DECtapeSkewAdj

This document is a section of a Field Service Tech Manual for PDP-8 DECTAPE Systems, providing detailed procedures for adjustments, primarily focusing on "skew" measurement and correction.

Key aspects of the document include:

1. Skew Definition and Importance: Skew is defined as the time difference between signals on two tape channels. The manual details how to measure and correct it for optimal DECTAPE performance.
2. Required Equipment: The process utilizes a dedicated G500 Skew Tester connected to the DECTAPE head, along with an oscilloscope to observe waveforms.
3. Procedure Steps:
   • Setup: Plugging in the skew tester and configuring its switches (S1, S2, S3) for calibration and specific winding selection.
   • Cleaning: Detailed instructions for thoroughly cleaning tape heads, guides, and the tape itself, as dirt is a common cause of skew.
   • Calibration: Calibrating the skew tester to compensate for internal drift and phase shift.
   • Measurement: Running the tape (either a "zero skew" tape or the actual tape) and observing the time difference between channel crossovers on an oscilloscope. The tester indicates twice the real skew.
   • Correction ("Deskewing"): If skew exceeds a specified tolerance (e.g., 2 µsec), physical adjustments to the tape head are required, often involving shimming to minimize the skew.
4. Tape Handling: Instructions and diagrams for correct tape threading in both normal and reverse modes are provided, as skew characteristics can change with tape direction.
5. Visual Aids: The document includes waveform diagrams illustrating how skew appears on an oscilloscope, as well as diagrams for tape threading, tester connections, and internal circuit schematics of the skew tester.

In essence, it's a comprehensive guide for technicians to diagnose, measure, and correct tape head misalignment (skew) in DECTAPE systems to ensure accurate data operation.