Thursday, July 19, 2018

Finished repair of ASR 33 punch, worked on model 15s, and prepared for exhibition at VCF West


Wednesday night, the eBay perforator arrived and I disassembled it to have the replacement parts for my ASR 33 tape punch. This arm with the teeth has a spring that attaches - the choice of tooth determines the tension on the paper tape as the punch operates.

The part that will replace my broken one
The part after removal from the spare perforator
I removed the broken part from the ASR 33 perforator and slid on the replacement part. The last step in restoring this unit was completed when I used a spring hook to pull the spring up and fasten it on one of the teeth.

Broken arm on the ASR 33 perforator, before removal and replacement
The new arm is in place now and I can begin to punch tapes necessary for the exhibition at VCF West. 


After studying the manuals and thinking about the symptoms of my model 15 (printer doesn't move print or function bails), I have zeroed in on the section I believe is frozen in place and needs some work.

Most likely mechanism causing symptoms - Clutch Throwout Lever and Stop Arm
There is a lever which rotates to engage and stop the clutch for the two cams which drive the print and function bail movements. A cam on the selector mechanism pushes the Cam Arm, causing the Clutch Throwout Lever to rotate on its axis, allowing the spring to push the clutch teeth together. This rotates the two cams that push the bails.

I confirmed that the driving clutch member is not sliding on the main axle and meshing. It took 90 minutes of manipulation of the newly oiled member before it would slide into mesh and pop back crisply. Hundreds of movements of the member, using a screwdriver as a prybar with gradually lighter pressure once it began to behave properly.

I took the carriage assembly off to work on the main shaft and other linkages. When I reinstalled it, I somehow pinched the strap that pulls the carrier back to the left. This strap is wrapped around a circular spring assembly and its other end is clipped to the right side of the carrier.

Marc repaired it, making a slightly shorter strap by opening the clamped metal, looping the remaining strap through the end hook and reclamping. It was installed and I once again have carrier return on the printer.

I hand rotated the main shaft and set up various baudot code patterns on the vanes. For example, I set them to SPACE, SPACE, SPACE, MARK, SPACE which is a CR command, then watched the mechanism trigger the return and it complete successfully.

I then triggered LF and verified the platen advanced one line. I triggered 'Space' and saw the carrier move one column to the right. Figures and Letters caused the entire platen to move up and down so the selected half of the typebar will strike the platen.

The computer interface then drove a pattern of alternating R and Y characters, a classic pattern used to adjust teletype reception. The results weren't perfect, although I had a decent number of R and Y strikes, there were plenty of miss-selected characters. I shot some slow motion video to examine further. It appears I have a slow vane number 5, sometimes settling in an intermediate position between MARK and SPACE - which is invalid.

I put a new ribbon on the printer but something is wrong with the ribbon advance mechanism as it isn't moving. I will review the documentation and then work on my model 15 some more tomorrow morning.

Marc worked a bit on the "Bob Erickson" model 15. First up was the need to adjust the speed to 45.45 baud, since this particular printer uses a governor to control motor speed. He hooked up an oscilloscope to the keyboard circuit, drove it with a power supply, and watched as he typed various characters on the keyboard.

He adjusted the motor speed until the bit cells took up exactly 22 ms. We then connected the computer interface and printed various patterns until it was working perfectly. When the interface listed its menu of choices, such as news stories or weather reports, we tried to select a choice by pressing a key.

The computer interface registered the wrong character - when Marc pushed W the interface saw a Z. A quick check with the oscilloscope confirmed that the keyboard is not encoding correctly. The contacts were quite dirty. After time spent cleaning the contacts and checking the results, we had the keyboard working well.

Unfortunately, I we ran out of time and couldn't continue with the computer interface testing. Next session, we plan to hook the keyboard and printer in a local loop, exercise the Erickson machine and hopefully have it ready to use at VCF if I can't get my unit working in time.


With my paper tape punch working correctly, I hooked up the USB to serial module through the MAX3232 board to drive the teletype. Using a terminal program that will feed text files to the teletype, I simultaneously listed and punched a few basic programs to use as demonstrations at the festival. These will work with the Altair 8800 clone.

I am sketching out ideas for scripts to use at the festival, allowing my helpers to give the same experience when I am not at the booth. These are essentially all examples of booting 4K Micro-Soft basic and loading the paper tape for a particular game or demo. In most cases, the attendees can interact with the programs using the teletype.

I have Adventure somewhere on the PDP 8 clone, along with OS/8, so I just need to script out the startup procedures and give them some aids to help attendees get through some early challenges in the game.

I also need to script out the method of switching the teletype between the two clone computers - it is pretty straightforward but worth documenting just in case. There is no such switching necessary for the model 15 teletype and its connection to a PC, although I will script how to restart the BaudotRSS program if something goes wrong.

For the model 15 demo, I did some experimenting to find interesting RSS feeds that could be printed. I will test this tomorrow; hopefully having something interesting to type out. 

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