This week I was busy with family visiting me, but today I found some time to revisit a computer I looked at quite some time ago. I have an Amstrad 386sx 16Mhz “laptop” that stopped working. I tried to fix it here, but couldn’t because of a lack of equipment. Now that I have an oscilloscope and ESR meter I thought I might use them both to see if I could work out what was wrong.
I disassembled the machine again and rechecked the Nicad clock/CMOS battery for leaks. The issue I’ve normally had with the machine was the graphic card not working, so I checked there first. I used the oscilloscope to check the clock crystals on the video board and that data appeared to be moving across the bus. The data bus didn’t seem very busy, but data was there, and the clock signals for the video board and the CPU all appeared normal. Given the video wasn’t working it’s not completely surprising that there was minimal data arriving at the video board. I performed similar checks on the RAM board as well and it seemed ok.
At this point I thought it best to check the obvious, like the power supply. I checked the output and all seemed well, with minimal ripple. This has left me scratching my head a little. I have some spares from a parts machine I bought, so some swapping may be in order. I have another screen and keyboard, but they are in a pretty beat up chassis, so I might use them to check the screen I have is operational.
The other thing I may try is replacing any and all electrolytic capacitors I can find. Most of the boards have either no caps, or surface mount ones only. I don’t know anything about surface mount tech, so I won’t be able to diagnose/replace any of those. I did however find some through hole caps on the riser card that connects the video board, so there is a possibility they could be the problem.
It was an interesting afternoon, and I’ve learned something about the operation of this machine. However I still feel it’s unlikely I’ll be able to fix this old beast, but it will certainly be good practise for using my tools and diagnosing problems.