Repair on Symbolics LISP Engineering

Ivory CPU Heatsink and Instability

Sat, 06 Aug 2016 14:13:58 +0000

One of the theories about the instability of my Symbolics XL1200 is that the CPU has no heatsink and so it is suffering from thermal problems after a while. (15-30-ish minutes)

David K. Schmidt was kind enough to send me an old heatsink. It’s 38x38x5 mm and has 11 fins on it, as you’ll see on the pictures; slightly smaller than the Ivory CPU itself.

First I cleaned the old epoxy from the heatsink using acetone, which was surprisingly easy to do.

XL1201 Moment of Truth

Sun, 24 Jul 2016 21:17:33 +0000

UPDATE: No joy. Still self-reboots.

So, I put the Merlin II card back into the XL1201, connected the power and console, and powered it up.

Nothing. Blank screen (which means the monitor was at least getting a signal) but no boot, nothing. I hit the reset button a few times and powered it off and on a few times, and tried to reseat the card. Not a thing.

So, removed the Merlin II and took a very close look at it, comparing my before pictures to everything, to make sure every chip was in the right place and every pin was in its slot.

XL1201 Merlin II Chip Reseating

Sun, 24 Jul 2016 19:48:20 +0000

UPDATE: I clipped and removed the broken capacitor. Replacing it will be the task of another day.

I spent two days removing, cleaning and reseating each DIP chip on the Symbolics XL1201 Merlin II Main (CPU) board. Earlier I closely examined the solder side of the board and found a few minor discrepancies. This time I also encountered three discrepancies:

Bent pin on chip BB27 - unbent
Broken bypass cap next to chip GG7 - I clipped this one off entirely and will replace another day
One long bodge wire running along the entire top has come loose. This could get snagged on something and fall off.

Additionally, I removed and resocketed the Weitek Floating Point co-processor without doing any cleaning. It looked sparkling clean.

Broken SCSI Pin on XL1201

Fri, 24 Jun 2016 00:03:00 +0000

Well, I was installing a new hard drive into the XL1201 today and just before I connected the SCSI cable, I noticed a pin seemed to be missing on the 50-pin SCSI male connector on the backplane. On closer inspection, it’s definitely missing.

Fortunately, it seems to be pin 1, which is a Ground pin on both single-ended and differential SCSI. Since half the pins are ground on single-ended SCSI, that’s fine, and there are still quite a few ground pins on differential, so we’re fine. It’s the bottom left pin in the picture here.

Repairing Symbolics 3600 Keyboard

Wed, 22 Jun 2016 14:09:40 +0000

When I purchased the XL1201, it came with a Model 3600 keyboard. Unfortunately, several keys didn’t work, including both hypers, and the left control and super. I disassembled it and couldn’t see anything obvious wrong, so I assumed the key switches went bad. However, this keyboard uses hall effect switches, which have no electrical moving parts. (The keypress moves a magnet, which is sensed by an unmoving electrical component.) So, there aren’t many things to break. I put it back together to deal with later.