Wednesday, March 29, 2017

Continued work on the 1401 (DE) machine at CHM

Have been very busy with many tasks outside of the restoration hobby, which left a bit of a gap in the blog. Starting back up but will still have many distractions.


Today we began testing the 1401 whose Start Reset switch I recently replaced. I had some suspicions that the short circuit weeks ago had damaged more than the circuits I replaced. Alas, I was correct. 

Two of three pins that deliver -6V to the 01A2 A backplane and then to the control panel were dead. Last week I ran new lines to deliver +6V to two of four pins on the same slot (A24), as those were dead. Now, I had to add two wires to provide the missing -6V. All these extra wires run over to the power distribution strip on gate 01A3 which is adjacent to the control panel.

We finished adding the lines, pushed the power on button and were surprised that the system failed to power up. Unhooking the two lines I added didn't fix anything, we still had problems powering up. Some quick checking showed a failure of the +6V supply during the power sequencing.

After lunch, when we were trying to shoot the problem, we saw the +6 supply to the sequence logic yoyo a bit then suddenly the machine was powering up fine. About 30 minutes later, it failed to come up again. This is a problem we will have to address, one that seems to have cropped up independently of the earlier issues.

With the machine up, we found that some of the controls on the control panel are not working properly. Regardless of how the rotary address switches are set, we get all lines high and an error condition. These switches, like most controls, run past a resistor divider hooked to +6 and -6. This tells me that our voltages are not getting from slot A24 to the other A slots that hold the resistor cards.

We need to get access to the resistor cards and paddle cards (connecting wire harnesses to an SMS card stub). These are blocked underneath the backplane in a very very challenging spot on the machine. We discovered a way to swing a cable trough down and out of the way to let us see the cards from the underside. 

Unfortunately, the A24 paddle card is blocked in place by all the cable harnesses running through the cable through and down into the card gates below the control panel. The crowd of attendees built up and the demonstration crew arrived, cutting our work off and requiring us to clean up prior to the scheduled demo at 3PM. 


We will be back together working on the Alto this Friday, after a hiatus due to work obligations for other members. There are nine cartridges from PARC that we will clean, insert, read and archive during the session. Too, we will collect some information to figure out why cartridges written by my tool cannot boot up on the Alto. 


  1. Hi Carl. Will the 1130 be visiting the VCF West show this year?

  2. Hi Bob. It will not, but we will exhibit a restored Xerox Alto and give a talk on the restoration.