CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [152]
Figure 10-38 Testing one of the 5-volt DC connections
Do not panic if your power supply puts out slightly more or less voltage than its nominal value. The voltages supplied by most PC power supplies can safely vary by as much as ±10 percent of their stated values. This means that the 12-volt line can vary from roughly 10.5 to 12.9 volts without exceeding the tolerance of the various systems in the PC. The 5.0- and 3.3-volt lines offer similar tolerances.
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NOTE Many CMOS utilities and software programs monitor voltage, saving you the hassle of using a multimeter.
Be sure to test every connection on the power supply—that means every connection on your main power as well as every Molex and mini. Because all voltages are between –20 and +20 VDC, simply set the voltmeter to the 20-V DC setting for everything. If the power supply fails to provide power, throw it into the recycling bin and get a new one—even if you’re a component expert and a whiz with a soldering iron. Don’t waste your or your company’s time; the price of new power supplies makes replacement the obvious way to go.
No Motherboard
Power supplies will not start unless they’re connected to a motherboard, so what do you do if you don’t have a motherboard you trust to test? First, try an ATX tester. Many companies make these devices. Look for one that supports both 20- and 24-pin motherboard connectors as well as all of the other connectors on your motherboard. Figure 10-39 shows a power supply tester.
Figure 10-39 ATX power supply tester
Switches
Broken power switches form an occasional source of problems for power supplies that fail to start. The power switch is behind the on/off button on every PC. It is usually secured to the front cover or inside front frame on your PC, making it a rather challenging part to access. To test, try shorting the soft power jumpers as described earlier. A key or screwdriver will do the trick.
When Power Supplies Die Slowly
If all power supplies died suddenly, this would be a much shorter chapter. Unfortunately, the majority of PC problems occur when power supplies die slowly over time. This means that one of the internal electronics of the power supply has begun to fail. The failures are always intermittent and tend to cause some of the most difficult to diagnose problems in PC repair. The secret to discovering that a power supply is dying lies in one word: intermittent. Whenever you experience intermittent problems, your first guess should be that the power supply is bad. Here are some other clues you may hear from users:
“Whenever I start my computer in the morning, it starts to boot, and then locks up. If I press CTRL-ALT-DEL two or three times, it will boot up fine.”
“Sometimes when I start my PC, I get an error code. If I reboot, it goes away. Sometimes I get different errors.”
“My computer will run fine for an hour or so. Then it locks up, sometimes once or twice an hour.”
Sometimes something bad happens and sometimes it does not. That’s the clue for replacing the power supply. And don’t bother with the voltmeter; the voltages will show up within tolerances, but only once in a while they will spike and sag (far more quickly than your voltmeter can measure) and cause these intermittent errors. When in doubt, change the power supply. Power supplies break in computers more often than any other part of the PC except the floppy disk drives. You might choose to keep power supplies on hand for swapping and testing.
Fuses and Fire
Inside every power supply resides a simple fuse. If your power supply simply pops and stops working, you might be tempted to go inside the power supply and check the fuse. This is not a good idea. First off, the capacitors in most power supplies carry high voltage charges that can hurt a lot if you touch them. Second, fuses blow