iMac screen problem - PSU schottky's failing?

[Crystaleyes]

Hi all,

have another iMac with flickering then black screen issue.

It is a 21.5" 2010 A1311 model.

When switched on, the screen flickers before going black. With a torch you can see the image is there, as are all four diagnostic LED's.

Measuring the PSU output only gives around 10.5v with the cable harness connected, although it measures 12.06v unconnected. This leads me to suspect something the power supply.

I saw that some people had found a shorted mosfet on the output however these seem fine, when tested for shorts on the board.
There are two power-Schottky rectifiers which although the datasheet gives the Forward Voltage at 0.57v, are only reading around 0.19v across all four.

So my question is, "could these low Schottky readings cause the PSU output to drop under load?


Thanks in advance

[Crystaleyes]

Re: iMac screen problem - PSU schottky's failing?

Just to add, that the inverter board is receiving the 3.3v ENA and PWM signals

And the Schottky's are these;

https://pdf1.alldatasheet.com/datash...TPS3045CT.html

[Crystaleyes]

Re: iMac screen problem - PSU schottky's failing?

Just in case anyone ever has the same question, the Schottky's are fine.

Testing a brand new one gives around 0.175v across each leg, so the problem lays elsewhere.

[R_J]

Re: iMac screen problem - PSU schottky's failing?

Check the inverter board, but I suspect you have a bad led in the panel

[Crystaleyes]

Re: iMac screen problem - PSU Schottky's failing?

Turns out the problem is the PSU.

This iMac has just been gathering dust whilst I had other projects and it wasn't clear where the problem lay, which potentially, meant a headache to locate the issue.

Fast forward a few months and by luck, I was asked to replace the logic board in an identical iMac, which gave the opportunity to swap over any parts necessary to identify the source of the problem.

Installing the known good PSU resolved the flickering display, so I set about having a closer look at the bad PSU and after testing a few things, think I may have found the problem, I floopin hope so anyway as this is the last transistor to be tested on the thru-hole side of the pcb.

Buried under a heatsink is a 15N60C3 MOSFET, and this is giving strange readings. On a T7 component tester is comes up as a N-Channel Mosfet, yet when performing the DMM test with -ve on Source and +ve on Drain, and touching Gate, instead of shorting, it starts with maybe 40Ω and just keeps rising to OL. Similar behaviour in Diode mode.

This Fet is out of circuit and being tested on the bench, so unless I am about to learn some funky Fet behaviour which was unknown to me, then I would say that this Fet is fecked. I mean, Mosfets shouldn't have a rising impedance across the Drain and Source should they?

It would also explain why the display deteriorates after being switched on.

It'll be great if this is the only problem. I haven't yet had the most success with SMPS's.