XL1201 Instability Next Steps

Well, the instability on the XL1201 has been disheartening. However, I have a few suggestions and plans to see if I can figure it out.

  • Kalman Reti would like to see backtraces to see if it's a software problem. I will definitely get these for him; awesome to have a Symbolics expert offering to help - thank you.
  • I'll put the Fluke 289 data logger on the 5V power supply rail and see if the voltage has any spikes or "events" when the computer reboots or acts strangely. If not, I'll try the other three rails too (12V, -12V, -5.2V).
  • There are a number of tantalum bypass capacitors on the RAM, Merlin II and I/O boards. If any are going bad, they could short, heat up, "heal" and then work again for a little while.
    • Test the tantalum caps for resistance (briefly, as the resistance will change as they charge) - anything under 6 ohms might be bad. This will be very difficult to do with the backplane caps, though; I may have to learn to disassemble the XL1201 chassis.
    • After the problem happens, quickly power down and remove the boards to see if any components appear overly hot on the FLIR one IR camera.
  • Remove the cards again and push on all the socketed chips to make sure they're all seated properly. Possibly apply contact cleaner to the socketed chips. Maybe the whole board - they seem dirty. Not sure if this "whole board" is safe or not.
  • Remove the cards and all socketed chips, clean the chip and socket, and reseat them. This would take a long time as there are a ton of chips and I haven't pulled chips in literally decades. I would need to get a chip pulling tool and be super careful as this stuff is, as far as I'm concerned, irreplaceable.

Unfortunately, I don't have a way to run the Merlin II without it being installed in the XL1201 chassis so I can't test the caps directly or watch the heat signatures directly without doing a massive disassembly and hack job on the computer chassis, backplane, power supply, etc.

Some things that are checked:

  • The boards are all seated as well as I can do it.
  • All the pins on the 4 connectors per board seem intact and unbent.
  • There is one broken screw in the I/O board that prevents the backplane connector from being screwed fully in, but I don't have any idea how I can remove that broken screw. No parts are sticking out.

I'm now wondering why the power supply has a -5.2V rail. That's just weird. Apparently Todd MAX-354-1252 means that it puts out 350W of power in 4 voltages including 12V and 5.2V.

Also, about seeing 6 Mwords of memory instead of the 8, DKS suggests that I remove the second memory card and put in my old Rev A memory card to see if it still doesn't see 8 MW. If still 6 MW then swap the other card. If neither swap causes any difference it's probably a problem with the motherboard. Also, DKS says the reason you don't want to run Rev A and B together is that there are apparently "reflections" on the data lines that cause the Rev A memory chips to fail over time. If

Douglas Fields

Writing LISP and Haskell since 1990