Why does the Q800 (and others that use that power connector) etc 5v have continuity to ground?

[Durosity]

I'm trying to trace a power issue on a Quadra 800 (Wombat) motherboard where I think I have a 5v short to ground. Or at least I did.. continuity between pins 2/3/4 to ground on the power supply connector. That was until I realised my fully working Q800 does that too.. and so does my Iici. So why does it do that, and why doesn't the 12v or other pins follow suit?

 

[Phipli]

I'm trying to trace a power issue on a Quadra 800 (Wombat) motherboard where I think I have a 5v short to ground. Or at least I did.. continuity between pins 2/3/4 to ground on the power supply connector. That was until I realised my fully working Q800 does that too.. and so does my Iici. So why does it do that, and why doesn't the 12v or other pins follow suit?

It's not continuity, but just quite low resistance. A lot of Macs will cause some meters to beep between Gnd and 5v. The 8100 and 840av do it. Can be tricky for troubleshooting. The more paths to gnd in parallel you have, the lower the overall resistance, but each component doesn't necessarily have that higher current flowing.

Colleague had a similar problem where... Erm... A large industrial machine, had so much wiring in it that it failed the insulation test, because even with good insulation, every cable is like a resistor in parallel and the more you have, the lower the resistance.

 

[Durosity]

Ahh that makes sense! Is there any way I can work around it?

 

[Phipli]

Ahh that makes sense! Is there any way I can work around it?

Read the resistance instead of using the continuity beep, or put a small (need to experiment with the value, but probably 5, 2.7 or 1ohm) value resistance in series with the probe.

A true short will be really really low, these false beeps are usually a few ohms

 

[Durosity]

I will try that! Thank you!