Computer Question for Evan

[The Doctor]

Okay, here's what I've been doing. My sister's kid has this old "Barbie" computer which suddenly stopped working. It's an older saying, I would guess at the end of the Pentium II or beginning of the Pentium III era. Well, it sounded like a power supply to me. It uses a smaller power supplies than a normal computer, although all the voltages and the connector are the same as the standard ATX supply. So I tried a new ATX supply off of a Pentium 4 system, and it would not power up.

Well, I decide I should know a little bit more about what I'm working on. So, I find some pages telling me what the ATX pinout is, and how the whole thing works with the standby power the power good, et cetera. Well, I start checking the power supply, I find it is not putting out the power good signal, but the 5 V standby power is present. I put a few amps of load on the 5, 3.3, and 12 V lines, and ground the power on line. All positive and negative voltages are right on the money, and while it is turned on the power good line is also active. So, I decide to make a small modifications and tried again. I cut the power good line and ties the connector end of it to the 5 V standby line with a 10 k ohm resistor. Try to fire the machine up again, nothing happens.

Well, now I have a better(or just stupid) idea to try. I activate the power supplied manually by grounding the power on terminal. The lights and fans come on, and the hard drive starts spinning, but the monitor does not turn on and there is no sign of the hard drive being accessed. Trying to use the power switch on the computer does not help.

My questions are as follows:

Is it likely that I destroyed anything by forcing the power supply to turn on?

Is it common to see a power supply and motherboard fail at the same time?

Are there any fixable things that happen to a motherboard that can cause it to not turn on? I already tried powering it up with all drives on clogged, and by that I beam data and power lines so that the drives could not be causing the problem. It was not effective.

Thank you
Ed

[BillH]

The ATX power cables that plug into the motherboard are different than the older AT style, im pretty sure you dont want to mess that part up. older AT style had 2 plugs where the black wires on each connector had to be next to each other.(I think it was black)
I Think you just complete the circuit for the power on lead.

[Evan]

Forcing the power good line with the standby power will make the motherboard try to initialize before the power supply outputs are stable. In cheap supplies the power good line is usually just on a short time delay, on better ones they actually test the output until it falls within spec. Once power good is on then the MB starts the reset sequence. It is possible the power controller IC on the MB has failed.

It's possible for the supply and MB to fail at the same time but unusual. Have a look at the MB for failed bypass capacitors. There will be a bunch of small electrolytic capacitors scattered around, especiall near the CPU. They should have FLAT tops, usually with a score mark to allow them to explode gracefully

If even one has a bulging top then the MB is toast. This was a huge problem with computers made from around 2000 to mid 2003. (long story)

Bottom line is that there are very few things that can go wrong that are repairable except by replacement.

[3 Phase Lightbulb]

Unfortunately, I'm sure your original power supply is fine.

This might sound unlikely or ludicrous, but try changing the CMOS battery.

These mattel motherboards will not power up with a CMOS battery that is below a certain voltage. The CMOS battery powers a soft power circuit identical to the MACs. If the CMOS battery is low, you will not get ANY indication of life from the motherboard. It's a very bad design.

-3Ph

[The Doctor]

3 Phase, thank you for your suggestion. I would never, ever, have suspected a Dead CMOS battery was the cause of a computer not working. This thing was ready to go in the can when you reply. I checked the battery with the meter, 0 V. Took it out, charged it for about five minutes(yeah, I know better, but.....), put it back in with about 2.7 V. Computer fired right up and worked perfectly :-) Found five brand-new CR2032 batteries on eBay for less than five dollars delivered, and now it is living happily ever after with its new battery.

Interestingly enough, the power good output from the power supply doesn't seem to matter one little bit. You can tie TO 5 V, tie it to ground, or just leave it disconnected, and the computer still comes on immediately when you press the button. So, I removed my jumper and put things back to the way they were with the power supply.

Yes, I agree with you this is a terrible design. I suspect it was not an accident. I figure it like this, those batteries usually last for a couple years, long enough that the computer is starting to get old when they go bad. I think it is intended at the computer stops working in a few years, and people figure it is not worth fixing so they go buy a new one. Either that, or the engineers that designed that motherboard are the stupidest SOBs on the planet! The computer I use for a network server here at the house has had a dead battery since I got it, the only problem is ever caused me is that I have to reset the time if I reboot. Well, I guess it will now get one of the remaining batteries :-)

Thank you all for your help
Ed

[This message has been edited by The Doctor (edited 01-30-2005).]

[3 Phase Lightbulb]

<font face="Verdana, Arial" size="2">Originally posted by The Doctor:
3 Phase, thank you for your suggestion. I would never, ever, have suspected a Dead CMOS battery was the cause of a computer not working. This thing was ready to go in the can when you reply. </font>

Very few people know how these things really work. I'm surprised you did try replacing the CMOS batery because most people would not believe me.

<font face="Verdana, Arial" size="2">Originally posted by The Doctor:
Interestingly enough, the power good output from the power supply doesn't seem to matter one little bit. You can tie TO 5 V, tie it to ground, or just leave it disconnected, and the computer still comes on immediately when you press the button. So, I removed my jumper and put things back to the way they were with the power supply.
</font>

The power good output is not used on "PC" type systems. We use that signal on systems that have redundant power supplies like a good industral CompactPCI chasses. PC reset logic is fanned out by fixed delays that allow signals and decoupling caps to charge up and settle before the reset lines get deasserted. The power good output has nothing to do with the reset logic on a motherboard. The power good output on a "PC" is sometimes just tied to a 300ohm resister and a LED on the motherboard.

<font face="Verdana, Arial" size="2">Originally posted by The Doctor:
Yes, I agree with you this is a terrible design. I suspect it was not an accident. I figure it like this, those batteries usually last for a couple years, long enough that the computer is starting to get old when they go bad. I think it is intended at the computer stops working in a few years, and people figure it is not worth fixing so they go buy a new one. Either that, or the engineers that designed that motherboard are the stupidest SOBs on the planet! The computer I use for a network server here at the house has had a dead battery since I got it, the only problem is ever caused me is that I have to reset the time if I reboot. Well, I guess it will now get one of the remaining batteries :-)</font>

The battery should last a good 10+ years. I'm sure the manual for the computer says something about replacing the battery at a certain frequency.

-3Ph

[snowman]

DAMNIT...

Where was this topic say....four months ago when I trashed my girlfriends pentium 266 because I was sick of moving it.

It would have been a perfect computer for the shop! Still fast enough to run old AutoCad and turboCNC.

-Jacob