No power till external USB attached

JoHubb · 2 Jul 2014 at 10:58

2008 homebuild: Asus Rampage Formula, Q9650, 4GB DDR2, W7 x64.

This has been running perfectly since I built it. It was lightly o/c to 3.9 GHz but since the problems arose I'm running at stock 3.5 GHz.

The issue is that the machine fails to power up until either an external USB drive is attached, or the CMOS is reset.

To try to fix this I have done these:

1. Renewed CMOS battery.
2. Thoroughly cleaned out all dust, disconnected all cables and cleaned pins, removed and reinstalled memory sticks, checked start connections on mobo and case.
3. Switched SATA cables to different sockets.
4. Checked BIOS boot priority.
5. Disconnected PSU and checked using the paper clip method - the fan runs normally.
6. Installed a different graphics card.

None of these has corrected the fault. When I reset the CMOS or attach an external USB drive the PC 'appears' to start and boot normally but clearly there is something very wrong somewhere.

I have run out of ideas and can only assume some major motherboard fault.
Before I junk this rig, I'd be grateful for any suggestions as to what the problem might be.

John

Tealc · 2 Jul 2014 at 12:42

Is the external USB device bus or self powered?

Do the paperclip test and measure the voltage on the 5v rail using a multimeter.

If you have a spare psu try the system with that.

JoHubb · 2 Jul 2014 at 17:14

Thanks.

It's bus powered.
5v rail measures 5.0v
No spare PSU unfortunately. Do you think a PSU fault could show up in this way?
I admit that 6 years must be close to the lifespan of a heavy use PSU.

John

Tealc · 2 Jul 2014 at 18:02

My thought was that if the usb were self powered it was providing some current to the 5v rail. Seeing as it is bus powered it eliminates that thought.

Here's another thought. Does it make a difference which USB you plug into? My thought us that the USB power is shorting to the case and when you plug in you break the short.

JoHubb · 2 Jul 2014 at 21:44

Tealc said:
Here's another thought. Does it make a difference which USB you plug into? My thought us that the USB power is shorting to the case and when you plug in you break the short.

It makes no difference which USB port. Immediately I plug in the drive into any USB port the machine powers up and begins its boot sequence.

Bizarre :confused:

JoHubb · 2 Jul 2014 at 21:57

Partially solved. Your thought about USB providing power got me thinking.

The PC is attached to a powered KVM switch.This allows me to switch to another PC whilst using the same screen.

When I disconnect the USB feed from the KVM switch the powering up problem disappears. The PC will start in the usual way from the start switch.

Up till a few days ago the KVM switch behaved perfectly - it is a high quality ConnectPro switch and only 2 years old.

Seems something is shorting somewhere, or what else could it be?

Tealc · 2 Jul 2014 at 23:48

I'd now be looking at the PSU for the KVM switch. If it is poorly regulated it could be sending spikes into the 5v rail and for some reason this is negated by the USB device which might be acting like a capacitor to smooth out the power, or something.

If the KVM power supply isn't 5v then it won't be that and it could be something internal, maybe a bad cap or 5v reg.

Guesses of course.

JoHubb · 3 Jul 2014 at 21:06

The problem was a powered USB hub, hidden away under a desk. The lid had blown right off the casing. Gunk oozing out inside.

I've no idea how this created my weird start up problems.

Thanks for your help, Tealc, I'd never have sussed this without your inspired guesswork.

John