[GreenKeys] Model 33 ASR adventures (continued)

[Jones, Douglas W]

The saga of the U of Iowa Model 33 ASR continues.

After last week's missive, someone suggested to me that the back of the selector armature might be physically sticky, so today, I pulled the selector armature and cleaned it.  It was just a bit sticky, nothing exciting, but the alcohol-soaked paper towel I wiped it and the magnet faces with came away much darker than it began.

We removed only two screws, the ones holding the two sides of the armature.  The screws holding the tops of the side plates together were merely loosened, and the spring at the back of the selector was rotated down (it has one end that's a long lever to permit this.  That wasn't enough to free the armature.  We also had to remove the screw at the end of the selector clutch so the cam cluster could slide sideways a millimeter.  Then the armature pulled out fairly easily.

Putting it all back together was "fun."  Pushing the cam cluster back required pushing all the cam followers down (9 of them) back (one follower) and up (one follower).  All at the same time, of course.  Then, prodding the selector magnet to seat it properly (it didn't want to cooperate).

With all screws tightened, we got good data except on the least significant two bits.  It turned out that the armature itself wasn't seated all the way down in its slots in the end plates.  With that job done, we got reliable data on the ends of the code bars.

Hurrah, except we didn't get a reliable linefeed.  Going over everything, we found that the code bars were correctly showing the code for linefeed.  The problem was in the decoding, so we need to loosen some screws to adjust the line feed pry points.  I think the linefeed operating link is just a bit too short.  Those two screws are positioned so you can only work them with an open-end wrench.  Even a short right-angle screwdriver or a short right-angle nut driver can't reach the screw heads.  A truly delightful design.  So, I need to dig up a 1/4 inch open-end wrench.

(I was asked, did these problems come on suddenly?  No!  The machine has sat idle since well before COVID, this semester is the first time students have wanted to focus on the 33 instead of on the PDP-8.  Last fall, in our first post COVID opening of the retrocomputer lab, we built some new logic probes for the PDP-8.)

            Doug Jones