Test SMD Capacitor. Beginner Question

Thread Starter

Chris Mac

Joined Sep 23, 2019
9
Hi. First post here. Total electronics beginner looking to learn how to repair all things electronics.

I heard that to test for a bad SMD capacitor, you should check for continuity to ground on both sides. It should only be grounded on one side, and if it's showing as grounded on both sides, there's a short somewhere on that line. Is that right?

So I have two identical tablets here. One with no power and one known-good. While testing the faulty one, I noticed a few capacitors on the main power rail (next to the battery connector) were beeping on both sides (black probe ground, red probe on each side one after the other)

Thing is, I took the known-good tablet to compare it with, and I'm getting continuity to ground on both sides of the same capacitors with that one too, but it works just fine. Am I completely missing something here or is the continuity test bs?
 

bwilliams60

Joined Nov 18, 2012
1,175
Whe you are doing continuity test, I assume you are on lowest resistance scale with audible tone on. When it beeps, is there any reading on the display or is it absolutely 0.00 ohms? I have seen where you get a beep but still show a resistance value as well.
 

KeithWalker

Joined Jul 10, 2017
411
Hi. First post here. Total electronics beginner looking to learn how to repair all things electronics.

I heard that to test for a bad SMD capacitor, you should check for continuity to ground on both sides. It should only be grounded on one side, and if it's showing as grounded on both sides, there's a short somewhere on that line. Is that right?

So I have two identical tablets here. One with no power and one known-good. While testing the faulty one, I noticed a few capacitors on the main power rail (next to the battery connector) were beeping on both sides (black probe ground, red probe on each side one after the other)

Thing is, I took the known-good tablet to compare it with, and I'm getting continuity to ground on both sides of the same capacitors with that one too, but it works just fine. Am I completely missing something here or is the continuity test bs?
You have been badly informed. Capacitors are not necessarily connected with one side to ground. It just depends on where they are used in the circuit. For example, in the circuit below, one side of C4 is is connected to ground but C1, C2 and C2 are not :
Tone_2.jpg
 

Thread Starter

Chris Mac

Joined Sep 23, 2019
9
Whe you are doing continuity test, I assume you are on lowest resistance scale with audible tone on. When it beeps, is there any reading on the display or is it absolutely 0.00 ohms? I have seen where you get a beep but still show a resistance value as well.
Just tested one and I'm getting anywhere between 0.3 - 0.8 ohms. It seems to fluctuate until it eventually stops between those values. Doesn't seem to be an option to change the resistance range with this MM
 

Thread Starter

Chris Mac

Joined Sep 23, 2019
9
You have been badly informed. Capacitors are not necessarily connected with one side to ground. It just depends on where they are used in the circuit. For example, in the circuit below, one side of C4 is is connected to ground but C1, C2 and C2 are not :
View attachment 186775
Ok thanks. That makes sense. So there's no way to reliably test without schematics?
 

narkeleptk

Joined Mar 11, 2019
366
Total electronics beginner.
Thing is, I took the known-good tablet to compare it with, and I'm getting continuity to ground on both sides of the same capacitors with that one too, but it works just fine. Am I completely missing something here or is the continuity test bs?
Just to be clear, you are removing all power sources before testing continuity/resistance correct?
 

dendad

Joined Feb 20, 2016
2,977
It is pretty difficult to test caps in circuit, without knowing what the circuit is.
For instance, a cap can be connected across a coil as a tuned circuit. Doing a continuity test on that cap will read a short circuit. But the cap is ok.
For a real test, the cap will have to be removed from the PCB and measured.
But if you can get a circuit, testing for shorted caps can help, when you have a bit of a clue what can be expected by reading the circuit.
 

Thread Starter

Chris Mac

Joined Sep 23, 2019
9
Would anyone happen to know where to start with a tablet that has no display?

I've used a known-good LCD (tested and working on the known good but not on this dead one) Known good battery. Still no luck.

Current draw is 0.45A precisely regardless of whether or not the power button is pressed or held. (10.1 inch tablet so should be 1-2A and the other identical working tablet draws around 1.5A)

LCD and battery connector on motherboard don't appear to be damaged. Also used the good cables from the good LCD to connect it. I've tested the mosfet nearest the battery connector. 3.8V at battery terminal, also 3.8V at both input and output pins of mosfet nearest the battery terminal. Which also rules out the main power fuse being blown(?) And that's where I'm stumped.
 

Thread Starter

Chris Mac

Joined Sep 23, 2019
9
It is pretty difficult to test caps in circuit, without knowing what the circuit is.
For instance, a cap can be connected across a coil as a tuned circuit. Doing a continuity test on that cap will read a short circuit. But the cap is ok.
For a real test, the cap will have to be removed from the PCB and measured.
But if you can get a circuit, testing for shorted caps can help, when you have a bit of a clue what can be expected by reading the circuit.
Got you. So without understanding the circuit, you're just clutching at straws. You'd need to have schematics and understand what's going on with them?
 

dendad

Joined Feb 20, 2016
2,977
First off, see if you can measure the various power supplies.
Take a picture of the board and note the readings of the good tablet and compare them to the dead one.
 

Thread Starter

Chris Mac

Joined Sep 23, 2019
9
First off, see if you can measure the various power supplies.
Take a picture of the board and note the readings of the good tablet and compare them to the dead one.
Ok, understood. How would I identify the power supplies? I've heard people talking about different power supplies on a motherboard before but to me, they all just look like the same chips and inductors as any other
 

dendad

Joined Feb 20, 2016
2,977
You may be lucky to find some marked on the board.
Another trick is to measure the volts across any electrolytic caps. They are often power supply filters.
 

narkeleptk

Joined Mar 11, 2019
366
Got you. So without understanding the circuit, you're just clutching at straws. You'd need to have schematics and understand what's going on with them?
You don't need a schematic but you need to know enough to be able look at what your working on and have an idea of what it does. It helps to work on sections at a time and even draw them out yourself if needed.

Ok, understood. How would I identify the power supplies? I've heard people talking about different power supplies on a motherboard before but to me, they all just look like the same chips and indicators ad any other
Start where your battery or power plugs in. Pick a part and google its marking with "datasheet" behind it. Study some datasheets and sample circuits of voltage regulators. After awhile you will start to recognize them easy enough. ANYTHING you don't know what it is, just search google and always add "datasheet" to your term. It helps a lot to narrow down the results even if you don't get exact matches.
 

Thread Starter

Chris Mac

Joined Sep 23, 2019
9
You may be lucky to find some marked on the board.
Another trick is to measure the volts across any electrolytic caps. They are often power supply filters.
Thanks again for the help.
You may be lucky to find some marked on the board.
Another trick is to measure the volts across any electrolytic caps. They are often power supply filters.
Thanks, I appreciate the help. Learned a lot. I'll stick to this forum rather than asking on Reddit!
 

narkeleptk

Joined Mar 11, 2019
366
Hah, yeah I saw your post on reddit, I rarely go there but its seems just as bad for getting actual help as /diy/

I hang around a lot of forums and lurk around even more places but AAC is by far one of my favorites. Everyone is pretty nice and extremely helpful here. Even the grumpy members are great.
 
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Thread Starter

Chris Mac

Joined Sep 23, 2019
9
Hah, yeah I saw your post on reddit, I rarely go there but its seems just as bad for getting actual help as /diy/

I hang around a lot of forums and lurk around even more places but AAC is by far one of my favorites. Everyone is pretty nice and extremely helpful here. Even the grumpy members are great.
Ha oh you saw the Reddit post too? Guess it's a small community. Thanks so much for the advice re: datasheets. I'll have a look tomorrow
 
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