Tuesday, October 31, 2023

Adjusted backspace function on 1053, built testbed to check out the printer under power


The way that backspace works on the Selectric is to implement a second rack just below the escapement rack - this one also with teeth but slanted in the other direction. A spring loaded pawl from the carrier will engage in the rack but just pops out of one spot and into the next as the carrier does its normal rightward spacing. When the rack itself is slid backwards, it grabs the pawl and pushes the carrier backwards. The escapement pawl will pop out of its rack and enter the previous column's position, completing the backspace operation. 

There are only two adjustments for the backspace mechanism, one of which sets up the proper play in the system while the second limits the maximum movement of the backspace rack. If the limit is too large, the carrier might hop more than one column back. If the play is too tight, wear can develop on the mechanism during normal forward movement of the carrier. I adjusted both of these. The main difficulty in making these adjustments is limited visibility, with the backspace pawl hidden below the escapement pawl, the tab pawl/latch and the tab gang clear bracket. 


So far my adjustments have been tested by manually cranking the main shaft of the typewriter. Some operations such as backspace don't work well with manual cranking; the need the extra momentum given by the rapid rotation of the shaft under motor power. Other functions appear to work properly with me pushing on a solenoid armature using a screwdriver. The most accurate test is to switch on the motor of the 1053 and to drive the functions using the solenoids and front panel pushbuttons. 

The solenoid is powered by 48V DC and one end is pulled to ground to cause the solenoid to trip. These activations should be relatively short, less than the time of one print cycle. Since the typewriter can operate at a maximum of 15.5 characters per second, this indicates around 60 milliseconds per cycle is consume. 

I decided to fire the solenoids for about half that, just to ensure that I don't request more than on space or character to print by holding the activation too long. To do this, I set up a testbed I can use to selectively fire solenoids during this phase of the testing. I will make use of a 48V power supply and a relay to switch the 48V feed to the typewriter on for only 30 milliseconds at a time. 

I set up a 555 timer with a button to produce the single 30 ms pulse, inverted it to match the particular relay board I have, and used that to control the duration of solenoid activation. This means that I need only connect a line to ground on the desired solenoid connection to the typewriter and push my button to request that function. 

I will begin with the operations that I feel are in the best shape - space, tab, index, backspace and carrier return. I will also test the upshift/downshift which spins the ball to the upper or lower case side, and test the color selection solenoids that would cause the red or black half of the ribbon to be struck by the typeball. 

I still don't have the ball rotation getting fully to the +5 and -5 positions, so I will not be powering any character typing until I think the issue is sorted out, just to be safe. 

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