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Build this satellite TV receiver; Pt.2 This satellite TV receiver is based on a prealigned module. By combining it with a dish antenna & an LNB, you can receive many of the satellite TV sign­als (both C & K-band) that are available in this part of the world. By JOHN CLARKE Last month, we looked at the basics of satellite TV recep­tion and described the equipment needed for a complete ground station. As shown in Fig.7 of that issue, a ground station consists of a para­bolic dish antenna, a low noise block (LNB) downconverter, a satellite receiver and a TV set. The satellite receiver is the one item amongst this equip­ment that can be easily constructed at home. This particular unit is based on a pre-built receiver module that comes fully aligned. All you have to do is add a few 12  Silicon Chip ancillary circuits plus a power supply and install the bits in a case. In operation, the receiver is used to tune the signals from the LNB. What happens is that the incoming satellite signal is first collected by the dish antenna and directed to a dipole antenna in the LNB via a waveguide, after which it is amplified and downconverted. Downconversion simply refers to the fact that the incoming satellite frequency (either in the range from 3.7-4.2GHz or 12.25-12.75GHz) is converted to a much more manageable signal in the range from 950-1450MHz. It is this range of fre­quencies that are tuned by our satellite receiver. Fig.8 shows the block diagram of the pre-built receiver module. It utilises a tuner module which initially amplifies and filters the IF signal from the LNB. This signal is then fed to a mixer stage where it is mixed with the signal from a varicap tuned local oscillator for second conversion to 479.5MHz. The “tuned” signal is then fed via a bandpass filter and two further amplifier stages to a PLL de­modulator. This demodulator stage produces a “baseband” output which contains both video and audio information. Composite video and audio output signals are then recovered using separate demodula­tor stages. These signals can be fed direct to a video monitor and audio amplifier. In addition, the composite video and audio output signals are fed to an RF TUNER IN METAL BOX INPUT FROM LNB AMP AMP AMP MIXER AMP FILTER BASEBAND PLL DEMODULATOR BP FILTER DC AMP RF MODULATOR AGC DETECTOR VCO PLL DEMODULATOR 5MHz-8MHz RECEIVER MODULE 14/18VDC TO LNB VIDEO TUNING AGC +18V Block diagram Fig.9 shows the block diagram of the complete receiver. The pre-built module forms the heart of the design, with the extra circuitry all on an auxiliary PC board which you assemble your­self. This second board carries the IF (video) and audio subcar­rier tuning controls, the band switching, the LNB polarisation circuitry, the skew controls, and the metering and power supply circuitry. The two boards are 14/ 18VDC 9501450MHz then linked using ribbon cable and connectors. The rear panel of the receiver carries an IF input socket (to accept the signal from the LNB) plus the following outputs: VHF Out (this goes to the antenna socket of a TV receiver), Audio Out, Video Out and Skew Out. A small slider switch is also pro­vided so that either channel 3 or channel 4 can be selected for VHF Out. To cater for the different equipment available on the market, we have included both “dual polarity switching” and mechanical feedhorn “skew” functions in the receiver. Let’s take a closer look at what these terms mean. RECEIVER MODULE VIDEO VIDEO AUDIO AUDIO OUTPUTS +5V LNB POL +18V 240V A N VIDEO TUNE VR4 AGC RF OUTPUT AUDIO OUTPUT As with terrestrial TV signals, satellite TV signals are polarised to minimise interference between adjacent frequencies. This means that the dipole antenna in the LNB must be oriented to match the polarity of the incoming signal – ie, horizontal for horizontally-polarised signals and vertical for vertically-polar­ised signals (see Fig.10). Although this could be achieved by physically rotating the LNB, it is hardly a convenient solution. Fortunately, the answer to this problem is quite simple and two methods are commonly employed. The first method involves fitting the LNB with two dipole antennas mounted 90° apart – one horizontal and the other vertical. Either one of these dipoles can then be selected at will (using electronic switching) to match the signal polarisa­ tion. In practice, RF RF INPUT VIDEO OUTPUT +5V Fig.8: block diagram of the receiver module. The IF signal from the LNB is amplified & then mixed with the signal from a VCO for second conversion to 479.5MHz. The “tuned” signal is then fed via a bandpass filter & two further amplifier stages to a PLL de­modulator. Composite video & audio output signals are then recovered using separate demodula­tor stages. modulator stage. This then provides an output which can be fed to the antenna output of a conventional TV receiver tuned to either channel 3 or channel 4. VIDEO DEMODULATOR AUDIO TUNE VR3 Fig.9: this is the block diagram for the complete receiver. It’s based on the pre-built receiver module & adds in the necessary power supply circuitry, the tuning controls, a signal strength meter & the skew control circuitry. IC2 POWER SUPPLY +18V S3 PULSE GENERATOR E REG1,D5, D6 +14V SKEW OUTPUT SIGNAL METER SKEW VR1 June 1995  13 HORIZONTAL POLARISATION LEFT HAND CIRCULAR POLARISATION Fig.10: satellite TV signals can be either horizontally polarised or vertically polarised, just like terrestrial TV signals. In addition, some satellite signals on C-band are circularly polarised & these are best received using a servo-controlled feedhorn. VERTICAL POLARISATION RIGHT HAND CIRCULAR POLARISATION this is achieved by selecting between two DC voltages (either 14V or 18V) and feeding this back up the coaxial cable to the LNB, where the dipoles are selected using diode switching. This “dual-polarity” type of LNB is used primarily for receiving linear signals (ie, signals that are either horizontal­ ly or vertically polarised) on both C-band and K-band. However, a complication arises when we wish to also receive circularly polarised signals. These signals can be either lefthand or righthand circularly polarised (see Fig.10) and, in This pre-built receiver module carries an outboard tuner module & forms the heart of the Satellite TV Receiver. 14  Silicon Chip this part of the world, are transmitted only on C-band. To cater for these signals, a servo-controlled feedhorn is often used. This type of feedhorn employs a digital proportional servo motor which rotates a probe through an angle of about 200° inside the waveguide. This probe is mutually coupled to a dipole antenna and is oriented using the skew controls for best signal pick-up. In operation, the servo motor requires +5V and ground con­nections, plus a continuous “pulse” (Skew Out) signal. The servo motor then “skews” to an angle that’s dependent on the width of the pulses. The Skew switch and Skew Adjust control on the front panel set the pulse width and thus the angle of the probe in the feed­horn. Either horizontal (H) or vertical (V) orientation is ini­tially selected using the Skew switch, while the Skew adjust control allows the probe to be rotated to suit the signal. Circuit details Fig.11 shows the final circuit details of the Satellite TV Receiver. The receiver module is nominally designed to accept centre-tapped 23V and 15V AC supply rails but we’ve simplified the supply arrangements to take advantage of a readily-available transformer. As shown in Fig.11, power is derived from the 0-17.5V secondary of an M-6672 mains trans­ form­er and this drives a bridge rectifier consisting of diodes D1-D4. The resulting 25V (nominal) DC rail is then applied to 3300µF and 2200µF filter capacitors and to separate 18V and 12V regulator circuits on the receiver module. Note that the input to the 12V regulator is fed via an external 6.8Ω 5W dropping resis­tor. This measure is necessary to reduce dissipation in this regulator. The 18V regulated output appears at pin 17 of the module and is applied directly to one terminal of switch S3 (LNB) and to 3-terminal regulator A F1 500mA POWER S1 T1 6672 D1-D4 4x1N4004 6. 