how to tell if something is shorted without a beep from the multimeter?

[Gmorb]

ive gotten so much good advice here but one thing i still need to know is how do i know when something is considered a short in ohms when the multimeter wont give out a beep.

you know those shorts that arent easily detectable.

I remember duke telling me to measure resistance to ground and that at a certain ohm/kohm resistance it meant mcp was dead and whatnot.

what resistances indicate a short on a circuit?

hope i explained myself ok. thanks.

 

[JohnB8812]

Zero ohms or very low ohms on a main rail like PPBUS. Basically best way is to compare to a known good board. Low voltage rails will be lower resistance such as CPU VCore where as higher voltage lines like backlight will be higher resistance.

 

[2informaticos]

Beep is not equal short!
Many multimeters, on buzzer scale, beeps till 060-075; higher frequency and low power for higher value.
Now a real short is 0 (zero) ohms.
But we (unproper) consider short a low value, which is to low comparing with expected value in a specific rail; being still much higher than 0 (zero).

As example, on the DDR power rail we normaly expect more than 100, even 400 (diode mode); higher values for boards with RAM module removed.
030 indication there is "titled" as short, because is too low for that rail; but is till far away of zero.
But is not a real short. For this reason we can't use current injection method in such case.
Nothing will go hot with 1.5V/30ohms (50mA, 75mW), as example.
With a 001 indication (diode mode), which can be 0.5-1 ohm (200 ohm scale), we expect 1-3A (2-5W), which can help us to detect the culprit.

But 3rd Gen iCore CPUs (Ivy Bridge) have very low values as normal.
i7-3xxx can indicate aprox 1 ohm and is no short at all.
Do not inject tension in such case!

Is good to create a database (simple txt file) with normal values on the current power rails.
Then we can compare with the values from a defective board.

CPU core, as GPU core too, can have very low values.
But other CPU classe could have much higher values; check 1st and 2nd iCore CPUs.

Nortbridge cores and first PCH cores can indicate even 010 as OK.
Others should give more than 100.
nVidia MCP core shows 005-010 as normal.

3V3 and 5V rails give 120-450 as normal; 020 there will indicate a problem.

On MacBook, PPBUS gives aprox 110-130, or 430-500; lower value for the boards which have Schottky diode integrated on the low-side MOSFET from charger circuit.
But these two values are tipcally for PPBUS rail.

Keep in mind, measuring the same rail, even in the same board, with different multimeter, will give different values; but in the same class value.
Measuring the same rail in different board will also give different value; but should be close reading.