The University of Iowa's DEC PDP-8

Restoration Log

Part of the UI-8 pages
by Douglas W. Jones
THE UNIVERSITY OF IOWA Department of Computer Science



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.

May 17, 2019, Reassemble TTY, fix G007 diode

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The Teletype
Bug 26: In preparation for the University of Iowa Tech Forum on June 6, where artifacts from the Retrocomputer Lab will be on exhibit, we put the cover on the Teletype for the first time since we began work on it. Looks good!

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Bad diode replaced
Bug 26: We used an ohm meter to check all the diodes on the G107 sense-amplifier boards, and found one of them that was bad, measuring as an open circuit in both directions. The diode in question was D15 on the G007 schematic in the Maintenance Manual. DEC's part number for this diode is D668, and their semiconductor substitution instructions in the Maintenance Manual (Table 10-1, page 10-5) say to use two D-664 (1N3606) diodes in series.

As can be seen in the photo, showing the repaired board (bottom) and a board with a good diode (top), the pair of diodes in series is a bit bulky. We painted the transparent glass body of the diode with black paint because the forward frop of this diode is used as a voltage regulator, and the forward drop of glass-encased silicon diodes is notoriously light sensitive.