Test Pattern Generator

Originally published by Paul Stenning in Electronics and Beyond (The Maplin Magazine), January 1997

• Diagrams and Schematics

Introduction
When servicing and repairing televisions and other items of video equipment, it can be helpful to have a stable and recognisable test signal. A colour bar pattern is often used for this, and indeed many television and video recorder service sheets use this type of signal as a reference.

A variety of other patterns are used for specific tests. For example, a crosshatch is useful for checking picture linearity and convergence (colour gun alignment), while a checkerboard pattern or dots are ideal for setting up the focus.

The test pattern generator described here features these patterns as well as horizontal lines, vertical lines, dots, blank raster and white screen. The three primary colours are individually switchable. So for example the white screen can be switched to red, green or blue for checking colour purity.

There have been several designs for test pattern generators published in the past. Many of these are monochrome only and are often based on the ZNA234E IC. Published designs for colour units are often complicated, and therefore probably too expensive for occasional home use.

The aim of this design was to produce a straightforward reliable piece of equipment with no unnecessary frills and features. It should be possible to construct the unit for around seventy pounds including the case, which is much cheaper than commercial units having a similar specification. Setting up is straightforward and requires no test equipment.

This unit is an improved and updated version of a project by the same author, which was published in the March 1994 edition of "Electronics in Action" magazine. The original design was one of the most popular projects in the magazine, with over one hundred PCB's being sold. Since "Electronics in Action" is sadly no longer being published, and a couple of the components used in the original design are now obsolete, it was felt that now is the right time to publish this revised version.

A major improvement with this version is that construction has been simplified. The circuit is built on two PCB's, and requires no interwiring. The white level can now be switched to either 75% or 100%, so that the colour bar pattern can give a proper white reference level. The original Astec modulator is no longer available, so this unit uses an Alps modulator from Maplin. Since this modulator has sound capabilities, a 1KHz audio tone generator has been added. Other modifications have been carried out to improve performance and reliability.

The instrument is mains powered. A standard composite video output is available (1V pk-pk into 75R), as well as UHF modulated (UHF channel E36). An oscilloscope trigger output is also available, which is switchable to either line or frame sync, or to the audio oscillator.

Circuit Description
The circuit diagram is shown in figure 1. Most of the complicated stuff is handled by IC1 and IC2, which helps to simplify the remainder of the design. IC1 (SAA1043) is a universal sync generator, whose timing is controlled by XT1. In this design it is configured for the UK standard 625 line interlaced PAL, and four outputs are used.

IC1 requires a supply voltage between 5.7V and 7.5V. The SAA1043 has been operated with a 5V supply in several published designs (including some Maplin projects and the original version of this design). However experience has shown that a limited number of devices will not operate correctly at this voltage.

Since the current consumption of IC1 is very low (less than 1mA), it is powered by a 6V8 supply, which is derived from the 12V supply by a zener diode (D3) and resistor (R14). The output signals from IC1 are reduced to suitable levels for the 5V logic circuits by means of resistors (R15 to R19) and diodes (D4, 5, 6 and 9).

A 5MHz clock signal is derived from XT1. This controls the first binary counter (IC3, 74HCT4040), which is reset at the end of each line by the CB (composite blanking) signal. 258 clock pulses are counted by IC3 between each blanking pulse.

A second binary counter (IC4, 74HCT4024) counts the 312 or 313 CB pulses within a frame. This counter is reset by the VERT signal at the end of each frame. The fact that the counter has a capacity of only 128 is irrelevant in this case, since it merely wraps round.

PCB Construction
The circuit is assembled on two single sided PCB's. The track layouts and component overlays are shown in figures 2 and 3. Start with the wire links (use 24 SWG tinned copper wire), and then fit the components in the usual size order (starting with the smallest). Sockets may be used for the IC's if desired.

On the main board, switches SW3, 4, 5 and 7 should be assembled onto a four position latch bracket before being fitted to the PCB. The interlocking plate must be removed, and the individual switch latches retained. The knobs should also be fitted at this stage to avoid stressing the PCB later.

The voltage regulators (IC9 and IC10) must be fixed to small heatsinks and the PCB using M3 screws, nuts and washers. Although no electrical insulation is necessary, an insulator between the device and heatsink will improve the thermal coupling.

SK1 must be fitted before the modulator, because of the (unused) phono socket on the side of the latter. SK4 (IEC mains inlet) should be fixed to the PCB with M3 hardware prior to soldering. The transformer should be fitted last, because of its size and weight.

Before fitting SW1 and SW2 to the front PCB, the loops must be cut off the end of the tags. When doing so, be sure to leave the remaining tags as long as possible. SW2 should be positioned with the front panel locating tab towards the upper right corner as shown. The locating tabs are not used, and should be cut close to the body of the switches.

The two PCB's are linked using a 19 way length of right angle SIL header strip. This will need to be cut from a longer piece. The angled end of the pins must be fitted into the front PCB, so that the straight ends can then be fitted into the main PCB.

Case Assembly
The PCB's were designed to fit into the specified case. In the base of the case there are two flat tags towards the back which must be removed.