8  5W 8 0V 6 240VAC OUTPUTS RF 18V REG 18 17.5V 14 N 4 BASE PLATE AUDIO 18V 17 OUT 20 RECEIVER MODULE 13 12 E VIDEO 12V REG AGC C IN +18V VIDEO TUNE VR4 10k 5V 15 OUT 16 10 25VW REG1 7815 GND D5 D6 OUT 10 16VW 2x1N4004 +5V LNB S3 TO LNB 14/18V POLARISATION AUDIO TUNE VR3 10k 11 18V 14V K BAND S4 120  10 470  5 SIGNAL METER 8 IC2a 6 LM358 4 VR5 1k 7 2 1 IC2b 250uA 2.2k 3 1k B E C VIEWED FROM BELOW A POWER LED1 ZERO SET VR6 2k Q1 BC328 E B 10k  C 10k H SKEW S2 V 2.2k SKEW ADJUST VR1 10k D7 1N4148 K 2.2ms SET VR2 20k +5V 10 8.2k 7 150k 4 3 100  IC1 555 6 2 I GO 8 1 SKEW OUT GND 0.15 SATELLITE TV RECEIVER Fig.11: this is the complete circuit for the Satellite TV Receiver. Q1 & 555 timer IC1 provide the skew pulses, while IC2a buffers the AGC line to drive the signal strength meter. IC2b & VR6 provide a no-signal DC offset adjustment so that the meter can be zeroed, while VR5 sets the meter sensitivity. REG1. The resulting 15V output from REG1 is then fed to the 14V terminal of S3 via dropping diodes D5 and D6. As a result, S3 selects either 18V or 14V and feeds the selected voltage to the LNB. In the 14V position, the vertical dipole is selected. Conversely, in the 18V position, the horizon­tal dipole is selected. IC1 forms the heart of the skew control circuit. This 555 timer is wired in astable mode and produces pulse widths ranging from 0.65ms (for vertical polarisation) to 2.2ms (for horizontal polarisation). The pulse repetition rate is about 66Hz. Looking at this more closely, the 0.15µF timing capacitor on pins 6 and 2 is charged via D7, potentiometer VR1 and its parallel 8.2kΩ resistor, a 2.2kΩ resistor and either Q1 or VR2. Switch S2 provides the horizontal and vertical skew control. When “H” is selected, Q1’s base is pulled high and so the transistor is off and the 0.15µF capacitor charges via trimpot VR2 to give a nominal 2.2ms charging time. During this time, IC1’s pin 3 output is high. Conversely, when S2 is in the “V” position, Q1 is on (since its base is now pulled down to 2.5V). As a result, VR2 is by­passed and the timing capacitor can charge in just 0.65ms. When the voltage across the timing capacitor reaches 2/3Vcc (ie, 2/3 of the supply voltage), pin 7 of IC1 switches low and the capacitor discharges via a 150kΩ resistor until its voltage drops to 1/3Vcc. During this period, the output at pin 3 is also low. At the end of the discharge period, pin 7 (and pin 3) switches high again and so the cycle is repeated indefinitely for as long as power is applied. VR1 is there to provide fine adjustment of the skew. This pot allows the user to adjust the skew to obtain the best reception. The +5V supply rail for this circuit comes from a regulat­ed output on the receiver module. This supply rail is also ap­plied to LED 1 via a 470Ω current limiting resistor to provide power on/off indication. Signal strength meter The AGC (automatic gain control) line from the receiver module is used to provide a measure of the tuned signal strength. This line drives the signal meter via op amp IC2a which is wired as a unity gain buffer stage. In addition, the no-signal DC offset of the AGC line is nulled using trimpot VR6. This trimpot applies a preset voltage (derived from the 5V rail) to pin 3 of unity gain buffer IC2b, which then drives the negative side of the meter via trimpot VR5. In practice, VR6 is adjusted during the calibration proce­dure so that the meter reads zero under no signal conditions. It simply sets IC2b’s output to the same level as IC2a’s output, so that 0V appears across the meter. VR5 sets the meter sensitivity. It is adjusted so that the meter reads fullscale on a powerful signal. June 1995  15 16  Silicon Chip VR2 23 +5V 22 GND 21 OUT 10uF 150k 1 VR3 6 8 4 S4 D2 D4 D6 S3 6. 8  5W S2 D1 D3 D5 10uF REG1 7815 10uF 25VW VR1 2.2k 10k 10k 8.2k 0.15 D7 1 K A LED1 Fig.12(b): check your board carefully against this full-size etching pattern before installing any of the parts. VR6 Fig.12(a): install the parts on the control board as shown here, taking care to ensure that all polarised parts are correctly oriented. Note that regulator REG1 is installed from the copper side of the PC board. 20 19 18 17 16 15 14 13 12 11 100  Q1 1k IC2 LM358 AGC 470  AC IN IC1 555 2.2k 120  LNB 144/18V METER + METER - VR5 10uF VR4 VR3 and VR4 provide the audio and video tuning controls. These 10-turn potentiometers are respectively wired across the +5V and +18V outputs of the receiver module and provide variable DC tuning voltages for varicap diodes in the tuner module. Finally, switch S4 selects between C and K band operation. When S4 is open, pin 11 of the receiver module is pulled high by an on-board pullup resistor and the unit operates on the C-band. Conversely, when S4 is closed, pin 11 of the receiver module is pulled low and the unit operates on the K-band. Construction The Satellite TV Receiver is built into a plastic case measuring 260 x 190 x 80mm. This easily accommodates the power transformer, the receiver module and an add-on control PC board coded 02305951. This add-on board carries all the