hi friends ,

i have this hifi 5.1 system from 90s which i got from someone , its main unit is not working . so i opened the box and checked the power supply .
a transistor Q954 was blown so i replaced it with new one and it worked for few mins then again shorted . please tell me the reason for this transistor to be shorting. following attached the pic of system and circuit .

 

Without having it on the bench its almost impossible to say, but its a reasonable guess that some other part(s) are also damaged. What voltages are you getting at the connector?

 

Usually when a component goes bad, there is a reason..
Please tell us the make and model of the hifi.

 

Usually when a component goes bad, there is a reason..
Please tell us the make and model of the hifi.

its Philips FWD 796 model . i have attached the service mannual pages also the pictures of powersupply

Without having it on the bench its almost impossible to say, but its a reasonable guess that some other part(s) are also damaged. What voltages are you getting at the connector?

Yes . it giving BK5V and it was giving 12v also but after the transistor short , 12v pin is giving 4V only

 

here is highlighted part i replaced , probably the problem is in secondary section ( in this area). i have also observed the board carefully and there are no capacitors or components that looks burned

 

Mostly, transistors fail from excess heat, which is caused by excessive current. We are only shown part of that power supply circuit in post #5, and while we are shown the failed transistor, without seeing the rest of the circuit we can not determine the cause of the excess current.