Testcard Generator

Originally published by Paul Stenning in ETI, December 1991 and January 1992

• Diagrams and Schematics

Introduction
There have been several designs of test pattern generators published in the past, invariably they produce the standard cross-hatch pattern, possibly horizontal and vertical lines, and may-be a grey-scale. Most current designs are based around the ZNA234E monochrome test pattern generator IC.

This is adequate for monochrome (black and white) equipment, however for servicing colour televisions and video recorders, something a little more comprehensive is really required. The instrument featured here provides an impressive range of patterns including a full colour Test-Card, colour bars, crosshatch, grey-scale, focus grid, colour purity patterns and more - for considerably less cost than a commercial unit of lower specification!

The unit also features a basic sine-wave audio oscillator, set at 800Hz. The audio and video outputs are available from the audio and video output sockets and, for more modern equipment, the SCART socket. An RF modulator is provided for connection to the aerial socket of UK standard Televisions and Video Recorders (readers in other countries using the PAL standard may need to obtain a suitable type of modulator locally). The audio and video signals to the modulator are individually switchable to either the internally generated signals or to the external audio and video input and SCART sockets.

A Scope Trigger Output socket is provided to give reliable triggering on an oscilloscope when trying to view the video signals, the output from this can be switched to either line or frame sync.

Features and Specifications

Construction
All the components except the switches, sockets SK1 to SK5, mains transformer T1 and resistor R4 are mounted on the PCB. This is a single sided board, about 185 x 165 mm in size, and is available from the ETI PCB service, order number ****, see page ***. There are 45 links and it is advisable to fit these first, using tinned copper wire, about 24 SWG. The remaining components can then be fitted in the usual size order. Note that there are tracks between many of the IC pins so a fine tipped soldering iron and due care should be used. All the IC's except IC12 and IC13 (the regulators) should be fitted in sockets, and should not be inserted until the power supply voltages have been checked - see Testing and Setting-up. Veropins may be used for the off-board connections if desired. Take care particularly when fitting the inductors, crystals and variable capacitor since they are easily damaged by rough handling and excessive heat. When fitting the modulator, ensure it is well down, then twist the mounting tags by about 45 degrees and solder. Do not connect the wires from the modulator to the board until the power supply voltages have been checked.

The prototype was housed in a plastic case approximately 170x70x190mm, however this was a little tight for comfort. Since the instrument is mains powered, a slightly larger earthed metal case would be more suitable. The PCB is mounted at the rear of the case, with cut-outs in the rear panel for the SCART socket and the UHF output socket on the modulator. The other sockets are also mounted on the rear panel.

All the switches are mounted on the front panel, a suitable legend is shown in fig **. Switch SW2 is a rotary type, 1 pole 12 way, with the stop set at position 10 (note that no connection is made to pin 10, although position 10 is used). The other switches are the Japanese type latching switches, and are mounted in two banks of four. The individual switch latches are left in place and the interlocking mechanism is not used.

The inter-wiring is shown in fig **, thin coax cable is used for all signal wiring (proper 75R coax could be used for the video but it is not too important with the lengths involved), and standard hook-up wire for the remaining connections. The mains connections should be insulated, and the flex secured in some manner. No mains on/off switch or fuse were fitted on the prototype, however they could be fitted if required. The mains plug should be fitted with a 3 amp fuse.

Testing and Setting-Up
Due to the cost of the modulator and some of the IC's it is a good idea to check that the power supply voltages are correct before fitting these parts. Make sure none of the IC's except IC12 and IC13 are fitted, and that the modulator is disconnected. Give the PCB a final visual check over, in particular watch for solder bridges between IC pads and the tracks between them.

Connect the instrument to the mains, and switch on. Set your test meter to 20 volts DC or thereabouts, and connect it's negative lead to the case of the modulator. Check for 12 volts +/- 0.5 volts on pin 11 of IC9 socket, and 5 volts +/- 0.25 volts on pin 20 of IC8 socket. If all is well switch off and wait a minute or so for the power supply capacitors to discharge, then connect up the modulator and fit the IC's. Most of the IC's are CMOS types but have protected inputs so static damage is unlikely, however the usual handling precautions against static discharge should still be taken.

