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I'm rebuilding a vintage PC (Pentium 133) and when I power on the computer, power supply fan, HDDs starts but not the CPU fan. The screen also stays black (orange light). After like 15-30 secs, the CPU fan start to spin and you can hear the starting "beep" and the memory test finally show up on the screen.

It's strange that (most of) everything start up right away on the motherboard except the CPU... Fist time I see this. If I ctrl+alt+del, it does the same thing the PC "freeze" for like 20 secs before showing any text on the screen (BIOS/Post).

The power supply is new (it's a Corsair cx400w). I have tested 5 different memory sticks and I get the same results every time. I tested 2 CPUs, same thing. Tested up with no PCI / ISA / IDE with no success. I aslo just changed the CMOS battery with a new one.

Anyone have an idea why I have this behavior? What can cause this?


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Are there fan speed settings in the bios that could be relevant? – RJFalconer Sep 6 '10 at 13:57
@RJFalconer No, I haven't seen any fan settings for the CPU in the BIOS. – AlexV Sep 7 '10 at 15:47

Since it's old PC, maybe some of the capacitors on the motherboard have failing? In some cases several on/off cycles may be needed to activate the computer. It's possible that they could need long time in order to start working properly. This could be the problem if the screen stays blank until the beep sound.

Can you tell us the year in which computer was made? And how do the capacitors look? Any signs of leaks or bulging tops or anything similar?

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In fact I just bought all those separate pieces on eBay. The motherboard is an EPOX EP-MVP3G5 (1999-2000). The seller told me it was new (old) stock. All seems new/unused. I just changed the CMOS battery right away (with the same brand - Sony CR2032 3V Lithium). – AlexV Sep 6 '10 at 16:55
@AlexV It could be affected then. Capacitors can lose their capacity by just being in storage, but I don't have any sources for that right now so I can't say if the amount of capacitance lost is significant enough to produce symptoms you described. You said that the board looks new, so it probably isn't the plague. Just to be safe, read this article: and compare capacitors on your board to capacitors in pictures. – AndrejaKo Sep 6 '10 at 17:25
@AlexV I found a some mention of degradation of electrolytic capacitors. Apparently, they degrade more when they are not in use. – AndrejaKo Sep 6 '10 at 18:24
Watched all capacitors on the motherboard and they look brand new. No leak, all in the right shape. Thanks for the links, I didn't knew about that problem. – AlexV Sep 7 '10 at 12:24
@AndrejaKo Cooling is not an issue as I have a newer case with this old computer and have high end fans even if not needed in this situation. I think I will have it "charge" for 24/48h to see if it does anything... – AlexV Sep 7 '10 at 15:56

If the POST messages are delayed too then it means the entire boot sequence is delayed so maybe the CPU fan starting later is in fact the effect to that and not the cause. What's the state of the BIOS chip, battery?

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I just changed the CMOS battery with a new one expiring in Aug 2019... I agree that the entire boot sequence is delayed, what I find strange is that the HDDs start right away. – AlexV Sep 7 '10 at 15:47
I guess they're just powered earlier. Do they actually start making "reading/writing" (so to speak :)) sounds? – CyberDude Sep 9 '10 at 15:10
Yes you can ear some I/O noise right away... – AlexV Sep 14 '10 at 0:19

If a floppy drive is connected (3,5inch should be enough for a vintage pentium pc :-), check if the connector is upside down. This will sometime delay or even abort the boot process.

Does anything show up, if no HDD is connected?

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Connector is OK when floppy is connected :) Floppy works. – AlexV Sep 6 '10 at 16:52
The floppy light stays on all the time, if the connector is upside down. For the other question: Does anything change, if no HDD / IDE is connected? – MaoPU Sep 7 '10 at 7:49
Yes I know that :) Always put the pink wire the same side as the power line of the floppy :) As far as I remember when no floppy and no HDDs are plugged in, it never starts. The HDDs starts, but the screen light stay orange all the time... – AlexV Sep 7 '10 at 15:46
@AlexV If no IDE is connected, at least the BIOS promt should show up and report an error. Well, its probably something else. Just wanted to clear out unusual cases I've seen... – MaoPU Sep 7 '10 at 21:16
Indeed when I plug nothing the BIOS report errors on the screen after the 30 secs "warm up". This is the delay the problem... – AlexV Sep 8 '10 at 11:38

Perhaps the fan only comes on when the cpu reaches a certain temperature threshold.

As it's such an old chip, could it be that the fan behaviour continues even after a quick reboot because the heat dissipates so quickly? Seems like a plausible theory.

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I don't think so because the screen stay OFF (save mode) until the CPU fan starts. As soon as the CPU fan starts, the screen opens (orange light to green light) and POST text shows up. – AlexV Sep 7 '10 at 15:45

I don't remember P133 ever having variable speed fans. Is the fan powered from the motherboard? If so do you have an adapter to power it from one of the molex connectors?

Have you tried with the IDE cables removed as well?

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Yes the CPU fan is powered from the motherboard. I've tried to have the CPU fan powered from power supply directly. When I do so, the CPU fan start right away when I power on, but screen power light stay orange for like 20 secs still. I've also tried with all IDE removed same thing. – AlexV Sep 7 '10 at 15:45

What video card/chipset do you have? I've seen slow startups on older PCs which have both onboard video and an expansion video card--the BIOS first activates the onboard video (invisibly since there is no monitor there), then the expansion card.

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I use a Tseng Labs ET6000 (PCI). There is no onboard video on the motherboard (in fact there is no onboard anything on this one!). The BIOS is set to start with PCI video 1st (and not AGP). – AlexV Sep 7 '10 at 15:46

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