ancillary circuitry described above. Fig.12 shows the parts layout on the control board. Begin the assembly by installing PC stakes at the following wiring points: the skew outputs (OUT, GND & +5V), the AC inputs, the AGC and LNB outputs, and the meter outputs. This done, install the resistors and capacitors, followed by the ICs, diodes and the transistor. Table 1 shows the resistor colour codes but it is also a good idea to check each value using a digital multimeter, as the colours can sometimes be difficult to decipher. Note that the 6.8Ω 5W resistor should be mounted about 2mm above the board surface to allow the air to circulate beneath it for cooling (this resistor runs hot). Note also that D7 is a small signal diode while diodes D1-D6 are all 1A rectifier types. The trimpots can be installed now. Take care to ensure that the correct value is installed at each position (VR2 = 20kΩ; VR5 = 1kΩ; and VR6 = 2kΩ). PARTS LIST 1 satellite receiver module (Av-Comm) 1 vented plastic instrument case, 260 x 190 x 80mm 1 PC board, code 02305951, 233 x 51mm 1 self-adhesive front panel label, 254 x 73mm 1 self-adhesive rear panel label 254 x 73mm 1 aluminium baseplate, 1.5 x 220 x 90mm 1 6672 30V 1A transformer (T1) (DSE M-6672) 1 level meter, 250µA FSD (DSE Q-2100) 1 10kΩ 24mm PC-mount potentiometer (VR1) 1 20kΩ (203) miniature vertical trimpot (VR2) 2 10kΩ multi-turn potentiometers (VR3,VR4) 1 1kΩ (102) miniature vertical trimpot (VR5) 1 2kΩ (202) miniature vertical trimpot (VR6) 1 15mm knob with position marker 2 15mm knobs without position markers 1 M-205 panel mount fuseholder 1 500mA M-205 fuse (F1) 1 mains cord with moulded plug 1 cordgrip grommet to suit mains cord 1 SPDT mains rocker switch (S1) 3 SPDT right-angle PC mounting switches (S2-S4) (DSE P-7686) 1 3mm LED bezel 5 6mm PC board standoffs 1 9mm standoff 1 10-way pin header socket 1 5-way pin header socket 1 6.5mm stereo panel socket 3 solder lugs 1 300mm length of green/yellow mains wire 1 300mm length of brown mains wire 1 50mm length of 10-way rainbow cable (2.54mm spacing) 3 500mm lengths of different colour­ed medium-duty hook-up wire 1 200mm length of 0.8mm tinned copper wire 9 PC stakes 6 cable ties The resistance codes for these pots are shown in the parts list. Next, install the Skew Adjust pot (VR1), switches S2-S4 and LED 1. The LED should be mounted at full lead length so that it can later be pushed into its bezel on the front panel. Watch the orientation of the LED – the anode lead is the longer of the two. The two multi-turn pots (VR3 & Semiconductors 1 555 timer (IC1) 1 LM358 dual op amp (IC2) 1 BC328 PNP transistor (Q1) 1 7815 1A 3-terminal regulator (REG1) 6 1N4004 1A diodes (D1-D6) 1 1N4148 signal diode (D7) 1 3mm green LED (LED1) Capacitors 1 10µF 25VW PC electrolytic 3 10µF 16VW PC electrolytic 1 0.15µF MKT polyester Resistors (0.25W, 1%) 1 150kΩ 1 1kΩ 2 10kΩ 1 120Ω 1 8.2kΩ 1 100Ω 2 2.2kΩ 1 6.8Ω 5W WW Miscellaneous Heatshrink tubing, machine screws, nuts, lockwashers. TABLE 1: RESISTOR COLOUR CODES ❏ No. ❏  1 ❏  2 ❏  1 ❏  2 ❏  1 ❏  1 ❏  1 Value 150kΩ 10kΩ 8.2kΩ 2.2kΩ 1kΩ 120Ω 100Ω 4-Band Code (1%) brown green yellow brown brown black orange brown grey red red brown red red red brown brown black red brown brown red brown brown brown black brown brown 5-Band Code (1%) brown green black orange brown brown black black red brown grey red black brown brown red red black brown brown brown black black brown brown brown red black black brown brown black black black brown June 1995  17 LNB 14V/18V TUNER AGC METER+ METER- METER BREAK CONNECTION TO PCB 20 RECEIVER MODULE 11 10 8 6 4 2 REAR PANEL FRONT PANEL 8 6 4 ALUMINIUM PLATE SKEW OUT 17.5V REG1 7815 23 POWER TRANSFORMER T1 21 22 EARTH LUG F1 CORD GRIP GROMMET ACTIVE BROWN 0V EARTH GRN/YEL EARTH LUG 23 22 21 NEUTRAL BLUE EARTH ACTIVE BROWN Fig.13: the transformer is mounted in the case on an aluminium plate which must be securely earthed – see text. Be sure to use mains-rated cable for all mains wiring & cover all exposed terminals with heatshrink tubing. VR4) are connected to the board via short lengths of tinned copper wire. At this stage, just install 25mm lengths of wire into their terminal holes on the PC board but leave the pots to one side for the time being. The 3-terminal regulator (REG1) is installed on the under­side of the PC board (see photo), so that it can later be bolted to an aluminium plate. This aluminium plate is