The University of Iowa's DEC PDP-8Restoration Log
Part of
the UI-8 pages
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The University of Iowa's DEC PDP-8Restoration Log
Part of
the UI-8 pages
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This is a chronological log of the progress restoring the University of Iowa's PDP-8 computer. Entries are added at the end as work progresses. Click on any thumbnail image to see full-sized image.
Bug 67: Continuing the work from Dec 16, 2024, we found a second G209 board that was equally bad. As with the first bad board, all of the bad diodes were on the bottom (B-side) of the board. Some diodes were shorted, some were pretty good but showed some reverse leakage. We replaced all dioes that tested less than perfect.
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| Melted ground clips |
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Bug 68: Evidently, one of the clips, which was supposed to be on Pin C (ground) of the backplane socket inadvertently came into contact with pin A (+10). The current flowed through the scope probes to the other ground clip, melting both ground clips (but fortunately, doing no damage to the probes or scope).
We ordered replacements, and extras, and the parts required to build new
ground clips that will end not in alligator clips but in insulated sockets that
push down over pins on the backplane, eliminating this risk.
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| Layer 3 | Layer 4 |
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The mechanism parts that we must move from one punch to another are explained in the diagram on page 6 of High Speed Tape Punch (BRPE Type) General Description and Principles of Operation (Bell System Practices Section 592-802-100 Issue 3, March 1964). The exploded diagrams in the parts list are a helpful guide for assembly and disassembly, particularly High Speed Tape Punch (BRPE 6 and up) Parts (Bell System Practices Section 592-802-800TC Issue 2, June 1966; the cited PDF contains other parts lists as well). Unfortunately, these manuals do not suggest the order in which to remove parts to mine one punch for parts to install on another.
So, we have worked slowly inward from the front, removing parts one small layer
at a time, and placing each layer of parts in a small plastic bag along with
a tag indicating what punch it came from and the order of removal. The
photos here show the two punches, one with 3 layers removed, one with 4 removed.
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| Wire-wrap ground clips |
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Note that grounding clips are commercially available that include push-on connectors for "modern" wire wrap pins that are 0.025" square, designed for 30 AWG wire. Our old pdp-8 backplane, however, uses larger pins designed for 24 AWG wire, so we had to make our own.
Molex 1501811020 connector sockets are desined go push onto .062" round pins that are intended for use in a broad line of Molex multi-pin connectors. These pins, as luck would have it, slide snugly onto the wire-wrap pins of the PDP-8 backplane.
We used these Molex .062" sockets to replace the alligator clips on two of the scope probe ground clips, with 1/8" heat-shrink tubing to protect the outside of the socket and provide strain relief. These should be safe to push onto backplane pins even when the power is on with no risk of shorting adjacent pins.
We also made two similar sockets that clip to the scope-probe tip to allow
secure and relatively risk-free cliping of the probe to wire-wrap pins and
we made a shorting jumper. We made the latter because the memory tuning manual
told us to short some backplane pins to ground for some tests and this seems
safer than using alligator clips.
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| Removing pawls |
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| Linkages to move |
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During extraction of the parts, we noticed that the drag link for the sprocket track on the tape is a different part than the drag links (part number 143080) for the data tracks. This is explained in note 3 on page 3 of the parts list, where it says that a different drag link (part number 146678) is used for punches that have an advanced feed hole. Our 6-bit punches have this feature, but the 8-bit punch we want does not. So, we also extracted the extra drag link shown in the photo.
While going over the parts list, we discovered that both of our punches are missing the felt wick (part number 142849) that sits on top of the drag link pivot bracket (part number 142846). The clip that is supposed to hold this felt is present.
Having extracted these parts, we began reassembling the punch from which they
were extracted, in part so that we will have some practice on this complex
job before reassembling the punch we intend to widen to 8-bits.
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| Side view of blocks |
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| Top and end views |
We noted some interesting differences between our 6-bit punch block and
Bob's 8-bit block. The most notable is that the cast-iron body of our
6-bit block shows the pebble texture of sand casting on the hollow of the block,
while the hollow of Bob's block has a machined surface. Evidently, the 8-bit
block was machined from a solid billet of cast iron.
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Measuring the block was frustrating. It was very easy to make clerical errors, and locating centers of screw holes is particularly difficult. What seemed to work best was to measure to the screw head and then add half the screw head diameter. This only worked because Teletype Corp. used cheese-head screws with heads that are very concentric and very uniform in diameter.
To find the distance between two screw holes, the easiest solution was to measure the distance across both screws, including both screw heads, and then subtract the head diameter.
Note that any time you add or subtract measurements, the error in the result is the sum of the errors in the individual measurements. We found that we had to repeat measurements many times before we were confident that we had a useful value.
The drawings at the left are the result of this effort.