Mega STE killed by dodgy PSU! (high voltage)

[MRAtari]

a while ago i had a Mega STE that worked fine until one day i switched it on and the fan on the PSU ran very fast! (like it was getting too much voltage) so quickly i switched it off but was clearly too late

anyway on removing the PSU to test it i found the 12v rail was outputing 22v!! didn't get a chance to test the 5v as it blew up in front of me!

anyway needless to say the Mega STE is dead, however i have found the scsi board to be ok and working still and so is the memory

so not wanting to scrap it my question is what would be the first 5v component to get hit by high voltage?

[Nikolas]

This was fixed psu? That you recently replace transistor?
I think my st machines uses 12 volts for rs232 to power up chips. Or simply go with chip by chip who got shurcuts inside chips..
Or it just may be capacitators, that may have inside chortcuts.. That make your psu dead before you fixing your dead psu.

[SofiST]

I remember some bad brand of PC PSUs. It had stoopid HW problem of having too high voltage on TL494 (or equ) IC. Voltage was even over 40 V in some cases there. And it may kill TL494 in very bad way - for instance to damage internal voltage controlling parts. What may result in worst case - too high voltage. And it happened not just once by my customers.

What is first hit by too much on 5V line ? I think that no rule. Mostly RAM, CPU ... but in fact any IC connected on .

[MRAtari]

no this wasn't a "repaired" PSU, just the standard Phihong Mega STE/TT used working one which worked fine until this "freak" day!

frankly i wouldn't want to repair it now in case it happened again

[MRAtari]

just out of interest though what does control the 12v and 5v rails on these power supplies?

i mean it worked fine but was just outputting high voltages...