
usually 10uF to 47uF and they are the feedback filter
caps in the smps. Usually low volts caps with about a
9v winding on the smps transformer.

They CAN be on the primary or secondary. Primary side
doesn't need an optocoupler. Both are common systems.
The rule I use is any small cap directly connected to
the smps transformer gets a good inspection, or simply
replaced if time is short.
:o)
-Roman

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