This lovely, small fellow has been brought by one of our customers after some failed repair at one of Birmingham’s computer shops. Manufactured in 1992, it was working perfectly fine until a small liquid has been spilled on the keyboard and partially on the trackball.
Computer was starting up with chime, however, there was no picture on the screen and no HDD noise. Customer stated that after an accidental spillage he could hear the chime and noise of a spinning HDD (this model does have a very capacious 120MB IBM drive). Unfortunately, the previous repair company gave up on this repair after over 4 months, so first of all we opened it to see what had been done by the previous company. We quickly discovered that there is still a residue of some substance on the logic board (how could that be overlooked?):
We cleaned a whole logic board to prevent from further corrosion spreading. Unfortunately, after reassembling, the computer was still not willing to cooperate. Logic board had been pulled out again and examined carefully under the microscope (especially the area which had a liquid residue), and we quickly discovered something suspicious – one of the vias was looking slightly different than others:
We checked with the schematic diagram that this via connects pin 3 (source) of Q7, dual P-Channel MOSFET (IRF9953), with a cathode of D22 (10BQ100T), which provides 5V for the logic circuit and… for the HDD. Measurement of continuity has uncovered that there is no electrical connection between cathode of D22 and pin 3 of Q7. That was a turning point of our diagnostics – we have restored that broken via to get the proper 5V voltage on pins 5 and 6 (drain) of Q7.
We removed a soldermask from the surrounding area of broken via (both sides) and put a silver-plated wire into the hole.
Then we were able solder the wire to restore an electrical connection between both sides of PCB. We had to move D22 diode slightly away of that via.
After restoring that broken connection, we moved D22 diode back in place:
The logic board had been connected to our bench power supply and we realised that it takes more current than before the repair. That was a good sign so we decided to reassemble the computer – it started up, loaded the operating system and we could watch this beautiful piece of electronics back in action!
It is always worth to examine the logic board under a microscope – especially if the computer had been liquid spilled. All signs of corrosion can be easily found if you do not attempt to limit the time of a visual inspection.