Set SW2 to position 1 (full ACW); SW3, SW4 & SW5 on, SW6 off and SW8 & SW9 to Internal (all buttons out). Connect the RF output socket on the modulator to your TV, cross your fingers and switch on. You will probably need to tune your TV to channel 36, although the channel you use for your video recorder should be somewhere near. You should get an audio tone from the speaker and a test-card picture. Don't worry at this stage if there is a buzz on the sound or no colour in the picture.

To avoid damage to components and misleading results, always use a proper trim-tool when making adjustments. Adjust RV1 for minimum buzz (or to the middle of the area of no buzz) on the sound. Adjust VC1 until the colour locks on, then set to the centre of the range where you get colour. Finally adjust L2. If you have a 'scope connect it to the Video Output socket and adjust for maximum amplitude on the colour burst, otherwise adjust for brightest colour and minimum "herringbone" pattern on the picture. All that remains now is to check the various functions of the unit as detailed below. Once the unit has been tested, the PCB can be fitted in the case, and the unit finished off.

In Use
Firstly a word of warning. Anybody attempting to repair or adjust Television Sets, whether using this instrument or not, must be aware of the hazards involved. Many sections of TV circuitry operate at dangerous voltages (in the range of 100V to 30KV), and in many cases the internal chassis and metalwork is also at a dangerous (mains) voltage. For maximum safety the set should always be powered via a mains isolating transformer. If a transformer is not used, no test equipment should be connected to the set, except to external sockets, and very great care should be taken when making adjustments with the power on (the practice of disconnecting the mains earth to all the test equipment is extremely dangerous and should never even be considered). In any event always work with one hand in your pocket when the set is on, even with an isolating transformer the high voltages can be lethal. Care should also be taken when working near the power supply of video recorders, although in this case the rest of the circuit usually operates at more civilised voltages. The use of an Earth Leakage Circuit Breaker on the mains feed to the bench is also a sensible precaution.

In most cases the instrument would be connected to the equipment under test via its aerial socket, since this enables the whole signal path to be checked, also a fault in the equipment is extremely unlikely to damage this instrument when connected this way. Once the equipment has been proved to be basically sound, the instrument could be connected by the video/audio or SCART connectors if required.

SW2 selects which basic pattern is produced, as follows:

If SW6 (Colour/Mono) is pressed in, the colour information will be removed from the picture, this will convert the Colour Bars to Grey Scale, and remove the "herringbone" patterning on patterns such as the Cross Hatch.

Pressing SW3/SW4/SW5 will remove the Red/Green/Blue colour completely, this is the same effect as switching off that colour gun inside the TV (with all three off there will be no picture). If this is used in conjunction with the White Raster pattern, the sets colour purity can be adjusted. It is also useful for removing the blue from the Cross Hatch when starting to set up the convergence. (If SW3,4 & 5 are all switched off when SW2 is set to the test-card pattern, a faint pattern will still be present - this should be ignored).

The features of the Test Card require further comment. The overall size of the picture is slightly smaller than the minimum size of a transmitted picture. The TV's picture height, width and position controls should be adjusted such the whole test card is shown, but with the tips of the corners very slightly hidden by the rounded corners of the tube (you may find that some cheaper sets have the width adjusted wider than this, this is because the width of the picture on these sets varies with brightness). The overall brightness of the Test Card is about that of an average picture. The background is dark grey (25%) with a white Cross Hatch pattern, to enable the linearity to be checked.

The main area of interest on the Test Card is the centre section. Starting at the top we have a black rectangle within white to check the LF response, and a white needle pulse to check for reflections and instability. Next we have a 250KHz, 100% to 0% square wave to test the transient response; and below this are the eight colour bars. Next there is an six step frequency grating with frequencies of 0.5, 1.0, 1.25, 1.66, 2.5 and 5.0 MHz. These enable the frequency response of the video circuits to be checked, the gratings up to 2.5 MHz should be clear and sharp, but many sets will have trouble resolving the 5.0 MHz. Finally there is a five step grey scale for adjusting contrast, with amplitudes of 0%, 25%, 50%, 75% and 100%.

