A friend of mine brought some 3-4 old motherboards and said “They are all not working. If you could repair at least one of them, I’ll be very grateful to you”.
Except one old Pentium 4 mainboard, others were P3 mainboards, I told him they are not worth to repair, you have to throw them away. But tried to diagnose the P4 one even I’ve never repaired mainboard before.
With installed memory and VGA card, connected it to an ATX power supply and it beeped several times in certain interval like there is no memory installed at all. I cleaned the memory slot and gold contacts of RAM memory itself but it’s still the same. Then I found single big SMD transistor near memory slots and checked with DMM. To my surprise it tested short between the legs. I removed it and tested off board.
My previous testing was true, it shorted all through. I replaced it by same transistor with prefix APM from an old mainboard. After that the mainboard came to life. My friend was very pleased to return home with working motherboard in such a short period, it took just 15 minutes.
From my PC troubleshooting experiences I would say this problem was related with incorrect handling of RAM memories. One tries to push down RAM memory in wrong direction, some do it while the PC is still connected to power supply. Also there are people who even try old DDR1 type memory into DDR2 type slot.
Any of the above could bring the motherboard into above symptom, in worst case even more damages. Well that was an easy case of motherboard repair for my luck.