Haunted System

My son-in-law gave me his computer to fix for him when it wouldn’t start after he’d pushed the power button. It’s an old PC Partner M/B with an Intel i815 chipset, socket 370 Pentium 700mhz and 128mb ram.

After testing all components in another system and finding them to be ok, I put everything back together and assumed the problem to be with the motherboard and was prepared to write it off.

After hitting the power switch again and getting no response, I left the machine hooked up, with the monitor turned on and went off to do something else. When I returned half an hour later, I was amazed to see that the computer had powered on and was asking for any key to be pressed to proceed with bootup. I pressed the spacebar and it booted up like nothing had happened.

After switching the system on and off several times over a period of three days with no problems, I gave it back to my son-in-law. He used it once only before the same thing happened again. I took it back, checked the power switch, reset cmos by removing/replacing the battery, and still getting no response, I gave up in exasperation. Twenty minutes later, the bloody thing powers itself on again and has been restarted several times without any problem.

Any ideas anybody?

 

These symptoms may indicate a bad contact somewhere in the power supply (or power button). This would also explain why moving the computer makes the problem occur again. You could use some contact cleaner on the power supply and/or power button or replace it.

 

The likely culprit is the softstart circuit. PC power supplies use a switched ground through the start-button to enable the start relay. For the softstart circuit to work, both relay contacts have to close to enable current to flow from the mains through a thermister and through the rectifier to fill the large capacitors, just ahead of the switching transistors. This thermister limits the in-rush current with very high resistance until it heats up. The heating, then lowers the resistance of the thermister to very nearly that of a standard fuse. The relay contact that is in series with the thermister is damaged(burned). It is allowing only a portion of the expected current to flow, so it is taking a longer time for the thermistor to heat-up. However, once the thermister heats up, the rise in voltage across the damaged(burned) contact allows the power supply to start up normally. A technician or astute hobby-ist could effect a repair. However, finding another power supply will probably be best.
.

 

Thanks for the suggestions fellas. I haven’t tried contact cleaner yet, so I’ll give that a try. Your diagnosis was very interesting Prometheos, and I’ll keep it in mind for future reference, but I neglected to mention that my first thought was a faulty PSU so I tried a new unit first thing with no result. Also tried a replacement processor.

It’s an old system and probably not worth the trouble, but my curiosity has been aroused so that now I’d just like to find out what’s wrong with the damn thing.

For the moment it’s working fine, but I’ll clean all the contacts and leave it for a few days before trying to start it again. Maybe it just needs the sledge hammer treatment.