I will not go into further details of which adjustments should be made with which pattern. I you have the service manual for the equipment this will give suggestions, otherwise it's a matter of experience and taste - if the set will give an acceptable picture on all patterns there can't be much wrong! Some sets may have trouble locking onto the output from this unit, in this case try switching off the sets AFC and tuning manually. Remember that a test pattern is intended to show the worst in a piece of equipment. At the end of the day the picture should be acceptable to the viewer on a normal programme!

As its name suggests, the Scope Trig socket can be used to externally trigger an oscilloscope when trying to view the video signals inside the equipment. The output is 5 volts peak-to-peak at 10K, and can be switched to either line or frame sync by SW1. Bear in mind, though, that the outer of this socket is earthed inside the unit so you may be earthing the TV's chassis via the 'scope leads.

When using the video and audio output sockets it should be noted that the signals from these are always the internally generated ones, regardless of the positions of the Int/Ext Video and Audio switches; these switches only affect the output from the RF modulator. This can be useful if, for example, a video recorder under test is connected to the SCART socket, and the RF output connected to the workshop TV. The signal from the generator can be compared to that from the video recorder merely by operating the Int/Ext switches (no more fumbling with RF leads behind the TV!). The unit can also be used to enable the composite video and/or audio outputs from equipment such as video cameras and computers (and certain satellite decoders!) to be fed to a normal TV via the aerial socket.

Care should always be taken, particularly when working on faulty equipment, to ensure that no excessive voltages are applied to the unit, as this will almost certainly cause expensive damage.

Diagrams

• Circuit Diagram Page 1
• Circuit Diagram Page 2
• Circuit Diagram Page 3
• PCB Artwork
• PCB Component Layout
• Interwiring Diagram
• Front Panel

Zip Files

Tips
Tom Payne has developed a method of replacing the SAA1043 with a SAA1101, and kindly supplied details.

Parts List

IC sockets (1 x 8 way, 4 x 14 way, 1 x 16 way, 1 x 18 way, 1 x 20 way and 3 x 28 way), Knob for SW2, Buttons and brackets for SW1 & SW3-9 (Maplin FH61R & FH78K), PCB, Case 170x70x190mm (WxHxD) or larger, Coaxial cable, interconnection wire, tinned copper wire (about 24 SWG), 3 core 3 Amp mains flex, 13A mains plug with 3A fuse, Phono plug to Co-ax aerial plug lead (Maplin FV90X), other leads as required, grommet, nuts, screws & spacers.

Buylines

All Components except IC6, IC7 & IC8 could be obtained from Maplin Electronic Supplies Ltd when the project was originally written. The Maplin Order Codes for the more obscure components are given in the parts list. The 74HC574 (IC6) & 27C128 (IC7/8) were listed by RS/Electromail and several other suppliers.

The data for the EPROMs is provided in Intel-Hex format - see link above. If you require a different format for your EPROM programmer, you should download the software for the EPROM Programmer Mk2 since this contains a utility to convert the data to many popular formats.

Update (May 1992)
Update notes were published in the May 1992 edition of ETI. Many of these have now been incorporated into the main text and diagrams, the remaining points are detailed below.

Modification
The wire from colour switch (SW6) to the PCB should be co-ax to prevent stray pickup affecting chrominance level, a hole is available on the PCB for the screen. The 0v link between SW6 and SW3 should be removed and the switch mounting brackets connected to 0v at SW5.

Buylines
It has come to the author's attention that 150nS 27C128 EPROM's are not too easy to come by. The prototype has since been tested with 250nS EPROM's which work fine. It should be noted that the EPROMs must be the CMOS 27C128 types, as normal 27128 devices consume far too much power and will overload IC13 (the 78L05 regulator) causing it to current limit.

Maplin have changed their range of 0.1" pitch ceramic plate capacitors to ceramic disks with a 0.2" pitch, without changing the order codes! These will fit the PCB if the leads are bent, or the correct type can be obtained from RS/Electromail.

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.