Troubleshooting spooky & intermittent instability after power-up: Symbolics XL1200

[Ian Finder]

Hi folks,

I know this is a long shot but... I'm having a rather curious issue with
one of my Symbolics XL 1200 CPUs, which I suspect may have a far more
general cause.

I have narrowed the issue down to the specific Merlin II CPU card I'm
running- not the RAM, Jumper, I/O, backplane, power supply or any of that
good stuff.

My friend Doug has a rather pretty picture of the board in question on his
site:
https://symbolics.lisp.engineer/koken/albums/merlin-ii/content/img-4316/lightbox/

It's basically a big wedge of PGA ASIC encapsulations and PALs, for those
unfamiliar.

The symptom is this. After a cold boot- a fresh power-up from no power
applied- the system will hang after 30-40 seconds into the startup with a
hard lock and sometimes memory bus issues. If I wait 10 more seconds or so,
and hit reset, the system usually comes up fine for hours.

If I hold the system in reset for a few minutes during the cold boot before
proceeding, the system works fine.

The repro rate is close to 100%. If the machine is off for a few minutes,
then cold booted, the issue repros.

It sounds as if something has gone thermally sensitive in a very
deterministic manner. Can that happen with the tantalum caps on the board?
Perhaps they are shorting intermittently for the first few dozen seconds?

Again, the rest of the system is good- the issue follows this board and
only this board no matter which chassis it runs in. All socketed ICs have
been reseated.

I know this isn't much to go off of, but anyone out there have experience
with spooky "intermittent" yet 100% repro-able cold-power-up issues?

I'll debug further of course, but thought I'd ask here.

Thanks,

- Ian

-- 
   Ian Finder
   (206) 395-MIPS
   ian.finder at gmail.com