3.3V Short Circuit on ATX PC Motherboard

[mmmk]

Hi everyone,

Today, I wanted to put two of the four RAM bars back into my wife's PC from where I borrowed them until my own new kit arrived. Additionally, I replaced the GPU with another one that I took from a third PC. Both RAM and GPU are known to work (or have worked up until now). Unfortunately, after reconnecting power and trying to boot, the PC is completely dead. Not a single LED is turning on and I was puzzled--something like that has never happened to me before.

After trying all kinds of tricks like removing the BIOS battery and power for a couple of hours, I read trouble shooting guides online that suggested to test the MB for short circuits. I tried and found that the 3.3V circuit is shorted to GND. I measured resistances and volts, first when connecting the ATX connector to the MB and then directly on the PSU with a jumper between pins 16 and 17 (enable PS_ON):

ATX Pin Number	Resistance (Ohms)	PSU to MB (Volts)	PSU Direct (Volts)	ATX Reference
1	2.64	0	3.3	3.3
2	2.64	0	3.3	3.3
3	0.12	0	0	GND
4	0.378k	0	5	5
5	0.12	0	0	GND
6	0.378k	0	5	5
7	0.12	0	0	GND
8	188.52k	0	5	Power Good
9	145.84k	5	5	5
10	1.739k	0	12	12
11	1.74k	0	12	12
12	2.64	0	3.3	3.3
13	2.67	0	3.3	3.3
14	inf	0	-12.2	-12
15	0.14	0	0	GND
16	4.9M	5	-/- (JMP)	(PS_ON)
17	0.13	0	-/- (JMP)	GND
18	0.12	0	0	GND
19	0.12	0	0	GND
20	inf	0	-/-	NC
21	0.375k	0	5	5
22	0.375k	0	5	5
23	0.375k	0	5	5
24	0.13	0	0	GND

It seems that with only around 2.6 Ohms, the 3.3V circuit is shorted to GND. Also, the PSU does not even power on when connected to the MB because PS_ON is not activated. Could this be a fail-safe of the MB because of the short circuit?

I hope there is a chance that I can find the issue that is causing this short circuit and fix it. I (probably) have the tools to do it but I lack the understanding of what exactly to measure or what to look for to find the root cause. I hope you guys can help me there! Is there any specific pins that I can inspect to track down the cause?

The motherboard is a Gigabyte GA-Z270X-Ultra Gaming (rev. 1.0). All peripherals are removed right now (no CPU, no RAM, no SSD/HDD, no GPU, ...), it's the plain motherboard with the PSU.

All ideas are much appreciated!

 

[betwixt]

Those resistances are not unreasonable. 3.3V into a 2.64 Ohm resistance is only a 1.27A load ( = about 5W). When you consider PSUs are rated at hundreds of Watts you can see it is an almost insignificant load.

I would check the PSU is operational first. Solder some wires to a car headlight bulb and poke the other ends into the GND and +12V PSU outputs at the output plug. Then link pin 16 to GND with a short wire. The lamp should light when the link is in place and go out again when the link is removed. Note it will not work with the -12V output as it will not have enough output current capability. If it lights on the +12V output, measure the voltages on the other outputs with your DVM to check they are OK.

Brian.

 

[ZASto]

Do you have +5V Standby?
Without that voltage your PSU will not start at all.
Brian wrote pin 16 - it is a green wire on connector and GND is any black (just by the green )

 

[mmmk]

I see, good to hear those 3.3V resistance values may be fine after all. I forgot to mention that I also tried using a second power supply without success. Nevertheless, I tried the light bulb method using a 12V 5W car light bulb and tested both 12V pins successfully. After that I left it connected to pin 11 (12V) and tested all voltages with the light bulb lit. All values (3.3V, 5V, 12V, -12V) are fine!

Yes, the pin labeled 5VSB (stand by +5V) does also read 5V--it is pin 9 though. Port 16 on my PSU is called S_ON (soft On/Off) and is connected to GND (port 17) to power on the PSU. If not active/on, pin 16 also has 5V (as noted in the table above).

BTW: All my cables are black and I have to count the pins and use the manual to determine which pin is which (instead of using the mentioned colors).

I also forgot to mention that the primary PSU is a Corsair RM560x (650W).