There are also a number of PCB fixing posts, of which only seven are used. The remaining posts must be shortened so that they do not foul on the underside of the PCB. This may be easily achieved using a sharp 8mm drill bit in a hand operated or SLOW electric drill (such as a rechargeable battery drill). Locate the tip of the drill bit in the hole at the top, and cut away each post until it is about half the original height.

Drilling details for the front and rear panels are shown in figure 4. The rectangular holes may be cut by drilling holes in two opposite corners and then cutting out the remainder using a fret saw. This can then be finished with a small file.

Suitable overlays for the front and rear panels are shown in figure 5. These may be photocopied onto a white A4 self-adhesive label and fixed to the appropriate panels. For durability they should then be covered with clear self-adhesive vinyl. The label and vinyl can then be trimmed around the holes using a sharp craft knife or scalpel. You may need to chamfer the inside edges of these panels slightly to allow them to fit into the case, because of the extra thickness on the outside.

Set the end stop of SW1 to eight positions, and SW2 to three positions, and then fit the front panel to the switches. The shakeproof washers supplied with the switches should be positioned between the switches and the panel. Cut the shafts down to about 8mm, then fit the knobs.

The rear panel should be fixed to SK4 using M3 hardware. The supplied nuts should be used to fix the panel onto SK1 and SK2 (discard the shakeproof washers).

The whole assembly should then be slotted into the base of the case. Depending on the accuracy of your drilling you may need to ease the mounting holes in the PCB slightly, so that they coincide with the posts in the case. The PCB is held in place using seven 6.35mm long No 4 self tapping screws. Do not overtighten otherwise the screws will strip the threads they have cut in the posts.

Testing and Setting Up
Providing the unit has been carefully constructed there is no reason why it should not work first time. Connect the unit to the mains using an IEC mains lead. If a test meter is available, check the +12V and +5V rails are correct on the appropriate pins of the voltage regulators.

Take care as there are mains voltages on the PCB. These are limited to the area around T1, FU1 and SK4, and are fairly inaccessible from above providing the cover has been fitted to the fuse holder - but due care must still be taken.

Set the small slide switch on the back of the modulator to the upper position. Connect the UHF output to a television and tune in to UHF channel E36 (often used for video recorders and video games). You should be able to tune in two white vertical bars (this pattern is generated within the modulator). An audio tone should also be heard.

Set the switch on the modulator to the lower position. Ensure the four front panel push switches are out. Set SW2 fully clockwise and a set of colour (or monochrome) bars should appear. You may need to adjust the tuning on the television slightly.

Adjust C6 with a suitable trim-tool until the colour locks on. Adjust it back and forth to find the range of adjustment that gives a colour picture, then set it to the centre of this range.

Now adjust L1 for the clearest colours and a well defined edge where the colours change. This will probably be about a third of a turn up from the bottom. If an oscilloscope is available this can be connected to the video output and L1 adjusted to peak the amplitude of the colour information on the signal.

If SW2 is turned to the centre position, the direction of the colour bar pattern should reverse. Operate SW7 (White), and the brightness of the white bar should reduce.

If SW2 is now set fully anti-clockwise, the pattern displayed is controlled by SW1. Check SW1 works as expected, then set it to the white screen position and check SW3, SW4 and SW5.

If the unit is left on for a while, the voltage regulators (IC9 and IC10), the TEA2000 (IC2) and the mains transformer (T1) will become warm. You may also notice a faint smell of varnish from the transformer the first couple of times it warms up. This is normal, and should not cause concern.

Finally fit the top cover of the case, with the two plastic tabs to the rear.

In Use
The UHF modulator has a pass-through facility, similar to that on video recorders. The unit may therefore be connected between an aerial and the equipment being tested, so that the normal channels may be obtained without changing connections. Due to the digital nature of the unit and the signals produced, it may cause some interference on the normal channels unless the signal from the aerial is fairly strong. If this occurs you could try using an aerial amplifier between the aerial and the unit, otherwise you will need to use the unit separately.

The modulator UHF output is tuned to channel E36 when supplied. An adjustment screw on the back allows this to be adjusted between channels E30 and E39. This adjustment may be necessary if you are using the unit in conjunction with a video recorder or if you live in an area where Channel 5 is (or will be) broadcasting on channel E35 or E37.

The unit should be disconnected from the mains when not in use.

One final important comment - if you are working inside a television set or other piece of equipment with the power on, please be very careful.

Acknowledgements
The author would especially like to thank the former editor of "Electronics in Action" magazine, for granting permission to publish this design. He would also like to thank Maplin/MPS for supplying the components used to construct the prototype. The Maplin codes for the more critical components are given in the parts list).

Diagrams

• Circuit Diagram
• PCB Artwork - Front Board
• PCB Artwork - Main PCB
• PCB Component Layout - Front Board
• PCB Component Layout - Main PCB
• Front and Rear Panel Drilling
• Front and Rear Panel Artwork

Zip Files

Tips
Tom Payne has developed a method of replacing the SAA1043 with a SAA1101, and kindly supplied details. Link is in the list below.

Specification of Prototype

Parts

This project, including all text, images and diagrams, is copyright 1991 - 2003 Paul Stenning. No part of this article may be reproduced in any form without prior written permission from Paul Stenning and WallyWare, inc. All details are believed to be accurate, but no liability can be accepted for any errors.