Mac mini 820-3228-b no power,Bios

[reset1986]

hi guys, i have this mac mini mainboard 820-3228-b that doesn't turn on,

Primary power supply 12V ok

3.3v is ok

On the bios chip I have a variable voltage ranging from 0 to 3v constantly

The bios chip is ok there are no schematics for this motherboard, I wanted to understand what can cause the voltage drop to the bios chip

There are no shorts or burns on the motherboard, as the chipset carries the intel SLJ8C, can it be causing this voltage variation?
or to try the flash to the bios chip with a different image?

Thanks

[mon2]

Re: Mac mini 820-3228-b no power,Bios

hi.

Place your meter in resistance mode, turn off / remove all power and then check the resistance to ground on all coils on your logic board.

You may have a local bad / shorted capacitor that is causing a short circuit event due to the low internal resistance. This will force the local switching regulator to turn off or enter chirp mode (ON / OFF / ON cycling) to prevent damage.

Post your results / pix if you find something that has a very low resistance to ground.

[reset1986]

Re: Mac mini 820-3228-b no power,Bios

hi.

Place your meter in resistance mode, turn off / remove all power and then check the resistance to ground on all coils on your logic board.

You may have a local bad / shorted capacitor that is causing a short circuit event due to the low internal resistance. This will force the local switching regulator to turn off or enter chirp mode (ON / OFF / ON cycling) to prevent damage.

Post your results / pix if you find something that has a very low resistance to ground.

thanks for the advice, I have a low resistance on the black coil, generated by this component, I do not know if it is a diode, I am attaching a photo of the faulty component,

the red circles are low bga power supply resistances, they are correct

[reset1986]

Re: Mac mini 820-3228-b no power,Bios

ok, it's a capacitor, datasheet placed for those who need it,

I'll post the results of the repair

[mon2]

Re: Mac mini 820-3228-b no power,Bios

hi.

1) with the power OFF, what is the resistance reading on this coil to ground ?

2) with the power ON, what is the voltage reading on this coil to ground ?

It is possible for a rail to have a low resistance to ground if the rail is a low voltage supply such as 1v0 or 1v05, etc.

Just the same, if there is a bad cap on the rail, you can face a very low resistance to ground so please post the value you are reading on this coil. If the voltage reading is not normal then this rail is defective and could be linked to:

a) bad cap - certainly you can try to remove this cap and then check the resistance to ground - if the resistance is higher then this cap is bad and needs to be replaced

b) coil not soldered / solder cracks over time due to heavy currents through this rail - you can reflow with flux and hot air to resolder each such connection

c) local mosfet is defective ( the square looking device with IR = international rectifier is the mosfet )

Assuming there was no liquid damage to the logic board.

[reset1986]

Re: Mac mini 820-3228-b no power,Bios

hi.

1) with the power OFF, what is the resistance reading on this coil to ground ?

2) with the power ON, what is the voltage reading on this coil to ground ?

It is possible for a rail to have a low resistance to ground if the rail is a low voltage supply such as 1v0 or 1v05, etc.

Just the same, if there is a bad cap on the rail, you can face a very low resistance to ground so please post the value you are reading on this coil. If the voltage reading is not normal then this rail is defective and could be linked to:

a) bad cap - certainly you can try to remove this cap and then check the resistance to ground - if the resistance is higher then this cap is bad and needs to be replaced

b) coil not soldered / solder cracks over time due to heavy currents through this rail - you can reflow with flux and hot air to resolder each such connection

c) local mosfet is defective ( the square looking device with IR = international rectifier is the mosfet )

Assuming there was no liquid damage to the logic board.

thanks for the advice, the capacitor was defective, the machine was repaired

[mon2]

Re: Mac mini 820-3228-b no power,Bios

Good job!

[djdblaster]

Re: Mac mini 820-3228-b no power,Bios

Hi, I have 2 units, both same problem. Replaced caps, checked for shorts, and plugged cable - cap goes bad again.
Stumbled. Any insight would be great!

[rookieme]

Re: Mac mini 820-3228-b no power,Bios

Hi, I have 2 units, both same problem. Replaced caps, checked for shorts, and plugged cable - cap goes bad again.
Stumbled. Any insight would be great!

That caps are at 5V line so beware if you use 2,5V capacitors

[natbuk]

Re: Mac mini 820-3228-b no power,Bios

Thanks for the tips and photos (reset1986) - helped me quickly find the shorted cap on my 2012 MacBook mini.

[ric8002]

Re: Mac mini 820-3228-b no power,Bios

hello from france

I have the same failure on a mini mac of 2012 I7 2.3ghz
I had a 0.22ohm resistor between this coil and the ground I desoldered this capacitor and miracle the Mac starts

thanks for the information

[natbuk]

The Mac mini died again - in a different way now - it chimes, but no display. cap that was bad before is OK (5V rail good) - looks like one of the CPU rails is chirping - it's hard to debug this board as I can't find board view, only schematic, so have to guess what is what. I have quite low resisitance (0.7 ohm) across what I think should be 1v05 rail - on the scope I can see it come up briefly, then goes away. Looking at node marked red: - any tips? should it be steady rail after chime?

Thanks for any advice.