Wednesday, June 13, 2018

Worked on 1401 systems at CHM, two fun kits


Today we met to work on issues with the 1401 systems, particularly the inability of the German system's 1402 to read cards successfully. During a special visit of the "Fortran Geezers" on Sunday we ran Fortran compilations using a low density tape we had created. We discovered that one of the tape drives would not switch between high and low density.

Investigating the tape drive, Alan Palmer and Marc Verdiell found the contacts of the pushbutton were not working properly. A bit of work corrected that, allowing the drive to switch between Low and High density. At that point, a second flaw was discovered, a bad light bulb in the High Density display; a replacement bulb was put in place.

The card reader has a central shaft that rotates once for each card feed cycle, activating a number of cam based timing switches and the picker knives that grab the bottom card and shove it through the hopper throat and into the maw of the reader. The picker knives are driven by two cams, metal objects with hard plastic molded over them.

Due to improper adjustments in the past, the picker knife mechanism would slam back against the metal frame, applying excessive shock to the picker cams. The plastic was dented, fractured and gouged. This caused the picker knives to move at variable times, leading to the physical card moving out of sync with the rotating shaft and its timing switches.

The reader was disassembled and the bad shaft removed. We don't have a spare for this in our workroom. We have two ways to address this problem. If we can find a machine in the museum storage area that is surplus, thus amenable to removal of parts, we can swap the damaged shaft with a good one. If not, we have to machine a replacement.

To be surplus, the museum must have several of the machine and designated one or more as available for these purposes. Machines with only a single example, by contrast, cannot be touched as they must reflect the exact condition they had when donated.

The card reader mechanism is used in several IBM products, not only the 1402 reader/punch. The 2540 reader/punch and the 088 collator are so similar that they would have the same shaft with picker knife cams. We will explore this next week in the hope that we can repair the 1402 easily.

If we are forced to machine a replacement, it must also have a similar very hard plastic over the cam which makes the fabrication much harder than simple metal machining. 


I bought kits from Evil Mad Scientist to build large versions of the 555 timer and 741 op amp integrated circuits, using discrete transistors and other components matching the circuitry on the silicon of the real ICs. These work when wired in place of the 555 and 741 (with some performance limitations due to the large size). Mainly, the look cool and are a fun.

The ubiquitous 555 timer chip

Widely used 741 Operational Amplifier chip

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