Teletype Model 33 ASR Ribbon Feed

In November last year I attended DEC Legacy 2018 and as usual I took along my treasured Teletype Model 33 ASR. I hooked it up as usual to the PDP10 simulator in SIMH to run DEC’s TOPS-20 operating system. You can see it working here.

During the event I noticed that the ribbon was not feeding correctly. In fact most of the time it was not feeding at all. Resulting in fading characters as the same bit of ribbon was used over and over again. A few of us had a look, and we could see that the feed pawl was not engaging properly with the teeth on the ratchet wheels holding the ribbon spools. However, we couldn’t really see why this was happening, it was a bit intermittent. I thought it might just be a lack of lubrication, but in any case I resolved to check it when I got back home.

I didn’t look at it for a while, and then recently I asked on the Greenkeys mailing list and the ever helpful Wayne replied with a couple of emails. It took me a couple more weeks before I could look at it, but I finally did so today. Following Wayne’s advice I checked the springs. At first I couldn’t see any springs that had come undone, but then I noticed a spring  under the arm of the feed pawl that was disconnected, the two arrows in the picture below show the detached spring and where it should be attached.


I removed the ribbon feed mechanism so I could fix the problem. I attached the spring again, which was a bit fiddly, lubricated the mechanism and then refitted it. The ribbon feed now works perfectly.

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