I am prety sure the PSU in my slim is ok. I wasn't able to hook up a PC ATX power supply as I wasn't willing to mess with the 5.5v 4 plug lead from the PS3. I did however get a volt meter over the main large pins and they had 11.96 volts and the other small 4 pins had 5.5v. These seem perfectly inline what what is documented about the PSU.
The Fuses and Thermal cutouts on the mainboard are very small but was able to test most of them and they seem ok.
I did notice that where the 12v two large prongs on the mainboard. When taking off the PSU, the movement acts as a lever and this may cause a breakage somewhere in this area. The Standby button gets a 5.5volt feed, but Hypothetically if there was an issue with the 12v supply, would that give the same symptoms as I am experiencing. I.e 5.5volts to the Standby button which is ok so light comes on, but when the PS3 switches on the PSU and the 12 volt supply isn't reaching parts of the mainboard would that cause it to power off?