TU20

This document is an Engineering Change Order Log from Digital Equipment Corporation, detailing various modifications and corrections for the TU20 Datamec 2020 Mag Tape Transport, applicable to all processor types. Spanning from August 1971 to August 1973, the log addresses several issues:

1. Electrical Correction (TU20-00019): An incorrect N59M transformer output voltage was fixed by adding a wire from the transformer output to chassis ground.
2. Bus Drive Enhancement (TU20-B0020): Resolved insufficient bus drive when the TU20 operates on the same bus as other tape units (TU10-A, TU40, TU41) by changing specific bus driver modules and resistors, primarily for DECSYSTEM-10 use. This change required 0.5 hours for installation.
3. Documentation Update (TU20-00021): An administrative change to update the Accessory List.
4. Read Amplifier Overhaul and Associated Updates (TU20-B0022, TU20-D0023, TU20-C0024):
   • TU20-B0022 (Main Change): Replaced non-adjustable G084 read amplifiers with adjustable G087s and added a G0870 slice level control to fix problems with read/write thresholds. This was a significant design change requiring 3.0 hours for installation and specified costs for parts (G0870 at $66, G087 at $61). It applied to TU20s on DECSYSTEM-10.
   • TU20-D0023 (Specification Correction): A follow-up to TU20-B0022, this corrected margin specifications that were incompatible with the newly installed G0870 Slice Control and G087 Read Amplifiers. This was primarily a document correction.
   • TU20-C0024 (Documentation Correction): Also a follow-up to TU20-B0022, it corrected errors in the ADD/DELETE sheets, Wire List, and a logic drawing associated with the previous change.

The log details problems, specific corrections (often involving module replacements), in-plant and field effectivity, and estimated installation times or associated costs. Many changes are noted to be retrofits for existing TU20 units.