In keeping with current Department of Defense policies on integrated diagnostics and a reduced reliance on paper-based documentation, the concept of a portable, expert-system-based job aid and training device was proposed to assist inexperienced electronics maintenance technicians in learning to maintain sophisticated equipment. A prototype was designed and implemented for the troubleshooting portion of the F111 6883 intermediate-level avionics test station in order to investigate a variety of issues, e.g., hybrid diagnostics, knowledge engineering, and user interfaces. The phases of the project included conceptual design, development, and delivery software programming; delivery hardware prototyping; knowledge base development; field demonstration; and analysis of lessons learned. The design for the prototype incorporated both human-machine interfaces and end-user interfaces to promote incremental skill acquisition and assess the reasoning behind the diagnostic process in a troubleshooting situation. In a field demonstration, the prototype received high ratings for ease of use, speed of operation, troubleshooting accuracy, and usefulness for job aiding and training. Implications for future development focused on realizing the training potential of the system, enhancing user interfaces, and expanding the problem domain. Several illustrations are provided and the appended material includes technical data, data collection instruments, and 52 references. (DJR)