Help with no-boot diagnosis

[kranky]

It's one of those deals where a friend called me to fix their PC. Normally I would beg off but they are hurting for money and can't really afford to take it somewhere.

PC is about 3 years old. Was working fine until they plugged their printer into the USB port with the PC running. At that point the PC locked up. They pressed the power button to restart the PC but nothing happened.

I went over to check it out. With the power cord connected, there's a green LED on the mobo that is lit. When I press the power button, the PSU fan and the CPU cooling fan both start running, and the hard drive hums a bit, but nothing else - no beep, nothing on the monitor at all. If I hold the power button in for four seconds, it will power down.

Any ideas where to look? Thanks in advance.

 

[acole1]

start taking out system devices like video, ram, all attached devices and see if it will post and beep then.

if it does start adding stuff back in to narrow down a possible problem.

(it actualy sounds like it maybe video?

 

[furballi]

Reset CMOS. Retest. If problem persists, then test the MB outside of the case. Reset RAMs. Disconnect all devices from the MB including PCIs and USBs except for the power connection(s) and the HDD. Reset CMOS. Short the power pins and check for proper operation. Still no boot? Try a working PSU. Also try known good RAM.

 

[Jiggz]

OK, I know this sounds stoop but did they already unplugged the printer from the USB and tried starting again? Although I do not see any problem in plugging a USB with the PC running since this is normal and by design. If that was already done, take a close look on the USB port to make sure the contacts are looking normal. It could be that when they plug in the printer the contacts got shorted and tripped the PSU. If the contacts are messep up, straighten them up with a tiny screw driver and tape over the port since it's no good anymore. Try again. No go? Next is to disconnect all external peripherals, remove all add in cards except for the video card, and ram. Make sure the monitor is connected to the video card and use only a stick of ram. So you should only have KB, Mouse, video card, ram and monitor. Now boot. Did it post? If it did shut it down and reconnect the hdd and then boot each time until all the peripherals are back or the problem shows up again. I suspect either a bad mobo or PSU.

Oh by the way, when you held down the power button for 4 secs or more, yes that is normal it will shut down. You shouldn't have to hold the button down to power up. A single push is all it takes and it should power all along. Now if the system is not powering or maintaining power after pushing in the power button, you might want to try another PSU. If the same problem persists then the mobo is bad.

 

[velis]

In my company we lost 4 MBs to USB devices this winter. That's out of 13 computers total. Heating makes the air dry which allows for some good static to build up. As soon as you tuch the USB connector with a USB device cable, the mobo will fry beyond repair. We had to replace all of them
It could be the same thing happened to your friend.

 

[Ausm]

I would reset the Cmos personallyand see if that helps.


Ausm

 

[furballi]

The grounded AC outlet should take care of the static charge issue. The root cause is probably the current surge caused by the USB device. The rated output for each USB port is 2.5 watts. A poorly engineered USB product may draw a lot of current when first connected to the USB port, causing damage to the MB.

One way to minimizing this problem is the use of a POWERED USB hub. I would also connect/disconnect USB devices when the PC if OFF. Hot plugging is okay if you're working with low-powered devices like mouse or keyboard.

 

[Matthias99]

The grounded AC outlet should take care of the static charge issue

Um, no. Just because the computer is 'grounded' doesn't mean you can't fry components (especially connectors) with static discharge. The discharge temporarily spikes the voltage in the part of the circuit up into the kilovolt range, which can destroy diodes or small resistors/capacitors.

Now, if you have a device plugged into a grounded powered hub, you're probably OK, since it being grounded through the hub would probably keep it from building up a large static charge in the first place. But that doesn't mean that anything that is grounded is immune to static electricity.

 

[Luckyboy1]

Many of the cheaper mommaboards used in the supposedly "name brand" computers have little power available for the USB's. Combine this with the cheapest of power supplies and the fun is bound to happen sooner or later.

Reset the CMOS, but not until you've unplugged everything USB! Then if it boots, I'd suggest a powered UBS hub.

 

[furballi]

The outside of the USB plug is shielded. This is the first part that touch the MB. When this part makes contact with the MB, it is at the same voltage potential as the MB and case, which should be at the same potential as earthground if the AC and PC are properly wired. This happens at the speed of light.

Explain to me how it is possible to fry components via static build-up when the system is properly earth-grounded. There will always be a short period of time when the current will spike when one applies power to a device. If the impedance of the device is very low, then it is possible to damage the power circuit on the MB.