Busted computer

I spent the weekend doing computer repair in Santa Clara. Mrs. Bennett’s computer got itself into a mode where it wouldn’t boot. When the power was turned on, the fans all spun on the CPU, the video card, and the case, but it woudn’t complete the POST, didn’t make a beep, and didn’t sync the … Continue reading “Busted computer”

I spent the weekend doing computer repair in Santa Clara. Mrs. Bennett’s computer got itself into a mode where it wouldn’t boot. When the power was turned on, the fans all spun on the CPU, the video card, and the case, but it woudn’t complete the POST, didn’t make a beep, and didn’t sync the monitor. So no feedback from the monitor, and no beep codes. After swapping out this thing and that, I got it to work quite nicely, but the bad part wasn’t what I thought, so let’s have a contest and try and guess what the problem was.

7 thoughts on “Busted computer”

  4. The POST portion of BIOS shouldn’t need anything but power to the microprocessor, BIOS ROM, and a clear bus between the microprocessor and the BIOS ROM and the beeper I/O port. It doesn’t need good RAM or keyboard or videocard or SCSI or IDE to complete the POST.

    Since the fans spin, it’s probably not the power supply, so my guess is that one of the adapter cards was stomping on the microprocessor bus.

  5. POST

    When power is turned on, POST (Power-On Self-Test) is the diagnostic testing sequence that a computer’s basic input/output system (or “starting program”) runs to determine if the computer keyboard, random access memory, disk drives, and other hardware are working correctly.

    If the necessary hardware is detected and found to be operating properly, the computer begins to boot. If the hardware is not detected or is found not to be operating properly, the BIOS issues an error message which may be text on the display screen and/or a series of coded beeps, depending on the nature of the problem. Since POST runs before the computer’s video card is activated, it may not be possible to progress to the display screen. The pattern of beeps may be a variable numbers of short beeps or a mixture of long and short beeps, depending on what type of BIOS is installed.

    The patterns of beeps contain messages about the nature of the problem detected. For example, if the keyboard is not detected, a particular pattern of beeps will inform you of that fact. An error found in the POST is usually fatal (that is, it causes current program to stop running) and will halt the boot process, since the hardware checked is absolutely essential for the computer’s functions.

    http://whatis.techtarget.com/definition/0,,sid9_gci214577,00.html

  6. Turns out the problem was bad memory, in this case a DDR. There was no beeping, which was weird because back in the old days you’d get all the way through POST even if you had no RAM installed; the POST code is all in ROM and it tends to use registers instead of RAM to store results and stuff. But I guess in the newer computers the ROM code is copied into RAM and the addresses are mapped to make it all run faster, but if your RAM is totally hosed, it doesn’t run at all.

    One symptom that should have been telling was the Southbridge Ethernet (on the motherboard) lit up the 100 Mbps light on the switch, which means the power supply and the motherboard were basically OK, even though the powersupply looked ratty and one of its two fans wasn’t spinning. The CPU’s heat sink was all clogged with cat hair, and the die looked toasty, so I wasn’t confident about it all.

    Had I swapped out the cheapest part first – the RAM – I could have saved myself some trouble, but now I’ve got the parts for another Linux box, except for the video card and some RAM. No matter how fast I try and get rid of my junk, I keep getting more of it.

    Scott gets the prize here, which will be a brisket next time I’m in Texas.

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