Sunday, June 12, 2016

Xerox Alto II restoration begins, some work on 1130


Once the GUI is communicating successfully with the fpga, I can load the typewriter diagnostic to shake it down further. As is, the hand loop I entered into core lets me fire off steady sequences of two characters. That was the way I realized that my 1053 was not triggering the backspace operation when commanded by the program. All the others appeared to work.


I am helping a small team restore a Xerox ALTO II computer. Our first meeting was today to assess condition, begin on power supplies and plan out the restoration tasks.

The machine is mostly built of 7400 series chips, using four 74181 chips as the bit-slice ALU. Our power supplies are +12, +15, -15 , -5 and +5V.  The machine is a microcoded machine and the microcode makes the machine look like a Data General Nova minicomputer. We will first work on the microarchitecture and when that works properly, can debug the next layer up with the Nova-like microcode running.

Most of the power supplies came up to full power, showing good stability and almost no ripple. However, the +15V supply did not. It would wander in voltage, sometimes drop to zero, and the level it reached lowered as we increased the load. We removed the unit and took it to the bench.

The two main filter capacitors were essentially open, causing the problems on the supply. We did find some corrosion on traces on the edges of a couple of circuit boards in the supply, which we removed. In one case we rebuilt the trace due to the amount of copper loss it had suffered.

With replacement capacitors in place, we tested the supply again and found it fully operational. All supplies now back in the system and ready to deliver power to logic when we are ready for that. We decided we had a couple of tasks to finish first - verifying the boards are in the correct slots in the Alto card cage and testing the CRT.

The cards were verified and properly seated in the cage, but we discovered some anomalies. Card 21 should be the disk controller, but we found a disk controller card in slot 15. Several connectors with twisted wire cables were unconnected in the chassis. The Diablo 31 drive signal cable had an edge connector but it wouldn't fit on the disk controller card.

Looking closer, we found the disk controller card was for a Trident disk, not for the Diablo. The twisted wire cables fit onto the trident controller and run to the Trident disk sockets on the back of the Alto. Unfortunately, we have no Trident drive to connect to this. The Trident is a drive similar to the IBM 2311, several 14" platters on a shared axis, which are top mounted and are accessed by an arm assembly with multiple heads.

The trident controller belongs in slot 15, leaving us with a gaping hole where the Diablo disk controller should be. We will see if we have access to a controller, even temporarily, to bring up the system during the restoration. The owners will need to secure a disk controller card at some point if they want an operational Alto.


I ordered some candidate bulbs from and will see if they give me a way to get a more reliable and accessible set of lamps on the 1130. The main advantages of these bulbs are:

  1. longer bare wires allowing more maneuvering room inside the 1130 display panel
  2. wires are not brittle, won't break off when the bulb is moved

No comments:

Post a Comment