UNSUCCESSFUL HUNT TO IDENTIFY MYSTERY PART FROM TYPEWRITER
During the disassembly required to replace the broken motor belt in the console printer/typewriter on the IBM 1130, one part that dropped out confounded me when it came time to reassemble the mechanisms. I didn't see where it came from and I can't see any spot where it makes sense to be attached.
Yesterday I spent more than an hour scrutinizing the parts manual for the device, looking at every diagram no matter how unlikely it might seem that the part is associated with the diagram. Nothing that looked like the part was in the manual.
I then spent about an hour looking over my own IBM 1130's typewriter, hoping to find where the part was installed on my machine. As with the parts manual, I was unsuccessful. No sign of the part anywhere on the typewriter. The typewriter is composed of thousands of parts, with many deep dark nooks and crannies.
I had been adjusting my own typewriter when I moved to this new workshop, thus there were a few parts removed from it. I looked at the container holding those few parts and - voila - the mystery part was there. This confirms that it goes on the typewriter but gives me no clue as to where. Sigh.
WORKING ON THE TAB FUNCTION INTERLOCK SWITCH
A tab operation on a typewriter causes the carrier to move a varying number of columns to the right, stopping when it reaches a column that has a stop tab set. I have written extensively about the mechanisms involved in prior posts, which I won't repeat here, but I will mention just a few essential points to provide context for the work I did yesterday.
To start a tab, the operational clutch is triggered by the tab button or the tab solenoid fired by a write from the 1130 to the typewriter. During the fixed timeframe while the operational clutch rotates 180 degrees, it twists a torque bar that runs from right to left across the machine just behind where the carrier travels.
The torque bar has a roughly rectangular cross section, with the longer side positioned vertically. When the bar twists, the wide side pushes a lever away from the carrier and latches it in the out position. This pulls a pawl out of a rack of teeth which keep the carrier from moving to the right. Energy in a coil spring pulls on a cord that is tied to the right side of the carrier, thus with no pawl in the teeth the carrier is accelerated to the right.
A projection on the lever that is latched away from the carrier will strike a plate at any column where a tab stop was set (by moving the plate to the position where it will be struck). Hitting the projection forces the lever to unlatch, the pawl to fall into the rack of teeth, and the movement to stop.
Since the movement across from left to right takes a variable amount of time, based on the number of columns traversed, and because this takes longer than the time that the operational clutch was rotating, a means is needed to signal the IBM 1130 that the typewriter is mechanically busy. If the 1130 were to issue a character print command, that would print randomly during the tab motion.
To block commands being issued during busy times, the typewriter has a chain of four microswitches that can open their contacts to indicate that mechanical motion is underway. These cover printing a character, shifting between hemispheres of the typeball, carrier return and a tab movement.
The end of the torque bar on the left of the typewriter has a plate that rotates upwards as the bar is twisted. This allows a spring loaded lever to move upwards, transferring through another part to open the microswitch for tab motion protection.
The blue arrow points to the microswitch that is opened while the torque bar is twisted. The red arrow points at the torque bar and the green arrow is the lever that moves up when the torque bar is twisted.
The green arrow points at the plate on the end of the torque bar (see blue arrow), as viewed from the top of the machine. The lever is underneath the plate and not visible in this picture.
The torque bar should be held partially twisted as long as the lever is latched away from the carrier and the carrier is thus in motion. When the projection strikes the plate for a tab stop, the unlatching of the lever allows the torque bar to restore to its idle position. That pushes the parts down to close the microswitch again.
The problem I discovered was that the microswitch was not remaining open for the duration of a tab motion. It would briefly open during the time the operational clutch was rotating but then it closed in spite of the fact that the carrier was still moving to the right.
The lever for tab was not holding the torque bar partially twisted because it hadn't reached a critical angle of twist during the tab request. It turned far enough to latch the pawl out of the teeth, so the carrier moved, but not far enough to latch the torque bar in a twisted condition. A metal plate called a tab overthrow stop was what capped the maximum movement of the torque bar and lever. I adjusted this and the typewriter now holds the microswitch open until the lever is unlatched at the column where the tab movement stops.
I believe the erratic printing I observed while running the disk function diagnostic 309 was mainly due to the incorrect operation of the busy signal for tab movement. Attempts to print and to perform carrier returns were issued while the carrier was still in motion from a tab movement request. We will see how it behaves when I run the console printer/keyboard diagnostic at the end of this repair and adjustment project.
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