When installing or repairing video equipment or systems, a test pattern generator is a must. However, for this type of work you don’t need an expensive colour pattern generator with a myriad of options and settings. What is required is a simple signal source that allows a go/no-go indication.

While the specifications for this project don’t put it in the professional instrument class, it is light and rugged, can be carried in a toolbox and has the distinct advantage of being cheap. It also uses common components, is easy to build and should work first time. There is no setting up to do and there are no controls to fiddle with.

The circuit is powered from a 9V DC supply, which would normally be a 9V plugpack. It produces standard non-interlaced monochrome video and audio signals (see specifications) that are compatible with just about all TV sets that have a A/V inputs, VCRs and video monitors. Note, however, that this device is not suitable for testing most computer monitors.

Circuit details

Fig.1: clock signals for the circuit are provided by a 4MHz oscillator based on IC1a and these are divided down by dual 4-bit binary counter IC2 to produce most of the video timing signals. Dual decade counters IC3 & IC4 further divide the 500kHz output from IC2 to produce the vertical sync and audio output signals.

Fig.1 shows the full circuit details of the Audio-Video Generator. The circuit operation may not be obvious at first glance, mostly because the way in which the video signals are generated is a bit more complicated than normal. In addition, some circuit simplification and trickery has been applied to reduce the component count and keep costs down.

Clock signals for the circuit are derived from a 4MHz crystal oscillator formed around NOR gate IC1a. The 10MΩ resistor places the gate into linear mode and feedback is accomplished with the 4MHz crystal (XTAL1) and the two 22pF capacitors. Because we are using a crystal, the resulting clock signal is accurate and stable.

Inverter stage IC1b buffers the oscillator output which then clocks pin 1 of IC2, a 74HC393 dual 4-bit binary counter. In this case, IC2 has been cascaded to form a single 8-bit divider. Three of its outputs (pins 8, 9 & 10) are used for video timing, while a fourth output at pin 5 (500kHz) is fed to a divider circuit to derive a 500Hz audio signal. The signal at pin 5 is also divided down to produce a 50Hz vertical sync signal.