Retronics' DIY Digital 6U5 Magic Eye Tube Breathes New Life Into Vintage Radios
A circular display meant for a smartwatch enables this DIY replacement for a visual vacuum tube used in vintage radios.
Retronics, a pseudonymous vintage radio collector, faced a challenge when restoring a Tandberg Sølvsuper 4 radio. The radio's front panel uses a Magic Eye, a no longer manufactured component. So, Retronics developed a modern Digital 6U5 Magic Eye replacement using a circular IPS display, an STM32 microcontroller, and two PCBs to address the restoration community's needs.
Magic Eye Tubes are vacuum tubes that visually indicate a signal's strength. Vintage radios use them to help users tune into specific frequencies by visualizing the strength of a signal. Since these tubes are no longer manufactured, there is a dwindling supply of used parts available.
The DIY Digital 6U5 Magic Eye replacement has two printed circuit boards. A base PCB plugs into the socket meant for the original 6U5 tube. Its primary function is to get the automatic gain control (AGC) voltage from the radio and pass it to a microcontroller (MCU) board. The MCU board interfaces with the display and, as the name implies, contains an STM32F103CBT microcontroller and an analog conditioning circuit.
The radio's AGC signal is proportional to its strength. Unfortunately, its voltage range is from 0 to -22 volts. A conditioning circuit based around a Microchip MCP6001 op-amp converts the signal into a range suitable for the MCU.
Retronics acquired a functional 6U5 for comparison. After a meticulous process revealed that the 6U5 does not have a linear or logarithmic response, Retronics devised an algorithm approximating the original. The result is a fully functional drop-in replacement "tube." The STM32 microcontroller runs code written around the Arduino framework. Retronics developed the project in VSCode using PlatformIO.
All of the components for the DIY Digital 6U5 Magic Eye replacement are housed inside a 3D-printed enclosure. For those interested in replicating this project, the KiCad design files, 3D models, and Arduino code are available for download from the Digital 6U5 GitHub repository. For detailed information on the design and implementation, check out this blog post.