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By JOHN CLARKE RGB-to-PAL encoder replacement for the TV Pattern Generator Since publication of the TV Pattern Generator in Novem­ber 1991, the TEA2000 RGB-to-PAL encoder IC used in the circuit has gone out of production. This add-on board using a diff­er­ent encoder IC can be used as a replacement. The TV Pattern Generator described in the November and December 1991 issues was a very popular project. It produced a variety of patterns, including checker board, crosshatch, dot, white screen, greyscale, red screen and colour bars. The colour bar and red screen patterns relied on 58  Silicon Chip the RGB-to-PAL encoder functioning correctly to give the colour burst and chroma wave­forms in the composite video signal. In recent months, we’ve heard from quite a few readers who want to build this project but have been unable to do so because the TEA2000 encoder IC is no longer available. This drop-in board is the answer to that problem but there is a performance penalty which we’ll discuss shortly. It can also be used to restore a circuit to working order in those few isolated instances where the TEA2000 has failed. The add-in board is based on the Motorola MC1377P RGB-to-PAL/ NTSC encoder. This device has been available for many years and after being assured by the Motorola distributors in Australia that it is still in production, we decided to use it. Although the MC1377P is equivalent in function to the TEA2000, it Fig.1: the add-on circuit is based on the Motorola MC1377 RGB-toPAL/NTSC converter (IC2). IC1 provides buffering and blanking of the RGB input signals. does not have the same pinouts and, in our circuit at least, it also requires a separate blanking facility. In addi­ tion, the TEA2000 IC operated from an 8.86MHz crystal to produce the PAL signal while the MC1377P uses a conventional 4.43MHz colour burst frequency crystal. As shown in the photos, the addon PC board is mounted on the rear panel above the main PC board using a couple of right angle brackets. This board accommodates the Motorola MC1377P encoder, its companion 4.43MHz crystal and an additional quad AND gate IC (4081) which provides the blanking facility. There are 10 external connections and these are wired directly to the original circuit. Note that the original TEA2000 and its associated compon­ ents must be removed from the main board – see construction. It’s not as good Unfortunately, the quality of the colour bar pat­tern is not as good with the MC1377P (at least not in this de- sign) as it was with the TEA2000. In particular, there are faint horizontal lines across the colour bars and much more noticeable herringbone patterns between the bars. While these effects are probably not important as far as the overall test pattern is concerned, we thought it only fair to warn readers of the poorer picture quality. The other test pat­terns are virtually unaffected. What we are saying is that this board solves a problem if you wish to build the TV Pattern Generator but don’t expect too much in the way of picture quality on the colour bar pattern. For the same reason, we don’t expect any of the retailers to supply a kit containing the add-on board, particularly as all the original kits have now been discontinued. Circuit details Refer now to Fig.1 for the circuit of the RGB-to-PAL Con­verter. IC2 is the main encoder IC and it accepts sync and RGB (red, green and blue) signals on pins 2, 3, 4 & 5 to produce a composite video output at pin 9. This composite video signal includes the horizontal and vertical sync, the colour burst and the luminance and chrominance information. The 4.43MHz crystal oscillator at pins 17 & 18 produces the timing for the colour burst and chrominance signals. VC1 allows the crystal oscillator to be precisely trimmed, while the posi­tion of the colour burst signal is set by the ramp signal gener­ated at pin 1. In this circuit, it is placed right in the middle of the back porch. Note that the chrominance output at pin 13 is fed back into the pin 10 input via a 3dB resistive divider and a .001µF capaci­tor. The divider reduces the high level at pin 13 which is in­ tended to compensate for losses if a filter were to be included. The pin 10 input connects to the main TV Pattern Generator circuit and is shunted to ground via a 0.1µF capacitor and switch S2b when either the checker, hatch or dot pattern is selected. In other words, the colour burst and chrominance information is removed from the composite video output. March 1997  59 TABLE 2: CAPACITOR CODES      Fig.2: install the parts on the PC board as shown in this wiring diagram, taking care to ensure that the ICs and the electrolytic capacitors are correctly oriented. The external connections can be run using rainbow cable. Fig.3: this is the full-size etching pattern for the PC board. Check the board carefully before installing the