fastened to the base of the case and, in addition to supporting the power trans­former, also provides heatsinking for the three regulators on the receiver module. Before installing the regulator, bend 18  Silicon Chip AC IN its leads through 90° so that they mate with its mounting holes in PC board. This done, push the leads through the board (from the copper side) and adjust the regulator so that its top surface sits about 1mm below the bottom of the PC board, then solder its leads – see Fig.13. The PC board assembly can now be completed by soldering a 50mm length of 10-way rainbow cable to points 11-20, followed by three 50mm lengths of hook-up wire to points 4, 6 & 8. The free ends of these leads are then terminated in pin header sockets. This simply involves lightly soldering POWER S1 ACTIVE ACTIVE each lead to a pin and then pushing the pins down into the socket until they lock in position. Take care when connecting the leads from points 4, 6 & 8 to their header socket. Assuming the use of a 5-way header socket, these leads should go to the two outside pins and to the centre pin, so that they mate with points 4, 6 & 8 on the receiver module. Case preparation The next step is to drill the case so accept the various hardware items. This job can be made easy by first affixing the front and rear panel adhesive labels to the plastic panels. This done, the labels can then be used as drilling templates. Don’t try to drill large holes in these panels. Instead, it’s best to first drill a small pilot hole and then enlarge this carefully using a tapered reamer until the relevant part is an exact fit. The cutouts for the meter and power switch can be made by first drilling a series of small holes around the inside perimet­ ers of the rectangles. The centre pieces are then knocked out and the cutouts filed to the correct shape. Both the meter and the switch should be a tight fit in these cutouts. The rectangular holes in the rear panel for the two slider switches are made in similar fashion. The hole positions for the earth screw, fuseholder and mains cord grommet are indicated by crosses on the righthand side of the rear panel. Note that the hole for the cordgrip grommet should be carefully shaped to suit, so that the grommet will later securely clamp the mains cord without any risk of it pull­ing out of the panel. The adhesive labels for the front and rear panels are made from aluminium and must be earthed to ensure electrical safety. Make sure that the earth screws make electrical contact with the panels by scraping away the top layer to expose the aluminium around the hole. Once the holes have been drilled, fit the front panel to the control board by slipping it over the threaded bushes of the toggle switches and the Skew Adjust pot. Note that the latter is secured using a nut on either side of the panel. The control board, along with the front panel, is then mounted in the case on five 6mm-long standoffs. These standoffs in turn sit on integral mounting bushes moulded into the base and the whole assembly is secured using self-tapping screws. If necessary, use adhesive tape to hold the standoffs in position while the control board is posi­tioned over them and the screws installed. The power switch, signal strength meter and multi-turn pots can now be mounted on the front panel, along with the earth solder lug. Use a round­head screw and two starwashers (one under the head of the screw and the other under the nut) to secure the earth lug and make sure that the assembly is tight. Finally, check that the front panel is indeed electrically connected to the earth lug by checking for continuity with your multimeter. The two multi-turn pots are connected to the control board via short lengths of tinned copper wire, while the 5W wirewound resistor in the foreground should be mounted about 2mm above the board surface so that the air can circulate beneath it for cooling. Note that this resistor normally runs hot. The existing link between the LNB terminal on the tuner module (ie, the one nearest the rear panel) & the receiver board must be removed. This can be done by cutting the top of the link with a pair of side cutters & bending it down onto the PC board so that it is out of the way. Next, install the LED bezel and push the LED into it (bend the LED leads at right angles). The front panel assembly can then be completed by wiring VR3, VR4 and the signal