Spent the evening continuing to work on the 486 motherboard I had previously been repairing. Last time I had repaired a trace with a temporary bodge wire. The board otherwise seemed to work, so I wanted to implement a more permanent fix.
The attachment 4DMU HL3-L4-VB motherboard bodge wire.jpg is no longer available
After removing the keyboard socket and sanding down a bunch of the corrosion, I installed a replacement trace using some enamelled wire. I also noticed that the via near C15 was clogged with corrosion, so I cleaned that out too.
The attachment 4DMU HL3-L4-VB motherboard enamelled wire.jpg is no longer available
After soldering a new keyboard socket, I tried booting the system thinking it would be fully repaired. Much to my surprise, it failed to POST. The only thing that was working was the reset line wasn't stuck on, but other than that it seemed completely DOA.
The attachment 4DMU HL3-L4-VB motherboard no POST.jpg is no longer available
I proceeded to spend the next hour trying to figure out what had gone wrong. I triple checked my repair work, testing for shorts, continuity, voltages, and inspecting for other damage I might have caused. Everything seemed fine. I even double checked the processor and RAM in another board to make sure they still worked.
After getting nowhere, I resigned myself to the fact that maybe something else on the board was faulty and I'd have to spend more time diagnosing it. On a whim, I figured I might try a different processor completely. That's when I checked the switch that controls the FSB speed.
The attachment Stupid Switch.jpg is no longer available
Oops.
Turns out that stupid switch was set incorrectly. The "3" position should have been set to on. It must have gotten nudged into the off position at some point. This resulted in an invalid setting and the reason the board wouldn't POST.
After fixing the switch, everything booted up as normal.
The attachment 4DMU HL3-L4-VB motherboard successful boot.jpg is no longer available
This makes twice in the past week I thought I screwed up a repair only to discover the solution was a simple settings issue.
Will I learn my lesson after this? (Probably not.) 😅