parts. The composite video output appears on pin 9 and is fed to a 360Ω and 470Ω resistive divider to give the correct video level. The RGB and sync inputs from the main board are fed in via IC1 which is a 4081 quad AND gate. In the case of the sync signal, IC1a simply acts as a buffer stage, the sync signal then going directly to pin 2 of IC2. The RGB signals, on the other hand, are gated with a blanking signal that’s derived from pin 1 of IC10c on the main PC board. This effectively blanks the RGB signals during the horizontal sync and colour burst periods. The gated RGB signals appear on pins 11, 13 & 14 respec­tively and are fed to voltage dividers (12kΩ & 3kΩ) to obtain 1V p-p signals. They are Value  IEC 0.1µF 100n .01µF 10n .001µF 1n0 220pF 220p then coupled via 22µF capaci­tors to the RGB inputs (pins 3, 4 & 5) of IC2. Power for the circuit is derived directly from the main PC board. Note that two separate supply rails are used. IC1 is powered from a 5V rail, while IC2 is powered from a 12V rail. Construction The RGB-to-PAL Converter is built on a PC board coded 02302971 and measuring 98 x 53mm. Start construction by checking the PC board against the published pattern for shorts or breaks in the tracks. Fig.2 shows the parts layout on the PC board. Begin the assembly by installing PC stakes at all the external wiring points, then install the two wire links and the resistors. Table 1 lists the resistor colour codes but it is also a good idea to use your multimeter to check each value just to be sure. The ICs can be installed next, taking care to ensure that they are oriented correctly. This done, complete the assembly by installing the capacitors, the trimmer (VC1) and the crystal (X1). The electrolytic capacitors must all be oriented correctly, while the crystal can be installed either way around. Installation If you are building the TV Pattern Generator PC board as well, the following components should be omitted: the TEA2000 (IC16), the 8.86MHz crystal, TABLE 1: RESISTOR COLOUR CODES          No. 1 3 1 1 3 2 1 1 60  Silicon Chip Value 56kΩ 12kΩ 10kΩ 2.2kΩ 300Ω 1kΩ 470Ω 360Ω 4-Band Code (1%) green blue orange brown brown red orange brown brown black orange brown red red red brown orange black brown brown brown black red brown yellow violet brown brown orange blue brown brown EIA 104 103 102 221 5-Band Code (1%) green blue black red brown brown red black red brown brown black black red brown red red black brown brown orange black black black brown brown black black brown brown yellow violet black black brown orange blue black black brown PARTS LIST 1 PC board, code 02302971, 98 x 53mm 11 PC stakes 1 50mm length of 0.8mm tinned copper wire 2 right angle mounting brackets 4 3mm screws and nuts 1 4.43MHz crystal (X1) Semiconductors 1 4081 quad 2-input AND gate (IC1) 1 MC1377P RGB to PAL/NTSC converter (IC2) Capacitors 3 22µF 16VW PC electrolytic 1 10µF 16VW PC electrolytic 3 0.1µF MKT polyester 1 .01µF MKT polyester 2 .001µF MKT polyester 2 220pF ceramic 1 3-30pF trimmer (VC1) The add-on colour converter board is secured to the rear panel of the TV Pattern Generator using right angle brackets and machine screws and nuts. It takes the place of the original TEA2000 RGB-to-PAL encoder (IC16). These oscilloscope waveforms show the colour bar composite video signal (top), the sync signal (centre) and the blanking interval signal (bottom). the associated trimmer capacitor (VC1), the two 5.6pF capacitors, the 1kΩ and 910Ω resistors at pin 8, the 390Ω and 470Ω resistors at pin 6, and the 330pF ca­pacitor and 36kΩ resistor at pin 15. If you have already built the board, it will be necessary to remove these components. Next, insert hookup wires (eg, rainbow cable) into seven of the vacant TEA2000 pads at pin positions 1, 3, 5, 9, 10, 11 & 16. Additional hookup Resistors (0.25W, 1%) 1 56kΩ 3 300Ω 3 12kΩ 2 1kΩ 1 10kΩ 1 470Ω 1 2.2kΩ 1 360Ω wires connect to pin 1 of IC10c, pin 16 of IC15 and to the base of Q1. The most convenient place to connect to the latter is at the junction of the 390Ω and 470Ω resistors. The add-on board is mounted on the rear panel above the BNC output socket and is secured using right angle brackets and machine screws and nuts. You will have to drill a couple of holes in the rear panel to mount the brackets. Once the board is in place, it’s simply a matter of connecting the various hookup wires to the PC stakes, as shown in Fig.2. To test the unit, first apply power and check for +5V on pin 14 of IC1 and +12V on pin 14 of IC2. It’s then simply a matter of connecting the unit to a TV set using either the video modulator or the direct video output and checking that the unit works properly. If the colour is missing, adjust VC1 on the add-on board to obtain the correct colour burst signal. Footnote: we have been informed that Rod Irving Electronics still have limited stocks of the original TEA2000 SC encoder. March 1997  61