strength meter. A small amount of epoxy adhesive can be used to secure the meter. Moving now to the rear panel, begin by installing the fuse­holder and the earth lug in their designated positions. As before, use star washers under the head of the earth screw and under the nut, and use your multimeter to check for electrical continuity between the lug and the panel after the screw has been tightened. This done, the mains cord can be passed through its access hole and securely clamped using the cordgrip grommet. Strip back the outer sheath of the mains cord by about 80mm so that you are ready to make the necessary connections later on. The receiver module (with its attached tuner) can now be attached to the rear panel. It is fastened by doing up the nuts on the two RF sockets (IF IN & VHF OUT) and by installing a small screw and nut adjacent to the audio and video RCA sockets. Check that the two slider switches operately freely when this has been done, then fit the 6.5mm stereo panel socket (Skew Out). Drilling the baseplate The aluminium baseplate measures 220 x 90mm and is posi­tioned so that June 1995  19 The back of the pre-built receiver module is secured to the rear panel via the RF input & output sockets, while the front is supported on the aluminium baseplate by a 9mm standoff & by the heatsink for the 3-terminal regulators. Use cable ties to secure the mains wiring. its front edge lines up with the rear of the control board. You will have to mark out and drill four or five holes in this baseplate so that it can be fastened using self-tapping screws to the inte­gral mounting posts moulded into the case. After drilling these holes, temporarily fasten the base­plate using a couple of screws, then install the rear panel The rear panel of the receiver carries an input socket to accept the signal from the LNB plus the following outputs: VHF Out, Audio Out, Video Out & Skew Out. A small slider switch is also pro­vided so that either channel 3 or channel 4 can be selected for VHF Out. 20  Silicon Chip with its attached receiver module. The following mounting holes should now be marked on the base­ plate: (1) two holes for the heatsink fitted to the regulators; (2) a hole for regulator REG1; (3) a corner mounting hole for the receiver module at front right (ie, near the 8-pin IC); (4) two holes for the power transformer (use the transformer as a template); and (5) a hole for an earth solder lug – see Fig.13. The baseplate can now be removed from the case and the various holes drilled. This done, smear the mating surface of the heatsink with heatsink compound and bolt it to the baseplate using machine screws, nuts and washers. Similarly, secure the power transformer and the earth solder lug to the baseplate. The front corner of the receiver module is supported on a 9mm standoff and is fastened with a screw, nut and lockwasher. The entire assembly – consisting of the baseplate, trans­ former, receiver module and rear panel – can now be installed in the case. Note that it may be necessary to temporarily loosen the mounting screws for the control board so that the baseplate can be slid under REG1. Use self-tapping screws to secure the base­plate to the integral standoffs moulded into the base and be sure to re-tighten the mounting screws for the control board. It will be necessary to drill a hole through the bottom of the case to install the mounting screw for REG1. Measure out and mark the position of this hole carefully prior to drilling, to ensure that it is directly in line with the mounting hole in the baseplate. Note that REG1 can be directly bolted to the baseplate without an insulating washer. Final wiring Refer to Fig.13 for the final wiring details. Take care with the mains wiring – the Active (brown) lead from the mains cord goes to the fuseholder, while the Neutral (blue) lead goes direct to one of the transformer primary terminals. The other side of the fuse­ holder goes to the power switch (S1), with a further lead then running from S1 back to the remaining trans­former primary terminal. Be sure to use mains-rated 250VAC cable for the connections to the power switch and for the mains earth wiring. In addition, heatshrink tubing should be used to cover the fuseholder and the mains switch connections – see photo. This involves slipping suitable lengths of heatshrink tubing over the leads before they are soldered to these parts. After soldering