iMac (21.5-inch, Mid 2010) strange power on problem(?)

[kostaszaf]

Hello everybody
I have a strange problem with the above iMac.
The facts are:
- when i plug in the power cord, the iMac starts automatically with the fans always on and don't stop.
- I can do as many reboots I want, with the same condition. The OS works fine. (High Sierra)
- When I do shutdown, it power off but it never starts again.
I try everything that the manual said at troubleshooting, (reset SMC etc) but nothing
UNTIL I remove the battery for a while.!!
Then. I reconnect the power cord and it starts at the same mode (full fan) and I can work fine, until I power off.
And that loop's continuously.

It looks like the power button is pressed when I plug in the power cord, but I check it out and works perfect.
I thing that the problem is at the circuit and especially around the signal sending from the power button.
Seems like something stuck the SMC or the power on signal.

Does anybody have any idea?
Thanks in advanced

 

[TwoH]

My first instinct with power-related issues is usually the power supply.

 

[kostaszaf]

Thanks for your reply.
I'll keep it my mind, but,
you rule it out to be the motherboard?

 

[TwoH]

I'd never rule out the logicboard in this scenario - sadly.

Might quite possibly be the battery on the logicboard?

 

[USB3foriMac]

The power button goes through 2 FPGAs who process i and derive some other signals from it.
The latest stage in front of the Power Supply is this:

The FET Q610 might be shorted.
You can check pin 12 of your supply (this is for your 21 model). It should be 0V (e.g. you can also short it to 0V) in order to switch ON, and left open (there is a pull-up in the power supply) in order to switch the main voltage (pint 10, 11) OFF. So when off, the voltage is likely around 12V.
The standby voltage (pin 4) should always be there, 12V.

Maybe you can measure at Q610, and check whether this D and S (pin 2, 3) are shorted. This needs to be done with AC unplugged.
You can use the diode tester, red on 3, black on 2. There should NOT be any conductivity. If you reverse, the diode test should show something between 0.2V to 0.7V or so. Search for 2N7002 datasheet.

If that's ok, you can check the signal on G (pin 1), with AC plugged in. Set multimeter to measure voltage.
If 0V on pin 1, pin 3 should be "High" (12V or close to it).
If >0V (e.g. 12V), then pin 3 should be ~ 0V.

 

[Hombre53]

If you had the cover off the iMac recently, you may have forgotten to reattach the system temperature sensor to the MB. This would result in the fans running at high speed. Just a thought.

 

[kostaszaf]

Dear USB3foriMac
First of all thanks for your reply.
It is very informative and useful.

Maybe you can measure at Q610, and check whether this D and S (pin 2, 3) are shorted. This needs to be done with AC unplugged.
You can use the diode tester, red on 3, black on 2. There should NOT be any conductivity. If you reverse, the diode test should show something between 0.2V to 0.7V or so. Search for 2N7002 datasheet.

All these are correct

You can check pin 12 of your supply (this is for your 21 model). It should be 0V (e.g. you can also short it to 0V) in order to switch ON, and left open (there is a pull-up in the power supply) in order to switch the main voltage (pint 10, 11) OFF. So when off, the voltage is likely around 12V.
The standby voltage (pin 4) should always be there, 12V.

These are also correct, but I can't find (or understand) where is the pull-up that you mention in the power supply.
As well, I can't find (or understand) where the first pin of power button goes. (the second goes to the G of FET Q610)
Because I think this circuit is somehow permanently closed and that's why always starts in full fan mode.

The last check that you told me to do, I'll make it when assembly back the iMAC.

Thank you very much
[automerge]1596087957[/automerge]

If you had the cover off the iMac recently, you may have forgotten to reattach the system temperature sensor to the MB. This would result in the fans running at high speed. Just a thought.

Thanks for your reply, but there's no system temperature sensor to my MB. (or I can't fint it)
I remind you that my iMac is 21.5-inch, (Mid 2010)

 

[Fishrrman]

"I try everything that the manual said at troubleshooting, (reset SMC etc) but nothing UNTIL I remove the battery for a while.!!"

