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94  Silicon Chip POWER SOCKET EXTERNAL INPUT VIDEO IN VIDEO OUT 11 10 8  SEE TEXT 4.7k 1.5k 1.2k Q3 5.6k 10k 6.8k  7 6 100uF 680  Q6 150  2.2k 0.1 100uF 5 470W 100uF 10uF 7805 0.1 D3 4 1 270pF 75k 6 7 8 Q5 9 10 4 100uF 470 0.1 VR3 4.7k 6.8k 1M IC1 4066 1 47pF 11 IC2 4070 100pF 9 10k 100  1k 2.2k Q4 1.2k Q2 D1 1.5k 1.2k 0.1 1 2 100k 3 D2 10k 4.7k 1k Q1 82  Colour Video Fader, August 1993: there are several anoma­lies between the circuit and the wiring diagram. Also, due to spreads in the 4030/4070 XOR gates, it has been found necessary to make a number of changes. These corrections and changes are included on the revised wiring diagram reproduced here and this must be followed if you are building the project. Kitset suppli­ers have been advised of these changes. The changes are as follows: the 22kΩ resistor between the base of Q5 and the +5V supply rail should be 2.2kΩ; the 1kΩ resistor between the base of Q5 and the emitter of Q3 should be 1.2kΩ; and the 220Ω resistor at the emitter of Q4 should be 100Ω. On the wiring diagram, the connections to the video input socket are reversed. The 220pF capacitor at pin 4 of IC2b should be changed to 270pF. The 10kΩ and 12kΩ resistors connected in series between the +5V supply and ground at pin 2 of IC1a should be replaced with a 20kΩ trimpot (VR3). This trimpot should connect between the +5V and ground supply rails with the wiper connecting to pin 2 of IC1a. A hole will need to be drilled in the PC board to take the trimpot wiper. The trimpot will allow adjustment for correct sync pulse triggering by IC2a. VR3 is set up by first applying a video signal to the video input and viewing the output signal on your TV set (via your VCR). Rotate the Fade and Wipe controls fully clockwise with the wipe direction switch in the R-L position. Initially, centre VR3, then adjust anticlockwise until the picture starts to roll. Note this position. Now adjust VR3 clockwise and note the position that the picture completely loses sync. Finally, set VR3 in-between these two positions. The picture should now be in lock and the Wipe and Fade controls should operate. Having set the sync levels with VR3, the 10kΩ resistor at the base of Q3 may need to be adjusted to set the black level. You only need to do this if the wipe and fade controls do not provide a satisfactory black picture. If the picture is still visible on full fade or wipe, reduce the value of the 10kΩ resis­tor to 8.2kΩ. If this value does not provide sufficient bright­ness when the fader control is fully anticlockwise you may need to use a value between 10kΩ Fig.1: this revised wiring diagram for the Colour Video Fader includes all the changes described in the text. Note that you will have to drill an extra hole in the board to mount trimpot VR3. 470  Notes & errata 5 2 3 S1 1 VR1 and 8.2kΩ. This is achieved using paralleled values; eg, 10kΩ in parallel with 100kΩ gives 9.1kΩ. We also recommend earthing the potentiometer cases with a lead back to the video input socket as shown on the revised wiring dia­gram. Studio Twin 50 Stereo Amplifier, April, May 1992: since this amplifier was published, it has enjoyed modest popularity in the marketplace although the kit has since been discontinued. Part of the reason is that the original Darlington transistors have become virtually unobtainable. A number of kits have been supplied with TIP142/147 Dar­lingtons made by SGS-ATES and these have been found to be ther­mally unstable. If a Studio Twin 50 using these Darlingtons is left on long enough, it will most probably burn them out. The reason appears to be that the SGS transistors do not have the same bias and thermal characteristics as the Philips TIP142/147 transistors used in the original design. So as originally pre­sented, the circuit is not thermally stable with these SGS tran­sistors. Our remedy has been to modify the Vbe multiplier (Q17) and to increase VR2 the source degeneration resistors in the output stage. To be specific, the Vbe multiplier (Q7) is now a BD679 Darlington transistor and the resistor between its base and collector has been reduced from 680Ω to 330Ω. The 0.47Ω emitter resistors have been increased to 1Ω. This will slightly reduce the maximum power output. We have also reduced the quiescent current setting to around 25mA. These changes make the amplifier thermally stable but even so, its quiescent current stability is still not as good as would be the case with the originally speci­ fied Philips TIP­ 142/ TIP147 Darlington transistors. Amateur Radio, August 1993: the article on satellites requires a number of corrections. In Fig.1(a) page 73, the equations for apogee and perigee are transposed. Perigee height = a(1-e) - 6378km; apogee height = a(1+e) - 6378km. In Fig.1(b), the veloci­ ty of a low orbit satellite should be 26,000km/h not 13,000km/h. On page 74, in the paragraph beginning “AO-21 is a LEOS ... ”, the sentence referring to apogee and perigee heights should read: “Apogee and perigee heights are 1000km and 958km respectively”.