Thursday, April 20, 2017

1401 restoration work and continued curve tracer construction

I spent part of the day with the plumber as he finished up the tankless water heater installation and some other work, then spent the rest of the day with the 1401 restoration team. This evening, I attended a lecture by Paul Wesley at Stanford on the history of Silicon Valley. 


I slowly did the tedious wiring of the sockets - loops between each pin number, around all the sockets being used, through ferrite beads and connected to the commutator of the six rotary switches I had on hand. I also prepared the sites for the next three rotary switches to be installed, now that the stock arrived, for controlling pins 7, 8 and 9. 

Before I can mount the next three rotary switches into the holes where I removed tube sockets, I need to get to a hardware store for some regular and lock washers, since a tube socket is larger diameter than a rotary switch shaft. 

After they are mounted, I can wire up those three loops. Actually, one loop goes to all five sockets, a second loop goes to just four sockets, and the last loop goes to only two sockets, the 9 pin minis. 

Another issue is the mounting for the circuit board, power supply brick and the serial port DB9 connector. Besides working that out and doing the install, I have to wire the board produced voltages to the corresponding positions on all nine rotary switches. 

The voltages on the rotary switches are: 
  1. anode
  2. screen
  3. cathode
  4. grid
  5. filament A
  6. filament B
  7. anode reservoir
  8. screen reservoir. 
The regular anode and screen connections are only connected for very brief intervals during measurements, small fractions of a second, otherwise sit at zero. The reservoirs provide the same voltages continuously. 


SMS cards in the 1401 power supply circuitry drive a set of relays which sequence power on and off at startup or shutdown of the system. All the cards we have are scorched black underneath a pair of resistors that are extremely hot to the touch at all times. We investigated to see if this is a result of failed components or 'normal'.

The card schematics for this card matches exactly what is on the card. It is clear that the pair of 2W resistors are asked to handle more than 3.5 W of power, yet are mounted flat to the card with no space for airflow around them. Thus, the scorching and overheated resistors are by design.

More interesting to me is that the resistors in question are simply providing a voltage divider to bias a transistor, switching it on only if the two power supplies that fed the divider were working. They designed this to draw about 65 ma across the 42 V relative voltage, all to create a minute bias current in the transistor.

We then looked at the ALD schematic page, instead of the SMS card schematic, and found that the design had different and much more appropriate resistor values installed. 3,470 instead of 650 ohms, which dropped the power consumption down to a half watt. The schematic pages had the exact same drawing and part number, but the cards as they exist are the scorchers not the design in the ALDs.

We decided to take one of our spare cards and modify it to match the ALD schematic. I found almost every part needed on hand in the workroom, except that I had to parallel up two resistors to create a 43 ohm part. By the end of the day, I was partway through rebuilding the card. The tools on hand for solder removal were inadequate, so I will bring in my own gear next time.

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