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Why do capacitors near a CPU test OL both ways in Diode mode

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(@davehogue209)
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I'm testing two identical motherboards "Asus Crosshair Extreme". One of them boots into the bios with no issues however it won't boot to a boot device even though it was my main workhorse pc for 7 years, and the other board powers up and I receive a "CPU" error code light even though the cpu has been verified good.

Both boards have capacitors near the CPU and all of them test "OL" both ways in Diode mode even capacitors on the working mother board.

My question is; why is this? Are the capacitors near the CPU testing this way because the value of the capacitors near the CPU are so low the DMM cant measure them in Diode mode or is it because the capacitors near the CPU serve a different function and as a result have no voltage drop thus I'm testing them the wrong way?


   
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(@Guest 10257)
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Hello Dave! Please note that capacitors should be tested via resistance measurement not diode voltage drop and good ones will always read open. Their failure mode is almost exclusively to short vice open and values do not drift with age/temp as resistors do. You usually find em with one lead to ground unless being used in a few specific applications. Slightly advanced topic but worth noting that caps cannot be reliably tested in circuit with a DMM. To get a valid reading one needs to either use a Effective Series Resistance (ESR) meter or isolate at least one side. (I'm not getting into leakage current testing lol).

As far as your specific board issues I think you're missing the forest for the trees and need to back up.

For the workhorse board are you able to boot from a USB, optical disk, network, etc? Is this a M.2 or does a different SATA port give different results? Have you verified your drive in another machine or swapped the cable?

On your CPU problem board what do you mean by CPU light? What is the specific Q code? Remember that the CPU interfaces with the Northbridge on your MoBo first, then to other things, and then through the southbridge to even more things.

 

Good Luck!

 

Dan

Second Chance Repairs LLC


   
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(@davehogue209)
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@2ndchancereps First off let me say thank you for taking the time to respond to my question and second thank you for being so knowledgeable and sharing your time, I'm very grateful to say the least.

I'm new to troubleshooting electrical components so thank you for helping me understand. I took a course on using my DMM on Udemy but I'm still very confused. lol
So the right way to measure capacitors is to set my DMM to Resistance Mode or Ohms mode and then test it and a good capacitor will come back as being open or OL?

Thank you so much once again for helping me to learn more. Have a great weekend.


   
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(@Guest 10257)
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Dave,

       Just seeing your reply so I apologize for the delay in getting back to you. Yeah man you are quite welcome, anytime. Been fixing broke stuff and keeping the working stuff working for 17 years now so like to think I know a thing or three lol.

       HOW you test is dependent on WHAT you are testing AND the test CRITERIA. Not sure how versed you are in electronic theory but remember that capacitors will pass AC but block DC. (The exact opposite of inductors that block AC and pass DC) Your multimeter is running off a battery, and doesn't have an inverter circuit to convert the DC to AC, so when you're measuring resistance you're actually measuring the DC voltage drop of the component. The multimeter knows the current it's outputting through the leads so it uses Ohm's Law to determine and display the equivalent resistance given supplied current and measured voltage drop. A capacitor cannot pass DC current from the one internal plate to the other, so there is no voltage drop, so the meter reads OverLoad because the value (infinity) is too large to display. Does all that nerd talk make sense?

       Remember that all the MIT courseware is free so you can go as far down the rabbit hole as you'd like with electronics! No, you don't technically need to know all that background to find an replace a faulty component BUT knowing the "WHY" behind things is what separates a mediocre tech from the good ones. Once you get into Field Engineering it's more expected but I've seen some knuckle draggers out there. By the Engineering Technician level it's 100% expected lol.

Happy fault hunting my dude,
Dan
Second Chance Repairs LLC  


   
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