A CRT requires a minimum neck length proportional to deflection angle. For a 40 mm screen width, a standard 90° deflection tube would require a depth of ~40 mm from screen face to yoke. However, the electron gun assembly alone adds 50–60 mm. Thus, the total depth exceeds 110 mm, fitting the 203 mm bay depth only if the PCB is relocated externally. A 110° or 120° deflection tube could shorten depth to ~35 mm, but such tubes exist only as experimental designs due to corner focus distortion.
A more feasible but absurd approach: Drive the tube in (vector display), eliminating horizontal/vertical oscillators. The electron beam would be steered by DACs, enabling oscilloscope-like graphics at low refresh rates (< 100 Hz). The bandwidth would be < 50 kHz, unsuitable for video but sufficient for retro system monitors. 5. Safety and Regulatory Compliance CRTs are vacuum envelopes under significant stress (atmospheric pressure ~10 tons/m² on a 40×40 mm faceplate). A 5.25-inch tube’s glass thickness would be < 1.5 mm to save depth. Implosion risk is extreme. Furthermore, the 8 kV anode would be millimeters away from the metal drive bay chassis. Creepage and clearance distances required by UL/CSA (minimum 6 mm at 8 kV) are impossible. The device would arc through air to the chassis, shocking the user and destroying the motherboard. 6. Proposed Theoretical Design (Unrealized) We present a specification for the BayTube BT-1 , a non-functional conceptual device: 5.25 drive bay crt monitor
Author: Retrocomputing Architecture Syndicate Publication Date: April 2026 Abstract The 5.25-inch half-height drive bay was the dominant physical interface for peripheral storage and device mounting in personal computers from the early 1980s to the mid-1990s. While hard drives, tape drives, and even LCD panels were successfully miniaturized to fit this form factor, one display technology remained conspicuously absent: the Cathode Ray Tube (CRT). This paper explores the physical, electrical, and thermal impossibilities—and theoretical workarounds—involved in constructing a fully functional CRT monitor designed to fit within the 5.25-inch bay (41.3 mm height × 146 mm width). We conclude that while a monochrome, ultra-low-resolution electrostatic deflection tube could theoretically be manufactured, the resulting device would be functionally useless for video output and inherently hazardous. 1. Introduction The IBM PC 5150 standardized the 5.25-inch floppy drive form factor, which soon became a universal mounting standard. As the PC ecosystem evolved, third-party manufacturers produced a bewildering array of bay accessories: hard drives, CD-ROMs, sound card front panels, USB hubs, and even tiny LCD character displays. However, the CRT—the era’s primary display technology—remained an external peripheral or a full-height desktop enclosure. The question is not why a 5.25-inch CRT was never made, but what would be required to make one. 2. Dimensional Constraints A standard 5.25-inch half-height bay provides a frontal aperture of 146 mm (W) × 41.3 mm (H). The depth is typically 203 mm (8 inches). For comparison, the smallest commercial monochrome CRT used in portable televisions (e.g., the Sony Watchman) featured a 1.5-inch (38 mm) diagonal tube. Even that tube’s neck length (electron gun + deflection yoke) exceeded 70 mm—before adding the PCB. A CRT requires a minimum neck length proportional
Use electrostatic deflection (like an oscilloscope tube) to eliminate yoke power, saving ~10 W. However, electrostatic deflection requires extremely high deflection plate voltages (±300V) and severely limits scan angle, reducing screen size to <25 mm diagonal. 4. Electrical and Signal Interface A standard VGA or composite video signal requires horizontal scan rates of 15.75 kHz (NTSC) to 31.5 kHz (VGA). A 5.25-inch CRT would need to support these rates, but the flyback transformer for even 8 kV at 31 kHz is physically larger than the bay. Option: Use a DC-DC converter and a custom ferrite-core flyback. Miniature flybacks exist (e.g., in camera flashes), but they cannot sustain continuous operation at CRT scan rates without arcing. Thus, the total depth exceeds 110 mm, fitting