Short theory

[TheAnswer305]

Hi guys, so i am in the process of repairing a couple motherboards and ive come across something that im not quite understanding. Maybe you guys can help me out.

So i am checking the motherboard and find i have several areas of caps that have a short to ground, being that both sides give continuity when testing against a ground pad. Now i know this might be okay for some caps but this is a lot of caps doing this so im thinking theres a short. So i injected a small amount of voltage on a larger cap. I sent it in at 3v 1a to be safe and low, i saw some mosfets get hot and the bench shows it pulling 1.1v @ .8 amps. They also light up on the flir so i pulled them off. Now the delivery goes to 3v @ .03 amps so im thinking the short is gone becuase the voltage is flowing and only consuming whatever items are on the line take, however, i still get short to ground on the caps.

Am i understanding that when the voltage doesnt get dropped it means there is no short, or is it that there isnt a BIG enough short to cause heat.

In this scenario, how do you locate it?

[stj]

Re: Short theory

next time, dont go over a volt - a lot of cpu's and ram now run at 1.2v or 1.8v

[TheAnswer305]

Re: Short theory

next time, dont go over a volt - a lot of cpu's and ram now run at 1.2v or 1.8v

Thanks. Yes I read this somewhere and started going at 1v. However, given that the bench doesn't limit and show less volts, it means there's NOT a short right? But why do I have continuity on so many caps from ground?

[stj]

Re: Short theory

caps can look shorted while they charge.

[TheAnswer305]

Re: Short theory

caps can look shorted while they charge.

They are showing continuity with ground even without power though.

[stj]

Re: Short theory

wrong, your meter is supplying power.
thats how it works, it applies a current limited voltage - usually 2-3v and calculates resistance from the volt-drop

[TheAnswer305]

Re: Short theory

wrong, your meter is supplying power.
thats how it works, it applies a current limited voltage - usually 2-3v and calculates resistance from the volt-drop

Okay the definitely makes sense, thanks! Would that voltage cause all caps to show continuity on both sides of the caps? from what ive usually seen itll only sound on both sides if a component is giving a short somewhere

[TheAnswer305]

Re: Short theory

wrong, your meter is supplying power.
thats how it works, it applies a current limited voltage - usually 2-3v and calculates resistance from the volt-drop

As a follow up, is there degrees of shorts? Can i have a short that wont get hot but is still grounding the power rail? i thought all shorts would end up showing on the flir.

[diif]

Re: Short theory

Are you measuring in resistance mode or continuity ?

[TheAnswer305]

Re: Short theory

Are you measuring in resistance mode or continuity ?

Continuity mode, well on this mode it beeps if it goes under 50ohms i think, but it also displays resistance. So it might just be resistance mode with the little continuity beep on my particular meter. I can also check without the beep but readings are basically the same just having the beep is the only difference.

[stj]

Re: Short theory

test, then test with the probes switched.
if it's charging caps then you will see a jump in readings for a second

[eccerr0r]

Re: Short theory

If this is the short theory, what's the long theory?
(ugh this thread's topic name "short theory" bothers me...)

[TheAnswer305]

Re: Short theory

If this is the short theory, what's the long theory?
(ugh this thread's topic name "short theory" bothers me...)

long theory is i cant find a short and dont know how unless i start pulling everything off lol