Try REPLACING the battery with a new one.
Have you checked the old battery for proper voltage?

A dead battery can introduce "gremlins"...

 

[USB3foriMac]

I can't find (or understand) where is the pull-up that you mention in the power supply.

It's on the power supply PCB of which I do not have a schematics for you. You can trace the PCB tracks, the circuit is close to the big connector.

I can't find (or understand) where the first pin of power button goes. (the second goes to the G of FET Q610)

As I said, the power button goes to an FPGA.

I don't think the problem is with the FPGA. The problem would more likely be with the discrete circuitry.

What I did to check my power supply was:
* remove the power supply entirely
* connect to 230V - BE CAREFUL NOT TO TOUCH ANYTHING OR CAUSE SHORTS WITH YOUR PROBES
* measure standby output; should provide 12V
* measure main 12V output; should be 0V (OFF)
* short the PS_ON pin to GND
* measure main 12V output; should be 12V (ON)

The pinning is here, but I believe this is not on the supply itself but on the logic board: Pinning
Compare with the pinning here: Pinning which is on the logic board.

The marking on the power supply is on the component side:

Take note of indication for pin 7 and 14. Mark the corresponding pins on the solder side with a permanent marker.

On the 310W supply, you have several pins on +12V as well as several pins on GND. This should also be the case with the 200W supply.
This is easy to measure. The rest is slight more difficult: there should be a total of 3 unknown pins left; 2 of them for SMC communication (my supply uses the chip EMC1404), and the last one is PS_ON. If you follow the three unknown pins, two of them should go to similar components (probably to pins 12+13 of EMC1404), while one goes somewhere else. That single one will be PS_ON.
This is exactly the way I figured out the 27" supply.

 

[kostaszaf]

"I try everything that the manual said at troubleshooting, (reset SMC etc) but nothing UNTIL I remove the battery for a while.!!"

Try REPLACING the battery with a new one.
Have you checked the old battery for proper voltage?

A dead battery can introduce "gremlins"...

I already check that. It was one of the firsts tries.
Thank you very much

 

[kostaszaf]

What I did to check my power supply was:
* remove the power supply entirely
* connect to 230V - BE CAREFUL NOT TO TOUCH ANYTHING OR CAUSE SHORTS WITH YOUR PROBES
* measure standby output; should provide 12V
* measure main 12V output; should be 0V (OFF)
* short the PS_ON pin to GND
* measure main 12V output; should be 12V (ON)

The pinning is here, but I believe this is not on the supply itself but on the logic board: Pinning
Compare with the pinning here: Pinning which is on the logic board.

Before I open this thread, I follow all the steps and instructions of the apple technician guide:

http://tim.id.au/laptops/apple/imac/imac_21_mid10.pdf

The pinning that you mention, are all there at page 28.
I also make the "Symptom Charts" from page 31.
I think that I already cover the checks that you suggest me.

Another thing is that my M/B doesn't have an FPGA like that you post (U4900) and also the power button scheme
does not help me.
Probably both are from another model of iMAC (you mention about 27" model Power Supply)

btw,
Looking more carefully with a magnifying glass at the M/B, I found something strange with an smd component
around the power button plug. So, I put some dry contact cleaner and I wiped the area.
Then I realize that I had lost a piece as you can see in the picture. (you can zoom in enough)
I'm talking about the part L5523.
Can you help me to find out what is this or how to alternative fix it?

Thanks again

 

[TwoH]

I will just make sure on some logicboards that there is meant to be an inductor there when I have access to some tomorrow if you dont get any answer - it does also look like the solder pads are completely gone it too..

Edit:

 

[USB3foriMac]

Ok, great.
L5523 is in-line with a temperature sensor. You can simply put a solder bridge if you do not have any inductor from some scrap electronics. The inductor is to improve noise immunity, so without it the value may be a bit fluctuating but no harm.
This will explain fan(s) running full speed.

