Testing continuity - getting temporary "read" errors at certain points

Thread Starter

RogueRose

Joined Oct 10, 2014
375
I've found some really odd things while testing the ATX & auxillary (26 pin signal) connector on an old Cisco motherboard and I've been getting continuity when touching say a 5V and 3.3v, or a yet unknown wires on the auxillary connector. I'll touch the probe to one wire and with the other probe touch each of the other wires and at times I'll get a short beep and when I check again immediately, it doesn't beep and checking 3-10 more times gives no continuity. Now if I come back and check a little later, I often get the same result on the first touch/check - and I'm certain I'm not accidentally touching the probes to each other.

It happens with a lot of wires, not just 1 or 2. It makes it really difficult to figure out the pinout for the power supply and controller board. I'm wondering if this is kind of normal when checking continuity b/c I think it's happened on other projects but I always brushed it off as personal mistake, but here I'm testing all female socket's and I know I'm not making mistakes.

The motherboard/PCB looks to be in perfect, near new shape and it's from 2004, and was in service for 5 years, so IDK any of that can cause shorts or something. Any ideas what is going on?
 

Thread Starter

RogueRose

Joined Oct 10, 2014
375
Sounds like its reading capacitance, do the numbers rise sharply and then “open”?
I don't think it is. The meter has the dial that has the resistance (ohms), diode, continuity & capacitance, all on the same setting. I push the "mode" to select the continuity, which also displays the omega/ohms symbol and I guess the resistance when it is measuring. When I'm not touching anything it says OL and when I get the temprary beep, it stays on OL.

Now since I posted, I've tested about 500+ readings and I've had the same error on the same post about 30+ times.

I'll check on pin on the ATX connector and then all 26 pins on the aux connector. Almost every pin on the ATX connector gives an initial first beep on the same pin on the Aux connector - as well as an occasional other pin on the Aux connector.

I'm trying to figure out how to power on a server power supply that connects to a power control unit that breaks all the voltages down to 2.5, 3.3, 5, 12 and -12 & -5. I'm interested in it b/c it has 30A of 2.5v which is unique to this supply but I'm not sure how to get it to power up and the controller board connects to the motherboard with an ATX and aux connector so I'm thinking I can find a way to switch it on if I know the connectors.
 

Sensacell

Joined Jun 19, 2012
2,852
What you may be seeing is the capacitance internal to the board or power supply.

A large enough capacitor will look like a short- until it charges to the tester output voltage, then i looks like a high impedance.
The continuity tester will beep once, then go silent.
 

Yaakov

Joined Jan 27, 2019
3,346
If it i really an ATX supply, then pin 14 (green) needs to be pulled low (shorted to ground) to start it up.

You can jumper it to a nearby black wire.
 

Thread Starter

RogueRose

Joined Oct 10, 2014
375
If it i really an ATX supply, then pin 14 (green) needs to be pulled low (shorted to ground) to start it up.

You can jumper it to a nearby black wire.
Thanks but it's not a normal power supply. It has a 12v supply that plugs into a conversion board that then has the ATX & auxillary outputs. The ATX didn't have any turn on - it was on the 58 pin connector board on the 12v supply. I found the pinout of that and it just need's PSON jumped to ground.

As for the continuity issue, IDK what is going on. It does it with more than one meter as well. I'm going to test it with other boards, I have a few that are identical and it will be interesting to see if it is a problem with the motherboard or if it is happening for a reason.
 

Thread Starter

RogueRose

Joined Oct 10, 2014
375
I decided to test some capacitors I have - 50v 4700uf and see if there is a short inside as I've found a number of caps that show continuity when both terminals are touched - I'm guessing these are bad. I found that for almost 1/2 of the caps out of about 200, I get a short beep for about 1/4 - 1/2+ seconds the first time I test the cap, but if I take the leads off (using aligator clips) and test it again, I don't get the beep. IDK if this is a result of the meter or what, because it seems to happen on a wide range of things I test - this temporary beep on the first contact.
 

djsfantasi

Joined Apr 11, 2010
7,769
Do you have another thread on this same problem? Because I’m certain I’ve already answered this question.

Move on. Nothing’s happening here.

All discharged capacitor will read as shorted when first checked for continuity. All of them!! As the cap charges from the voltage used for the continuity test, the measured resistance will rise until you measure an infinite resistance.

The time to go from a short to infinite resistance, for a given meter depends on the cap value. It’s what s cap does, don’t cha know...
 
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