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Shorted PC - need help


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#1
evalente

evalente

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I decided to pop out a troublesome DVD drive and fix a mechanical problem. I did this on the fly, while my wife was using the PC - a 5-6 year old HP/Compaq tower. All was well until I tried to plug in the 4-prong connector to put the drive back in. It was backwards of course, so I had a pop and a flash of light and the system went dead. Now of course it won't turn on.

Any ideas on what exactly needs to be fixed at this point? I'm not sure if it blew a capacitor, or if this falls into the category of "frying the motherboard".

Any advice is appreciated.

Thanks!

Edited by evalente, 03 November 2009 - 08:36 PM.

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#2
Digerati

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I sure don't know what would possess anyone to mess with live electronics thinking it would be safe. You do understand that anything that plugs into the wall can kill, right?

If lucky, all you did was destroy the power supply. My advice is to take the computer to a shop and let a professional check it out.
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#3
evalente

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Thanks for the tip. I'll have it checked out.

It was not a smart move, but I rationalized it to be about as safe as screwing in a light bulb to a plugged in/switched on lamp.

I didn't rationalize it well enough.
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#4
Digerati

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It was not a smart move, but I rationalized it to be about as safe as screwing in a light bulb to a plugged in/switched on lamp.

I didn't rationalize it well enough.

Not sure I would call that a safe move either - light bulbs are not sophisticated electronics devices so the comparison was probably an apples and oranges thing. Plus I note light bulbs are directly connected to deadly voltages. I had a 250W bulb burn out in a ceiling fixture years ago. I left the power on so I could see what I was doing by the other lights. The new bulb blew up in my face when I screwed it in and besides hot glass flying in my face, I almost fell off the ladder. Not a fun day. New shop policy after that - power off and use a flashlight.

Below is my canned text on testing PSUs. Hopefully, it is only the PSU. Below that is my canned text on sizing and selecting a new PSU.

To properly and conclusively test a power supply unit (PSU), it must be tested under various realistic "loads" then analyzed for excessive [url="http://en.wikipedia.org/wiki/Ripple_(electrical)""]ripple[/url] and other anomalies. This is done by a qualified technician using an oscilloscope or power analyzer - sophisticated (and expensive) electronic test equipment requiring special training to operate, and a basic knowledge of electronics theory to understand the results. Therefore, conclusively testing a power supply is done in properly equipped electronic repair facilities.

Fortunately, there are other options that are almost as good. I keep a FrozenCPU Ultimate PSU Tester in my tool bag when I am "in the field" and don't have a good spare power supply to swap in. While not a certain test, they are better than nothing. The advantage of this model is that it has an LCD readout of the voltage. With an actual voltage readout, you have a better chance of detecting a "failing" PSU, or one barely within specified ATX Form Factor Standard tolerances. Lesser models use LEDs to indicate the voltage is just within some "range". These are less informative, considerably cheaper, but still useful for detecting PSUs that have already "failed". Newegg has several testers to choose from. All these testers contain a "dummy load" to fool the PSU into thinking it is connected to a motherboard, and therefore allows the PSU to power on, if able, without being attached to a motherboard - great for testing fans, but again, it is not a true load or suitable for conclusive testing.

As mentioned, swapping in a known good supply is a tried and trued method of troubleshooting used for centuries, even by pros. Remove the "suspect" part and replace with a "known good" part and see if the problem goes away.

I do not recommend using a multimeter to test power supplies. To do it properly, that is, under a realistic load, the voltages on all the pins must be measured while the PSU is attached to the motherboard and the computer powered on. This requires poking (with some considerable force) two hard and sharp, highly conductive meter probes into the main power connector, deep in the heart of the computer. One tiny slip can destroy the motherboard, and everything plugged into it. It is not worth the risk considering most multimeters, like plug-in testers, do not measure, or reveal any unwanted and potentially disruptive AC components to the DC voltages.

And remember, anything that plugs into the wall can kill. Do not open the power supply's case unless you are a qualified electronics technician. There are NO user serviceable parts inside a power supply.

***

Use the eXtreme PSU Calculator Lite to determine your power supply unit (PSU) requirements. Plug in all the hardware you think you might have in 2 or 3 years (extra drives, bigger or 2nd video card, more RAM, etc.). Be sure to read and heed the notes at the bottom of the page. I recommend setting Capacitor Aging to 30%, and if you participate in distributive computing projects (e.g. BOINC or Folding@Home), I recommend setting TDP to 100%. These steps ensure the supply has adequate head room for stress free operation and future demands. Research your video card and pay particular attention to the power supply requirements for your card listed on your video card maker's website. If not listed, check a comparable card (same graphics engine and RAM) from a different maker. The key specifications, in order of importance are:
  • Current (amperage or amps) on the +12V rail,
  • Efficiency,
  • Total wattage.
Then look for power supply brands listed under the "Good" column of PC Mechanic's PSU Reference List. Ensure the supplied amperage on the +12V rails of your chosen PSU meets the requirements of your video card. Don't try to save a few dollars by getting a cheap supply. And don't count on supplies that come included with a case. They are often underrated, budget or poor quality models "tossed in" to make the case sale. Digital electronics, including CPUs, RAM, and today's advanced graphics cards, need clean, stable power. A good, well chosen supply will provide years of service and upgrade wiggle room. I strongly recommend you pick a supply with an efficiency rating equal to, or greater than 80%. Look for the 80 Plus - EnergyStar Compliant label. And don't forget to budget for a good UPS with AVR (automatic voltage regulation), as surge and spike protectors are inadequate.
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