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Quick Psu Question // Dvd Rom


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#1 Ron Devito

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Posted 22 July 2008 - 06:04 PM

I had a pal replace all the bad caps on an old Soyo Dragon mobo. Mobo works fine. I had a PSU from an old machine so just for testing I popped it in, and fooled around with it last Saturday. So far, so good. I shut the machine off for the night and came back to it yesterday. Now...the PSU has an on/off switch in the back. I had left that on, but the machine was turned off.

OK...I go upstairs and the room is boiling hot. Fine. I turn on the AC. I light the machine, it starts to boot and shuts off. Hmm...I light it again. CPU fan spins, HDD and DVD ROM spins, but no video, no beeps. I shut off. I put my hand on the PSU. You could fry eggs with this thing.

I'm not going to ask the question to which the answer is obvious. I know the PSU is fried. Third-railed. When it's that hot, I know it's over. My question is....why would it overheat when the machine was verifiably shut down for two days?

In the same machine, my DVD rom will boot DVDs, but not CDs, even though it's supposed to read CDs. This is an old TDK piece of junk unit. Any reason for it not booting to CD but booting to DVDs?

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#2 aommaster

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Posted 25 July 2008 - 02:12 PM

I'm not going to ask the question to which the answer is obvious. I know the PSU is fried. Third-railed. When it's that hot, I know it's over. My question is....why would it overheat when the machine was verifiably shut down for two days?


Only reason I can think of is a short circuit.

In the same machine, my DVD rom will boot DVDs, but not CDs, even though it's supposed to read CDs. This is an old TDK piece of junk unit. Any reason for it not booting to CD but booting to DVDs?

CD's and DVD's need to have something called a "Boot Sector" in order for it to be bootable. If they do not have that, then the media will not be booted.

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#3 hamluis

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Posted 25 July 2008 - 02:19 PM

There always power going to the PS...as long as the PS on-off switch is in the ON position and the PS is plugged in to an active/live power source. That's why some of us see those little lights on in our systems even when they are shut down (but still properly connected to a power source). Well...I guess you wouldn't see them if you have a case which does not permit viewing the innards or which has those all-metal system covers.

The excessive heat would indicate a faulty PS to me.

Louis

#4 Ron Devito

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Posted 30 July 2008 - 02:51 PM

Louis and Aonmaster,

Thank you.

Truth be told, the PSU was actually one I cannibalized out of an old Pentium II (that's right -- a PII) machine. I had used it once before to lifeboat this machine when that PSU blew up. Even so, this was a P4 with 2.5 GB, on HDD a DVD burner, and a GeForce 5700 card (using VESA drivers) -- no real demand here. I was just baffled as to how this PSU could go into a thermal runaway with the machine shut off, but since it was drawing power (the machine is open, so I saw one LED on the mobo lit), and since the room was boiling hot, and since the PSU was 10 years old -- well, it had to go sometime. No biggie.

The DVD situation is a little more mysterious. The CDs are in fact bootable. One is a Mandriva disk, the other was an Ubuntu disk. The device would boot DVDs, but not CDs. The CDs worked in other machines (bootable). I'm not going to sweat the DVD situation either. The DVD is and old TDK piece of junk -- I can replace it with a nice LiteOn unit for under $30, so I'm not going to spend a crazy amount of time trouble-shooting it. I was just wondering about it.

#5 hamluis

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Posted 30 July 2008 - 03:53 PM

Feedback appreciated, happy computing :thumbsup:.

Louis

#6 Platypus

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Posted 31 July 2008 - 04:54 AM

In soft-off state (ie computer shut down but mains power switch still on) a PS/2 power supply shuts down the SMPS (Switched Mode Power Supply) circuitry that provides the regular 3.3, 5 and 12V rails. The mains rectification and filering components are still active, the SMPS circuitry is still connected (just not running) and the standby 5V rail is provided by a small switched mode circuit. This is what causes the LED to remain lit on some boards (memory must not be changed in this situation to avoid possible damage). It enables the keyboard, mouse, LAN or any other device that needs to be able to wake the system from standby to still function. The small amount of heat generated by these still active components is normally dissipated just by air convection, as no fan is running. If a fault develops in any of these areas, causing increased heat generation, this will build up over many hours due to the absence of air movement, and has been known to become severe enough to cause fires. That's why the safety advice is not to leave unattended computers in standby mode.

The device would boot DVDs, but not CDs.

The unit utilises a dual laser - CD and DVD have different sized reflective dots that are read by the laser, so need a different wavelength. The section of the laser used for CD will have failed.

Edited by Platypus, 31 July 2008 - 04:58 AM.

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