Hi there, i just took a pristine mac i had in storage since 1992! out of the box, it didnt turn on, so i opened it up, and the first thing that strikes me is the rotten fish smell of dead leaky capacitors on the PSU. I also noticed a dead MOSFET IRFBC40AP, so i went ahead and i bought those parts and the computer turns on and enters the operating system pefectly.
HOWEVER and heres my issue: When i initially turn on the computer an extremely high pitched sound almost inaudible starts to appear, then the sound starts lowering its pitch slowly and gradually until it becomes EXTREMELY ANNOYING AND LOUD, the pitch seems to be lowering all the time, until the computer turns itself off. Ive tried to trace it with my ear (without moving the head very close to the PSU ouch!) and it seems to be around the aread where i replaced the dead MOSFET, however im not sure if its the MOSFET or the transformer.
So i thought i had more bad caps, and it turns out one of the main reservoir caps seemed leaky, so i just went ahead and replaced all the LOW ESR caps, including the 2 main reservoir caps, and still the high pitched sound, although this time my MOSFET died.
So i tested basically mostly every component with some exceptions, one being of the optocoupler and the other the TDA4605 which is a control PWM circuit and i have no way to test it. I noticed that the MOSFET was shorted between its dran and gate connections, which are also connedted to pin 1 of the TDA4605 which is the regulation entry. There is also a polypropilene cap in that path which i took out and measured, and it seems to be fine.
Im running out of ideas, im about to order a new TDA4605 and another poly cap just in case, im also going to replace the MOSFET but im running out of ideas.
Ohhh by the way theres a high voltage diode which seems to be heated up, since all the silkscreen markings are gone, but the diode tests ok, and i have no way to determine the exact model, so is it possible that the diode becomes leaky at higher voltages thus producing the high-pitched sound?
I know this is a somehow complex question but most switching power supplies have the same architecture and i would really appreciate any input
I also know this is probably completely useless but i would really like it to have it up and running.
HOWEVER and heres my issue: When i initially turn on the computer an extremely high pitched sound almost inaudible starts to appear, then the sound starts lowering its pitch slowly and gradually until it becomes EXTREMELY ANNOYING AND LOUD, the pitch seems to be lowering all the time, until the computer turns itself off. Ive tried to trace it with my ear (without moving the head very close to the PSU ouch!) and it seems to be around the aread where i replaced the dead MOSFET, however im not sure if its the MOSFET or the transformer.
So i thought i had more bad caps, and it turns out one of the main reservoir caps seemed leaky, so i just went ahead and replaced all the LOW ESR caps, including the 2 main reservoir caps, and still the high pitched sound, although this time my MOSFET died.
So i tested basically mostly every component with some exceptions, one being of the optocoupler and the other the TDA4605 which is a control PWM circuit and i have no way to test it. I noticed that the MOSFET was shorted between its dran and gate connections, which are also connedted to pin 1 of the TDA4605 which is the regulation entry. There is also a polypropilene cap in that path which i took out and measured, and it seems to be fine.
Im running out of ideas, im about to order a new TDA4605 and another poly cap just in case, im also going to replace the MOSFET but im running out of ideas.
Ohhh by the way theres a high voltage diode which seems to be heated up, since all the silkscreen markings are gone, but the diode tests ok, and i have no way to determine the exact model, so is it possible that the diode becomes leaky at higher voltages thus producing the high-pitched sound?
I know this is a somehow complex question but most switching power supplies have the same architecture and i would really appreciate any input
I also know this is probably completely useless but i would really like it to have it up and running.
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