Hi,
Really need some input for this iMac 2008 24" PSU. It's driving me crazy.
Right now I only get a single spike to about 7 to 10V on the +12VS line.
First, some voltage measurements on the primary:
main cap: ~320V
SMPS controller (TEA1750)
pin 1: oscillating between 16 and 21V
pin 3: 0V
pin 4: 0V
pin 5: spike to 0.5V about second after startup, then 0V
pin 6: spike to 2V about second after startup then 0V
pin 7: 3.3V
pin 8: oscillating between 0.28 and around 0.37
pin 9: 2V
pin 10: spike to 0.2V about second after startup then 0V
pin 11: spike to 0.1V about second after startup then 0V
pin 12: pulsating to 0.5V
pin 13: pulsating to 0.15V
pin 16: ~320V
Now the history of the thing (why it's driving me crazy):
Received the iMac in a no power condition.
PSU was obviously dead: blown fuse.
Quick check: main switching MOSFET shorted.
Replaced both, fireworks and both part blew again.
Proceded to do a more in-depth analysis.
Found the PFC diode shorted, the current sense resistor for the PFC MOSFET fried and obviously fuse blown and main switching MOSFET shorted. I replaced all of them and the SMPS controller too.
Great! +12VS was back. However I had quarter fan spin: the +12V would not come up.
Proceded to check the secondary of the PSU.
The 060N03L which switches between +12V and +12VS was found shorted. I replaced it with a 70T03H.
At that point, everything looked fine. I turned on the computer, the fans spun for like 5 seconds and sadly, it was dead again after.
And here we are now. No idea what's happened, but now I replaced:
→ main switching MOSFET, but it was still good
→ an SMD resistor marked 390 (should be 39 ohms) on the base of the main switching MOSFET. It was reading 275 ohms.
→ a N-channel MOSFET (2N7002) on the secondary after reading this thread: https://www.badcaps.net/forum/showthread.php?t=50803 . I was getting 5kohms between drain and gate.
No change in the behaviour. I still get a spike of 7 to 10V on the +12VS and nothing else. I can hear a single click sound at startup, but I don't know if it was hear before or not.
Really need some input for this iMac 2008 24" PSU. It's driving me crazy.
Right now I only get a single spike to about 7 to 10V on the +12VS line.
First, some voltage measurements on the primary:
main cap: ~320V
SMPS controller (TEA1750)
pin 1: oscillating between 16 and 21V
pin 3: 0V
pin 4: 0V
pin 5: spike to 0.5V about second after startup, then 0V
pin 6: spike to 2V about second after startup then 0V
pin 7: 3.3V
pin 8: oscillating between 0.28 and around 0.37
pin 9: 2V
pin 10: spike to 0.2V about second after startup then 0V
pin 11: spike to 0.1V about second after startup then 0V
pin 12: pulsating to 0.5V
pin 13: pulsating to 0.15V
pin 16: ~320V
Now the history of the thing (why it's driving me crazy):
Received the iMac in a no power condition.
PSU was obviously dead: blown fuse.
Quick check: main switching MOSFET shorted.
Replaced both, fireworks and both part blew again.
Proceded to do a more in-depth analysis.
Found the PFC diode shorted, the current sense resistor for the PFC MOSFET fried and obviously fuse blown and main switching MOSFET shorted. I replaced all of them and the SMPS controller too.
Great! +12VS was back. However I had quarter fan spin: the +12V would not come up.
Proceded to check the secondary of the PSU.
The 060N03L which switches between +12V and +12VS was found shorted. I replaced it with a 70T03H.
At that point, everything looked fine. I turned on the computer, the fans spun for like 5 seconds and sadly, it was dead again after.
And here we are now. No idea what's happened, but now I replaced:
→ main switching MOSFET, but it was still good
→ an SMD resistor marked 390 (should be 39 ohms) on the base of the main switching MOSFET. It was reading 275 ohms.
→ a N-channel MOSFET (2N7002) on the secondary after reading this thread: https://www.badcaps.net/forum/showthread.php?t=50803 . I was getting 5kohms between drain and gate.
No change in the behaviour. I still get a spike of 7 to 10V on the +12VS and nothing else. I can hear a single click sound at startup, but I don't know if it was hear before or not.
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