In Need of a Skilled Troubleshooter

[Fricardo]

I have a home-built system in a budget case. A while back the little plastic piece in the front usb port that keeps a usb cable from being inserted the wrong way fell out so that it was just the bare interconnects. Unfortunately I forgot about this and inserted a usb cable the wrong way (like an idiot) while the system was on. It immediately began smoking, and I pulled it out right away. The system seemed to be fine, so I kept using it. About an hour later, it shut itself off. When I tried to turn it back on, it would turn on but wouldn't post. I unplugged all the usb connections from the motherboard and everything non-essential like hard drives, but it's still the same story. The fans spin when you turn it on, but it won't post to the screen, and it can't be turned off in any way with the power buttons; you have to turn off the power supply.

Anyways, I think I fried one or more of the components. My guess is the motherboard was fried because that's what the usb port is directly connected to. My next guess is the cpu. It was overclocked, but not overvolted, the temps were fine (high 40s at full load) and it had been running like that for over a year. Any comments? I already tested the video card just because that was the easiest, and that's not the problem. Any suggestions on how I should go about finding what's wrong without buying a new component that I don't know I need? I can ask around, but I don't know if any of my friends have a 754 cpu or motherboard.

Any ideas or suggestions would be appreciated.

specs:
Athlon 64 3000+ (754) @ 2.4 GHz (stock 2.0 GHz)
chaintech vnf3-250 mobo
ati radeon 9700 pro
antec true power 430w
1x512mb 400MHz Kingmax memory

 

[Seekermeister]

I think that you are right about the MB being damaged, but if you want to be certain, first test the power supply with a DMM on all rails and connectors. If that is good, then remove the MB with the CPU and HSF installed. Check that the CPU is properly seated and firmly latched. Place the MB on a nonconductive surface. Install the video card, reseat just one stick of ram, connect to the monitor and PS, and short across the power pins on the MB. If it still doesn't post, since you are certain of the video card, try a different stick of ram. If still no joy, try a different monitor, if you have one. If none of this helps, then the motherboard is 99% probability of being the problem. The other 1% is the CPU, but if the fan is turning, there is no visible damage and it is properly mounted, I would really doubt that is the problem.

 

[Zap]

Perhaps the wires are still shorted out in the USB port? Try disconnecting them and seeing if it turns on. If not, my money would be on the motherboard.

 

[nineball9]

By inserting the USB backwards, you reversed the 5-volt power connector with the ground connector and reversed the data+ and data- connectors too. The shield, which was not reversed, is also ground. Reversing the data connectors might "fry" the motherboard's USB IC's, but I doubt they were affected. More likely, you shorted the 5V rail on your motherboard.

If you have the equipment and are a "skilled troubleshooter" yourself, you can make a quick check of your PSU's +5V rail. Assuming it's OK, start with a DMM (or scope if you prefer) and perhaps an ESR meter and follow the +5V circuit backwards from the USB connectors on the motherboard. Suspect you'll find something.

However, if you can identify the USB IC's on the mobo, you may be able to cross-reference them (with something like NTE's cross-reference) to identify their pinouts, and possibly, the internal circuitry. Try a quick resistance check with your DMM using a range that will not forward-bias a silicon PN junction on the IC's - this is kind of hit or miss with unknown IC's - but a low resistance reading may indicate a shorted IC. You might have to do this with units out of circuit - not a lot of fun unsoldering - soldering SMD devices.

good luck!