The Blue Ring Tester solves this by turning the component under test into a temporary parallel LC tank circuit.
Though CRTs are legacy technology, many repair technicians still service them for arcade machines, oscilloscopes, and vintage computers. The Blue Ring Tester is legendary for identifying shorted flybacks without discharging the HV cap.
When Q1 turns off abruptly, the magnetic field in the coil collapses, generating a flyback voltage spike. The coil and its parasitic capacitance form an LC tank circuit , causing the coil to ring (oscillate) at its resonant frequency. blue ring tester schematic diagram exclusive
The schematic presented below has been redrawn and refined from original service manuals and reverse-engineered vintage units. It includes component values that are often missing or incorrect in other online sources.
: The device counts the number of oscillations that exceed a specific voltage threshold. This count is then translated into the number of lit LEDs on the display. Practical Application and Limitations The Blue Ring Tester solves this by turning
A comparator compares the amplitude of the ringing signal against a reference voltage.
The exclusive hardware design of the classic Blue Ring Tester relies on highly stable CMOS logic chips rather than complex microcontrollers, ensuring high precision at low cost. Blue Ring Tester Installation Guide | PDF - Scribd When Q1 turns off abruptly, the magnetic field
The circuit works on the principle of —an LC oscillation created when a pulsed voltage is applied to an inductor.
Connect a known good inductor. The green LED should illuminate steadily. Short one turn of wire around the core. The red LED should now light.
The heart of the tester is an oscillator, often built around a 555 timer or similar IC, configured to produce a sharp square wave. This pulse triggers the coil being tested.
But here’s the secret: The real magic isn't in the LED. It’s hidden in a schematic so elegant, so counterintuitive, that it feels like electronic sorcery.