Frequently Asked Questions Index:
1. How do I test a power supply while it is out of the case?
Reader: How do I test a power supply while it is out of the case?
Computer Doctor: That depends on what you mean by test. No matter what, the power supply will not run without a load. PC Power and Cooling http://www.pcpowercooling.com makes an ATX Power-Tester that supplies a load to one 5V line and then lights an LED if the power_good signal trips. This basically tells you whether the supply is dead or not.
To test the power supply thoroughly, you'd want to load all of the 3.3V, 5V, 5VSB (standby) and 12V lines to the maximum ratings specified by the power supply manufacturer. you'll find information on how to construct power supply loads using common automotive bulbs or heavy duty wirewound resistors.
2. Can the fan on a power supply be disabled while my computer is running?
Reader: When using Windows 98's Power Management, my hard disk, monitor and CPU fan can be turned off after a while but not the fan of the power supply. Is there a way to turn off the power supply's fan?
Computer Doctor: The power supply fan is wired to run continuously; there is no way to turn it off, unless you custom modify the unit (not recommended) by adding a switch, or perhaps wire up a thermostatic or other type of circuit to control it. If you're concerned about noise, there are power supplies that are specially designed with extra-quiet fans. Check out the "Silencer" series of supplies at http://www.pcpowercooling.com.
3. How can I verify the good operation of a power supply with a multimeter?
Reader: How can I verify the good operation of a power supply with a multimeter?
Computer Doctor: By using a multimeter, you can check the output voltage. For this to work, the power supply must be connected to a load, meaning a motherboard and at least one hard drive. More of a load is better so it's best to test the supply while fully installed in a system. Use the meter to backprobe the connector terminals and check voltages between the terminals and the chassis ground. All voltages should be within plus or minus 5% of the rated amount. Grounds should register LESS than plus or minus 10 millivolts (thousandths of a volt).
To properly verify functionality a power supply manufacturing company uses a load tester which could switch in different load amounts, and also a scope would be used to verify the cleanliness of the signals.
Lacking this type of equipment, the best test device a technician can have besides a voltmeter is a "known good" spare. That means a high quality replacement supply you know works, and which can be swapped in place of the suspected bad unit to verify system operation.
Bad power can cause all kinds of symptoms, which is why it is good to swap in a known good spare if you suspect the power supply. Bad power can make you think your processor, motherboard, memory, drives, etc. are failing, when in reality it was the power supply all along.
The power supply is by far the single most failure prone component in a PC. It has probably 10 times the failure rate of most other components, maybe more. I replace more power supplies than all other components combined.
4. Can a bad power supply be the culprit for a lot of otherwise unexplained problems?
Reader: I was reading in your book that a cheap power supply can cause alot of problems and I live in a house that is not wired to today's wiring standards as far as grounding is concerned. Could this cause all types of intermittent problems, too? I also have many power drains due to the wiring in- efficiencies.
Computer Doctor: It could be the cause of the problems, but not if the errors you get are repeated and consistent. I think you are having other problems. Try removing items from your system and reconfiguring until it becomes stable. Try also playing with the timing, reset the CPU speed to 200MHz for example and try slowing down the memory timing. If these things help then you have an unstable or bad motherboard or CPU.
5. If there are no markings on the connectors, how do I determine how to plug them in?
Reader: How do I determine how to plug the connections between the power suplly and the motherboard? I have about the most generic of each - a 'Slimline' power supply with 2 6-pin connectors labelled P8 and P9, and 3 motherboards between 386 and Pentium socket 7. Each board has the 12 pins in a straight line, with no clue that I can read as to which goes where. Theres no keying either, apparently. I've tested each connector, and they provide the voltages described in the section on power supplies, but it seems to me that connecting them the wrong way around would be a good way to fry the system.
Computer Doctor: Ahhh yes, that keying was eliminated by cheaper system mfrs. to save money. The original IBM connectors were all keyed. The 8th edition has better diagrams for this, but I think they were in the 6th ed. also. If you look at the power supply pinout table, you will see that the P8 and P8 connectors each have 6 pins, each with two grounds. The connectors should be plugged in such that the grounds are adjacent on each of the connectors. "Black to Black" is the rule. If you plug them in backwards, or off by even one pin, you will destroy the motherboard the second you turn on the power! This is one reason the ATX boards are superior, they use a single keyed power connector that you can't mess up.
6. How do I set the jumpers for my voltage regulator?
Reader: I have purchased a voltage regulator. When I started to install it, I noticed two jumpers beside the CPU called the "CPU Voltage Regulator Output Selectors" which can be set to 3. 3 volts or 3. 5 volts (default). Now, when I install the voltage regulator and CPU, what do I do with the jumpers?
Computer Doctor: Those jumpers will become ineffective when the VRM is installed. In other words, it doesn't matter how they are set. When the VRM is installed, it will be supplying the dual plane voltage (2.8/3.3) to the CPU, not the motherboard.
7. Could a black screen and a long beeping noise at power up mean problems with the power supply?
Reader: After reassembling my PC, I got a black screen and the PC would make long beeping sounds off and on (Beeeeeep-beeeeeep-beeeeeep-beeeeeep) at startup. I switched it back off and checked all the cables and they were all ok, so I turned it back on and it worked properly. It did this a few times. Could this have something to do with a bad power supply?
Computer Doctor: Most standard PC power supplies are terrible. Startup errors like this are symptoms of substandard supplies. I recommend Astec or PC Power and Cooling. You might consider a high quality replacement from one of those companies.
8. After installing a new HDD my computer, it will restart seemingly at will, please help.
Reader: Recently I installed a second HDD into my computer. After a week of usage, my computer would randomly reboot, as though I was pressing the reset button every now and then. What could be the cause of this?
Computer Doctor: Normally, problems like this are caused by overloaded or defective power supplies. If that is indeed the case, then replacement of the supply with a better (higher output and more reliable) unit is what I recommend. See http://www.pcpowerandcooling.com for some of the best power supplies in the business.
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