iMac 2017 4K Problems

[jasonr2000]

hi guys I have the above iMac

and im trying to figure out where there is a short on a logic board / motherboard.


I have narrowed the area down to the highlighted sections in the attached photo's but I have no idea where to start or what to remove first to try and see if the fault goes ?

there is a short on all the caps in the area both sides of them

thanks for any ideas or help.

[mon2]

Re: iMac 2017 4K Problems

Start with the logic # etched on the circuit board.

It should be of the format '820-xxxx' - perhaps a schematic is available for your logic board.

[jasonr2000]

Re: iMac 2017 4K Problems

thanks for the reply I have the schematic and board view file.

but im trying to work out where to start with the shorts and so far its only in the area highlighted both sides of the logic board.

[mon2]

Re: iMac 2017 4K Problems

When you say short - what is the resistance to ground that you are reading ? Also be cautious that low voltage rails like the CPU / GPU will be with a low resistance reading. This is normal. Post more details on the resistance that you are observing.

What are the symptoms of this box ? How many LEDs are lit on the POST ?

[jasonr2000]

Re: iMac 2017 4K Problems

hi again

I can confirm no led's are lit on the logic board at all.

ive not tried resistance to ground ive just been testing continuity mode and found that all the caps in that area are shorted.

but when I test others on the logic board there is no beep.

what should I be looking for in the resistance reading ?

many thanks

[reformatt]

Re: iMac 2017 4K Problems

If you have no LED's, check you have 12V coming from the power supply first. If that is good, then usually means the PP3V3_S5 is missing (usually due to a shorted C7640 or C7641).

You are going up a dead end with your first post. All caps in this area will measure near short because they either feed the PCH, CPU or GPU.

[jasonr2000]

Re: iMac 2017 4K Problems

If you have no LED's, check you have 12V coming from the power supply first. If that is good, then usually means the PP3V3_S5 is missing (usually due to a shorted C7640 or C7641).

You are going up a dead end with your first post. All caps in this area will measure near short because they either feed the PCH, CPU or GPU.

hi thanks for the reply

I have checked the power coming from the PSU and the voltages are there.

ill check the caps you have mentioned when I can locate them lol (sorry im a newb at this thing)

thanks for the thing about the caps measuring near short. I didnt know that.

[jasonr2000]

Re: iMac 2017 4K Problems

If you have no LED's, check you have 12V coming from the power supply first. If that is good, then usually means the PP3V3_S5 is missing (usually due to a shorted C7640 or C7641).

You are going up a dead end with your first post. All caps in this area will measure near short because they either feed the PCH, CPU or GPU.

hi again I have checked C7640 as you said and its shorted.

and C7641 is showing shorted.