# New member, quick general question

#### LionsFan

Jul 9, 2014
4
Hi guys,

Been in the computer service/repair industry for the past 16 years, but have never taken an electronics course, (though I'm very interested too). I've learned pcb component repair on my own as I went.
Anyway, a slow month at the store has motivated me to go though several laptop mobos that I had piled up as "bad". I'm having varied success, but here's my quick question, (and please don't laugh.... remember, no formal training):

Modern pc motherboard resistors, (square flat sort) should always conduct voltage through, (both ends) correct? So, if a reading of 19.60 volts on one end, and nothing all on the other end, then this resistor is an issue right?

I've had limited success over the years with replacing resistors of this type, (and blown caps which are easy to find). But with all the research I've done today, I can find very little on the web relating to the voltage input/output of a resistor, (and not testing of resistance). I just want to know why when I test other resistors of the same type on the same motherboard I get 19.6 volts on both ends, but on a few, 19.6 and nothing. See my point?

Any input is greatly appreciated and thanks for reading this first post here.

LionsFan

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
So, if a reading of 19.60 volts on one end, and nothing all on the other end, then this resistor is an issue right?

First determine if the device is actually a resistor.

If it is, there is generally a code on top that will give you the resistance.

Now, with the power turned off on the board (and capacitors discharged -- just wait a few minutes) measure the resistance with a multimeter. It should be no more than the stated resistance. Other components nearby may make the resistor appear to be a lower value. Try the reading both ways in case there's a diode in the circuit. If in doubt, remove the resistor and test it out of circuit (where it should read within a few percent of the stated value).

The voltage you read across a resistor depends on where it's connected. without knowing that all of these resistors are connected the same, or a schematic, comparing them is unlikely to be helpful. Comparing the *same* resistor over a couple of motherboards *is* likely to be helpful. Anything that sticks out as an odd-man-out is something to investigate further.

#### LionsFan

Jul 9, 2014
4
So, are you saying there's no good way of simply testing with voltage output in general? For example: 19 volts at the power jack, (check).... 19 volts at resistors around the power jack, (check).... 19 volts over here halfway across the board, (check).... but then, oops... nothing at all in this area, must be around here. Then by checking the different components that were in line with the trace from the last working area....
I've been doing it this way for years, but maybe that's not good, (and why I'm so frustrated with this laptop board today). I've used this method to "get close" for a while, but wanted to ask a electronics wiz about the voltage in and nothing out issue. Does that make sense? Here is a link with a photo of the resistor:

http://lh3.ggpht.com/_hNBdeOT2Lm8/SyoH1_NRNrI/AAAAAAAAAHk/3BH0AjRBy1s/s800/r519_before.jpg

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
Yeah, what Steve said. And to answer your question directly: No, it's not necessarily wrong for a resistor to have voltage on one end and no voltage on the other end. Actually it's quite common. It depends on how the resistor is used in the circuit; there is no simple rule.

#### Gryd3

Jun 25, 2014
4,098
A fully charged capacitor would give you very similar readings of voltage on one side and nothing on the other as well.
To be certain if a voltage should be present on both sides of a resistor, you will need to follow the traces to determine if the side your reading 0 on is connected to something that is not pulling that side of the resistor low. This can be very difficult with motherboards.
Best of luck, and take advantage of using the camera to help make those tiny numbers a little more legible

#### shumifan50

Jan 16, 2014
579
Referring to your picture: you are talking about the resistors(the black parts with numbers on them), not the little yellow part with R519 right next to it - it is a capacitor.

#### LionsFan

Jul 9, 2014
4
Thanks for the input guys. I guess I should have known this would not be an easy answer, (though Kris was close)... I know now that it's not always a bad thing, (thanks Kris) and that the charged resistor can give this type of reading, (thanks Gryd3). Steve, you gave me some ideas as well. Thanks guys. That link to a photo is not mine, but one I pulled off the web as an example of the resistor type.

#### LionsFan

Jul 9, 2014
4
Yep shumifan50... the black resistor with the number.... I was hoping for a hard and fast rule on this, but I think I have my answer. This is not a customer board.... I would never waste my time on this, (unless I saw an obvious burn spot or cap) but it's been a slow week. I was hoping the no-voltage on one side was telling me something, but I'll have to keep looking.
This is a cool forum guys... I'm glad I did a google search.

Replies
3
Views
307
Replies
4
Views
504
Replies
2
Views
354
Replies
6
Views
615
Replies
5
Views
2K