Secondary side controller SMPS don't turn on by itself.

[Huram Arvit]

I'm having trouble with a SMPS. When I use a bench power supply only to start it's controller circuit, it works fine adjusting the PWM and keeping output voltage at 12v, however it doesn't start by itself. I've already checked and replaced the pulse capacitor at the primary, but the problem persists. The circuit uses a 494 as PWM generator.

[tbegal]

How is the Modulation controlled? POT or Digital?

[Huram Arvit]

It uses a voltage divider and the 5v ref and the error amp from the IC to adjust it.

[wraper]

Check components around power transistor bases, especially small electrolytic capacitors.