Version 2: Improved Solid-State Converter
Among the improvements on version 1 includes the use of the 2N3906 and associated resistors for improved linearity of the sawtooth output. More importantly, a loop is introduced in the circuit from the output to the trigger input to allow the 555 timers to operate in astable (free running) mode. The advantage is that a raster will be present in the absence of a video signal so one does not have to worry about all the precautions with the oscilloscope before and after using the converter. Below is a schematic of the slightly revised converter.
The 555 works by allowing the capacitor to charge until the voltage reaches a certain threshold voltage that triggers the 555 to quickly discharge the capacitor then the process starts again with the capacitor charging up again. The frequency is determined by the rate of the charge and is synchronized like a phase-locked loop. Synchronization is achieved simply by feeding the sync signal into the trigger input via a capacitor to create a crude AND gate that forces the 555 to reset whenever there is a voltage change in the sync signal. When the sync signal is absent, the 555 continues to free run although at a different frequency due to impedance changes on the trigger input with an absent sync signal.