I have a low voltage output supply, from 220 v to 9v dc, it stays dead till I touch it (on high voltage side) with multimeter probe to check voltages. once powered up it stays and take the load. checked all resistors, checked by replacing all electrolytic caps but no luck till now.

 

If it is a switched mode supply then it may have a 'startup' resistor which supplies enough power to get the primary side running. Once it is running the power from the resistor is replaced by the power from the transformer. Maybe the startup resistor has failed and the resistance of your meter is enough to kick it into life.
For instance: https://electronics.stackexchange.com/questions/163940/resistor-between-line-and-neutral-in-smps

 

If it is a switched mode supply then it may have a 'startup' resistor which supplies enough power to get the primary side running. Once it is running the power from the resistor is replaced by the power from the transformer. Maybe the startup resistor has failed and the resistance of your meter is enough to kick it into life.
For instance: https://electronics.stackexchange.com/questions/163940/resistor-between-line-and-neutral-in-smps

I've already checked all the resistors