Thursday, July 5, 2018

Finished checkout of ASR 33 printer unit and punch


Before I could install the errant spring, I had to find it. I believed it was somewhere inside the printer mechanism, which led to a very slow and careful examination from all angles. This went on for quite a while, turning the mechanism on the two stable ends and trying a third less stable orientation in the hope of dislodging the spring.

Finally, when I had all but given up hope, at 9:30 at night, the spring appeared underneath. I put it in the parts bowl for now, planning the next attempt to attach it for the next morning. I picked up a great idea from Jack Rubin - tie a thin thread onto the spring and connect the other end to a part of the machinery. This will allow me to locate it quickly if it again 'springs' away.

Loose spring with thread tied to it and to printer mechanism
Using the safety thread to protect me from any further hunts for a needle in the haystack, I maneuvered the spring with the gripping tool and two springhooks. I wedged the gripping tool and its end of the spring over the top bar near where the spring should attach.

I then pulled the bottom of the spring down, transferred it to a springhook with a 90 degree end, perilous since the spring can easily slide off the hook end unlike the normal curved ones. However, this right angle springhook allowed me to pop the spring loop over the bottom mechanism and ever so gingerly remove the springhook itself. Success!

Spring in place on the bottom of the mechanism
I now had the spring attached at the bottom but needed to transfer its top loop over the projection on the top bar. With just the gripping tool, I got the spring on the bar. Removing the gripping tool requires some twisting of the spring loop which can detach it. It took about five minutes moving a glacial speed but I did remove the gripping tool with the spring still attached.

Spring attached to top bar and thread still tied to middle of spring
Using scissors with great care, I managed to cut the thread around the midst of the spring and pull the thread away. Removing the other end of the thread, fastened to the top of the printer mechanism, was child's play. The top of my spring is hooked over the top bar, not looped around the projection intended for it. This positions it slightly to the right of its ideal spot but I think this is good enough.

With the spring restored to bit 4, I can use my power supply and timing chart to set up the characters or functions I want to test. A rotation of 12 vanes is one bit duration; about 140 is a complete spin of the selector cycle to handle 11 bits and trip the code bar clutch.

With this in mind, I was able to encode a carrier return character and watch the function section attempt to release the carrier to slam to the left. Indeed, the carriage sprang to the left as the encoded character was executed. There is a release lever that should latch the carrier to the spacing mechanism once it hits the left side, but it is not releasing.

After studying the documentation for the involved mechanisms, I discovered that this is normal behavior. The carrier return function disengages the spacing mechanism as the carrier slams back to the left. It remains disengaged so that the carrier can be freely moved to the right and snaps back.

When a subsequent spacing operation occurs, either a space character or any printing character, the spacing mechanism moves one to the right and reengages to hold the carrier at column 2. I encoded the character W, watched it print, the carrier space over to column 2 and stay there.

Following that, I coded for a line feed to test that function. It did take a few tries before I produced exactly the bit pattern I wanted. The platen rolled upward one line, exactly as expected. I even coded up the 'bell' function code and heard it ring.

With such good results, I decided to wire up the printer unit to 110V for the motor and to my 300ma low voltage DC supply for the selector magnet. The expected behavior is to have the unit settle into the idle condition. What happened was exactly that.

Hooking up 120VAC for motor and DC to operate selector magnet
Dropping out the DC supply should give me chattering of blanks without printing or advancing across the page. That also occurred exactly as expected.

Twisting the keyboard activation lever should fire off one round of encoding a blank and then taking a non-print cycle, with the machine in LOCAL mode. Instead, the distributor fired but since I don't have the cabling in place to implement LOCAL or loopback mode, the printer unit continued to sit in idle. Still, I verified that the distributor clutch was operating properly.

In addition to triggering a distributor cycle to serialize the keyboard input when I rotated the keyboard trip lever, I pushed on the Answer Back lever and saw the drum rotate through its positions and the distributor serializing the programmed characters.

I switched on the paper tape punch and dropped DC power, which would give me blanks and should attempt to punch them. Indeed, the punch unit was attempting to punch when switched on, although with no paper tape inserted and with nothing but blank characters, this isn't a full verification.

I still have a problem with the tape punch unit, because of the broken arm used to adjust the tape roller pressure. A spring is hooked to one of a number of notches on a projecting arm, which sadly was broken off and lurking inside my punch unit.

I have emailed a service that used to provide Teletype parts from an enormous inventory, but no word back yet. Since I emailed at the start of the holiday week, it is possible that the owner is away and will respond next week.

Even if the parts service is no longer in operation, I should be able to fabricate a suitable replacement and fully repair the punch. For the immediate future, I am happy using the printer without tape punching.

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