The RJ Journal - Electronics - Repairblog

2007-09-27

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2007-03-30 Fujitsu Siemens tower PC computer
This started to take very long time to start up and when doing so it continuously beeped. I first thought the beeping was the bios that checked the memory but then again not, since it never did this before. Later on, it not only took a long time to start up but it also restarted a couple of times during the startup. I then began to look at it more closely and saw that the video didn't come on until the beeping stopped. It tuned out that the beeping was in fact the initial beep the bios does right after a reset so it seems as if the power good signal got up, the mother board went out of reset but this caused the voltage to drop which caused a new reset and so on. This continued until something made the voltage to stabilize, probably the heat. This could either be the mother board or the power supply (I had fixed a SMPS voltage regulator on this mother board before). I tried the simplest first, which was to change the power supply with one from an older, unused computer. This turned out  to work so it was apparently the power supply that was broken. Probably would have needed to replace some electrolytic capacitors.

2007-09-27 Update
It turned out to that it wasn't the power supply after all. The computer didn't start one day so it was time for a new diagnose.

There where 3 electrolytic capacitors, 1500uF each, that looked a bit swollen. The capacitors where connected in parallel in a switchmode powersupply circuit for the low voltages to the CPU. When measured, their value was only about one fifth of their marked value.
 

Of course, I didn't have a direct replacement for the capacitors so I substituted the 3 original ones with 5 that had 1000uF each. It was a bit hard to get them in place but since to the PCB was made to take two different sizes if the capacitors and had two holes for one of the legs it was possible to get them in place. This is not the best way to mount capacitors for a high frequency switching circuit since the extra lead length can result in a higher impedance at some frequencies.

It seems to have worked though. Again, time will tell if this is a permanent solution. Hopefully it will last for the rest of this motherboards lifetime. It is already starting to get a bit old.

When the capacitors where replaced, the original powersupply worked OK again. It was probably the capacitors in combination with an output voltage on the lower side that made it fail in the first place. Using another powersupply with a little bit higher output voltage may have been enough then.

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