the leads, push the heatshrink tubing over the switch and fuseholder and shrink it down with a hot-air gun. Similarly, sleeve the transformer primary connections with small dia­ meter heatshrink tubing to avoid the possibility of accidental electric shock from otherwise exposed terminals. The Earth lead (green/yellow) from the mains cord connects directly to the earth lug on the aluminium baseplate. Additional earth leads are then run from this point to the earth lugs on the front and rear panels. Use cable ties to secure the mains wiring, so that there is no danger of contact with low voltage circuitry should a lead come adrift. Medium-duty hook-up wire can be used for the low-voltage wiring. This involves the transformer secondary Where To Buy The Parts A short-form kit for the Satellite TV Receiver is available from Av-Comm Pty Ltd. This kit (Cat K-1000) is priced at $150 & includes the receiver module, an etched PC board (code 02305951), two 10-turn 10kΩ pots, the aluminium baseplate (undrilled) & the front & rear panel labels. The case, meter, power transformer, control-board components & other minor parts are not included & will have to be purchased separately from parts retailers. In addition, Av-Comm is offering the following packages to SILICON CHIP readers at special prices: (1) For K-band reception: 1.6-metre dish with ground mount stand, dual-polarity LNB, 25 metres RG-6/U coaxial cable & K-1000 short-form Satellite TV Receiver kit (see above). Price $684.00. (2) For C-band reception: 3-metre dish with tracking mount, servo-controlled feedhorn, 20°K (noise temperature) C-band LNB, 25 metres RG-6/U coaxial cable & K-1000 short-form Satellite TV Receiver kit (see above). Price: $2092.00. For further information, contact Av-Comm Pty Ltd, 198 Condamine St (PO Box 225), Balgowlah, NSW 2093. Phone (02) 949 7417; fax (02) 949 7095. connections, three connections to the Skew Out socket, and two connections from the control board to the tuner module (LNB & AGC). Twist the leads to the Skew Out socket together and lace the leads to the tuner module to keep them tidy – see photos. The transformer secondary leads should also be twisted together. Note that the existing connection between the LNB terminal on the tuner module (ie, the one nearest the rear panel) and the receiver board must be broken. This can be done by cutting the wire with a pair of side cutters. Finally, connect the header sockets to the receiver module and fit the knobs to the control pots. Test & adjustment Before applying power, go back over your work carefully and check that all the wiring is correct. In particular, check the mains wiring carefully and check that the front and rear panels have been properly earthed. Now apply power and check that the power LED lights. If it does, check the output voltages from REG1 and the three regula­tors on the receiver module. REG1 should have an output of 15V, while the other three regulators should have outputs of 18V, 12V and 5V (you should get readings within 10% of these nominal voltages). Note that the 14V rail for the LNB (ie, the cathode of D6) will not necessarily read 14V until a load is connected to this output. Assuming that all is well, adjust trimpot VR6 so that the meter reads zero. If an oscilloscope is available, use the following proce­ dure to adjust trimpot VR2 to obtain the correct skew pulse widths: (1) Connect the oscilloscope probe to the skew output (pin 21 on the control board); (2) Set the Skew Adjust pot (VR1) to mid-position and the Skew switch (S2) to horizontal (H); (3) Adjust VR2 until the skew pulses are 2.2ms wide; (4) Flick the Skew switch to vertical (V) and check that the skew pulses are now 0.65ms wide If you don’t have an oscilloscope, use the following proce­dure to adjust VR2 instead: (1) set VR1 to mid-position, S2 to “H” and temporarily connect a 47kΩ pullup resistor between the skew output and the +5V rail; (2) Connect a multimeter between the skew output and ground; (3) Adjust VR2 for a DC reading of 618mV (note: this is an aver­age reading of the skew pulses) (4) Flick S2 to “V” and check that the meter now reads about 200mV DC; (5) Remove the 47kΩ pull-up resistor. That completes the adjustment procedure. The unit can now be tested for proper operation by setting it up with a dish, an LNB and a TV set. We’ll cover that procedure and describe how the SC unit is used next month. June 1995  21