The pinning that you mention, are all there at page 28.

This is on the motherboard. You need to check the supply itself, and that's unfortunately not mentioned. As I said, it is easy to figure out.
If you have measured on the motherboard already, maybe you can post the values after shut down, as well as when running.
You can then identify whether the 12V standby is present, and whether the man 12V switch off and on, or are always on.

Another thing is that my M/B doesn't have an FPGA like that you post (U4900) and also the power button scheme
does not help me.
Probably both are from another model of iMAC (you mention about 27" model Power Supply)

I am looking at the schematics 820-2784, which is for 21.5" mid 2010. It's identical to the 2009 and the 2011 models, too, both 21.5 and 27". I am sometimes comparing with the 2011 models as I have both of them.
So it will be here, maybe hard to find, despite the general expectation that ICs should be easy to locate. But I think this is pointless anyway, the fault will likely not be with this IC.

 

[kostaszaf]

Ok, great.
L5523 is in-line with a temperature sensor. You can simply put a solder bridge if you do not have any inductor from some scrap electronics. The inductor is to improve noise immunity, so without it the value may be a bit fluctuating but no harm.
This will explain fan(s) running full speed.

Ok, but should be a coil inductor and how many henries must be?
And what about the PCB that is destroyed? Where must put the bridge or the inductor?

 

[USB3foriMac]

Ok, but should be a coil inductor and how many henries must be?
And what about the PCB that is destroyed? Where must put the bridge or the inductor?

The value of the component is not given in the scematics. As it is 0402 size, the value will be very small, some microH. If you have a scrap PCB where you can harvest such inductor, go for it. The value is not very important here, as I already descrbed. If you don't have one, replace it with a tiny wire. If you have a cable at home, if it is stranded cable, remove the insulation, and simply take a single wire strand and solder it onto the two points marked in red. If you still want to put a component, you can also measure the ohmic resistance of L5522 (same part), and use a resistor instead.

I solder such things using this: Microscope on Aliexpress

 

[kostaszaf]

I'm sorry my friends, but things are going from bad to worse.
As you can see in the photo, the other point where the bridge would become was also destroyed.
When I assembled it, it fell in a startup loop and the starting chime was constantly heard.
And as if that were not enough, the graphics card also broke. (perhaps re-balling problem)
I'm completely disappointed and I stop dealing with it anymore.
I found a used motherboard and graphics card together at $ 70.
What is your opinion about Pre-Owned parts?
btw
thank you all for your help

 

[USB3foriMac]

So sorry to hear about all your trouble. I can understand the frustration.
The PCB tracks are really tiny, but don't blame it on yourself that you were not successful. You can be proud of yourself that you dared to attempt the repair. Most wouldn't even do that.

If you have the chance to get a new PCB at low cost, that is a fair option. Otherwise, you could also solder a wire also to other components.
However, I noticed there are more components gone, see pic.

L5010 is coming directly from the power button, and goes to the R-C filter R5010/C5010. So with this gone, the power button won't work.

C3130 is a 2.2uF/6.3V cap across the RAM reference voltage. I'd fix that last.
L5523 is now a bit more difficult to fix, as you need to look for a suitable pad. Best if you have some copper wire from an old transformer. The wire is lacquered, so you can cut a suitable length and just wire it to wherever needed.
L5523 is between the ambient temp sensor 6-pin CPU fan connector J5700, pin 6) and the common point of C5551/5552/5553. This common point could have some spare pads marked as XW5551/XW5552/XW5553. Those XW... seem to be solder bridges, so maybe they would be easy to access.

The graphics card might also still be working; it is possible that the graphics card was just not properly inserted. Happens many times, and can be fixed by re-doing the dis- and reassembly. I know that this is a real PITA... again not your fault, only Apple's, to design a nearly unrepairable product.

So all in all, still lots of difficult work ahead of you if you want to avoid spending money and have sufficient spare time.
Otherwise, keeping the board for spare parts and getting a replacement would be the easier option.