Sharing this for the common good.

Ran into a weird issue with what I think is an early revision of the BEC Precision 286-12II motherboard. That's the one on the retro web that it most closely matches, though mine lacks any silkscreening of jumper maps, and has no external battery header, for two minor differences.

After clearing up the usual issues - putting on a CR2032 adapter with diode, replacing the tantalum on the 12 volt rail that exploded on first power up - I ran into a power issue.

On cold start - refuses to POST. Card shows straight dashes and the ready indicator never changed
On a restart - two scenarios:

• If you wait too long, same result
• If you do a snap shot restart, powering back on right after the lights go out, it boots

Traced this to the Power Ready line - on those failed boots, it was floating between 1.7 and 2.1 volts, and on the quick restart, hopped right up to 5 volts like normal. Thought it was an issue with the TC4069UBP chip pulling down the voltage but after tracing that back through the resistor/diode array nearby I found it was sharing a line with the adjacent HM6818P - a thankfully socketed chip. Pulling that chip out, I was unable to get it to hang. It started, though clearly failed (lacking an RTC chip) to POST under every circumstance.

Thankfully I had a 386SX board in my stash that I could "borrow" the same chip from and put it in place to see if it fixed the issue, and it did. 286 boots perfectly every time with that chip in place.

Was about to swap them back and order a replacement when I thought to try the "bad" chip in the 386SX board - and it worked too! Every time, without fail.

So what I've taken from this is twofold:

1. If it doesn't post and there's voltage being held from reaching critical levels on the power good line, look at that clock chip.
2. If it proves to be the chip it might not be that it's bad, it might be that the board is sensitive to a something about that particular chip, where another board might not be.

nb - I did try to put the old chip back into the 286 and see if it was just that removal/reinsertion had cleaned the connectors on the chip or the socket, but it immediately started failing again after a single successful start. But has yet to give me any trouble on the 386.