This video details the teardown and reverse engineering of a Racal Avionics Control Display Unit (CDU), an aircraft avionics system featuring a Cathode Ray Tube (CRT) and keypad. The presenter aims to get the unit operational by understanding its internal hardware and software.
The teardown reveals a multi-board design, including a sophisticated power supply, a dedicated CRT driver board, and a video processor board. The power supply utilizes an Eri Power DC-DC converter, providing regulated +/-12V and +5V from a 19-35V input, and incorporates a unique mechanical "tilt switch" for shock detection to disable power. The CRT board handles high voltage generation for acceleration and focus, along with deflection circuits and a video amplifier; notably, significant post-manufacturing soldering issues were identified and repaired. The video processor board features a Motorola 6802 microprocessor primarily acting as a counter for scanning video memory, an EPROM storing character fonts, and a parallel-to-serial converter to generate the video signal. The font EPROM was successfully dumped, revealing 143 characters, with a subset displayable on screen.
An initial power-on test after repair proved successful, with the unit displaying its firmware version and "SELF TEST OK," demonstrating the feasibility of operating such complex legacy avionics hardware outside its original context.
Exploring a Racal Avionics Control Display Unit (CDU)
0:00 Introduction to the CDU: The video introduces a Racal Avionics Control Display Unit (CDU), an aircraft avionics component, with the goal of reverse-engineering its hardware and software to get it operational.
0:44 Power Supply Analysis: The unit contains a power supply board with high-temperature capacitors and a flyback transformer for the CRT's high voltage. The power supply provides +5V, +12V, and -12V from a 19-35V input.
2:49 Eri Power DC-DC Converter: The power supply incorporates an Eri Power DC-DC converter, and its datasheet and internal hybrid circuit design are detailed.
3:50 Shock Detection Mechanism: A unique "tilt switch" is discovered, designed to detect mechanical shocks and disable the DC-DC converter, functioning as a safety feature. The internal mechanism, including a mass and spring, is shown.
6:15 CRT Assembly Details: The CRT assembly, including deflection coils, a high-voltage flyback transformer, and the CRT board, is examined.
7:26 Soldering Issues Identified: The presenter finds and highlights several defective solder joints on the CRT board, particularly on resistor R41, which were likely caused by heat and poor pad design.
8:32 CRT Video and High-Voltage Circuits: The CRT's video circuit, including the filament, cathode amplifier (using BSX20 transistor), and high-voltage power supplies for acceleration (1-2kV) and focus, are reverse-engineered.
9:53 Firmware-Controlled Luminosity: The Wehnelt grid, responsible for brightness, is controlled by an amplifier and a ZN428 Digital-to-Analog Converter (DAC) on the processor board, allowing luminosity adjustment via firmware.
11:21 Deflection Circuits: Detailed schematics of the vertical (Y) and horizontal (X) deflection circuits are presented, showing the use of power transistors, sawtooth generators, and inductors for controlling the CRT's electron beam.
14:59 Video Processor Board: The video processor board features a Motorola 6802 microprocessor (used unusually as a simple counter for scanning video memory), an EPROM for program code, another EPROM for character fonts, and a parallel-to-serial converter that generates the video signal.
19:24 Font EPROM Dump & Analysis: The font EPROM is successfully dumped, revealing 143 characters. The presenter demonstrates how these characters are rendered on screen using an Excel sheet and custom macros.
22:14 Successful Power-On Test: After repairs, the unit is powered on with 28V, and successfully displays its firmware version ("S302") and "SELF TEST OK," confirming its functionality.