Fullscreen HTML5Med Res (??MB)HTML4

Build your own Super-7 AM RADIO RECEIVER Part II – by John Clarke All on a single PCB – and no SMDs! In this second and final article on the new Super-7 AM Radio, we show you how to assemble it, then align it for best performance. Then you can put it into its superb acrylic case . . . and your friends won’t believe you built it! A ssembly is not at all difficult – everything is mounted on one large PCB and we don’t use any SMD components – so it’s standard soldering all the way. And don’t be put off by alignment: it’s not hard to do and can be done using quite basic equipment, as we will explain shortly. Of course, it can also be even better using specialised equipment, such as the Dead-Easy DDS Superhet IF Align66 Silicon Chip ment Unit we published in the September 2017 issue (www.siliconchip. com.au/Article/10799). As its name suggests, this makes alignment, or adjustment of the IF coils, on the Super-7 AM Radio . . . dead easy! (see the panel on page 73). But if you can’t justify building a device such as this, there are other ways to do it; maybe not quite so simple or elegant but effective nevertheless. We will cover other approaches to align Celebrating 30 Years the radio set shortly. There are a number of test points on the circuit board which can be used for voltage measurements or to provide signals to be displayed on an oscilloscope. We will show some typical waveforms in this article, so you will know what to expect. Fortunately, you don’t need an expensive ’scope for this – indeed, there are any number of 1MHz bandwidth siliconchip.com.au kit models available on ebay and similar (ie, you build them first!) for well under $100. And if you’re at all into hobby electronics (or above) you really do need an oscilloscope on your bench. Spend a little more and you can get a really good, higher bandwidth scope which will suit your needs for many years. Construction The Super-7 AM Radio is built on one double-sided PCB coded 06111171 and measuring 313 x 142.5mm. It is housed in a multi-piece acrylic case, available from the SILICON CHIP Online Shop. This also includes a transparent tuning dial. Station call signs (eg, RN for Radio National) and frequency markings that are screen-printed on the PCB can be seen through it. The Super-7 AM Radio uses some special AM radio parts. These include a coil pack, a mini tuning gang capacitor and ferrite rod with coil. Otherwise, most of the parts are pretty common – you may have many of them in your “junk” box. Fig.1, the circuit, was published last month. Fig.2 (overleaf) is the overlay diagram and this shows where all the components go on the PCB. Use this (and the photos) as a reference while following these instructions to fit the components to the board. Begin construction by installing the resistors. Their colour code table is shown on page 70. We suggest that you also check each resistor value with a digital multimeter before it is inserted – some colour bands appear close to others (eg, red, brown and orange) so it is always wise to double check, especially before you solder them in! Resistors are not polarised – they can be inserted either way into the board but it is a good idea to install them so that their colour codes all align in the same direction. This makes it so much easier to check their values later on. Fit the PC stakes for the GND (TP GND), two near CON2 (for the speaker), one at TP1 and five for VR1. Three of the PC stakes for VR1 are to wire it to the board, while the remaining two are to solder to the potentiometer body to hold it more securely. This pot is installed later. Next, install the capacitors. There are three types used in the circuit. One type is MKT polyester (plastic) and these can be recognised by their rectangular shape. These are not polarised. The second type is ceramic and these are also not polarised. Fortunately, they are all the same value too, so you can’t get them mixed up! Generally, small capacitors are not marked with their actual value – instead, they use a code which you need to decipher. We make that particular task easy with the small capacitor code table, also on page 70. The third type of capacitors used in this project are electrolytics – they are polarised and must be inserted the right way around – follow the markings on the PCB overlay. Electrolytics are (usually) cylindrical in shape, with a polarity stripe along one side for the negative lead. The opposite (positive) lead is usually the longer of the two. Almost invariably, electrolytic capacitors will have their actual value printed on them, along with their voltage rating. One point which often confuses beginners: it is normally OK to use an electrolytic capacitor (or indeed any capacitor) with a voltage rating higher than that specified, as long as there is room (capacitor size normally increases with voltage). However, it is not OK to use capacitors with a lower voltage rating than that specified. For example, if a circuit calls for a 10µF, 16V electrolytic capacitor, you can normally use one of the same value and type – 10µF electrolytic –with a 25V, 35V or even higher rating, as long The Super-7 AM Radio Receiver in its purpose-designed acrylic case. The majority of the case panels are high-gloss black but the rear panel is crystal clear, (hence the reflections), just so others can see your handywork in all its glory! siliconchip.com.au Celebrating 30 Years December 2017  67 Fig.2: this PCB overlay diagram shows where to fit the components onto the board before soldering. Ensure polarised components (diodes, electrolytic capacitors and transistors) are the right way around. Also pay careful attention to ensure each component installed is of the correct value and type. The four transformers have colour coded slugs, as shown. as it will fit. However, you generally cannot use a 10µF electrolytic capacitor with a 6.3V rating – it is liable to explode! But in this circuit, you could use capacitors with a 10V rating, since the battery voltage is only 9V. OK, back to construction: install diodes D1, D2 and D3. While they may look identical, each diode is a different type so don’t mix these up. Diodes are also polarised. The polarity band or stripe, which indicates the cathode (k), is oriented toward the bottom of the PCB as shown on the overlay diagram. The transistors go in next. Again, make sure you put the correct transistor in each position. Transistors Q6 and Q7 are mounted horizontally with leads bent over at 90° so that their holes line up with the holes in the PCB. The Q6 & Q7 transistor bodies are attached to the PCB with M3 x 10mm 68 Silicon Chip screws and nuts with the screw placed from the rear of the PCB and the nut on the transistor. (The copper of the PCB acts as a “heat sink” to keep them from overheating). The remaining transistors don’t handle as much power so they are smaller types which are mounted vertically on their leads. You may need to splay their leads out to fit the mounting holes on the board (eg, using small pliers). Make sure the “D”-shaped packages (looking down on them) go the same way around as shown on the overlay diagram. IF transformers Now you can install the oscillator and IF transformers. They will only go in one way with three pins on one side and two on the other. However, these all look the same except for the colour of the slug at the top. Celebrating 30 Years The colours are as follows: the oscillator transformer (T2) is red; both the (identical) IF transformers (T3 & T4) are white; the third IF transformer (T5) is black. The mounting positions for each of these transformers are clearly indicated on the PCB. By the way, resist the temptation to twiddle the slugs of the IF transformers and oscillator coil, especially using a small screwdriver. There are several reasons not to use a small screwdriver to adjust the slugs. First, it is all too easy to crack the slug since these are brittle and once broken will be jammed in the transformer core. Second, the blades of screwdrivers are often magnetised and this can affect the magnetic characteristics of the slugs. Third, when you are aligning the rasiliconchip.com.au off with sidecutters. We want to solder the pot body to the PC stakes to hold it securely in place but the body is normally “passivated” to prevent corrosion. This makes it almost impossible to solder – so you will need to scrape the pot sides with a hobby knife to remove some of the passivation before soldering. Pass the potentiometer through the PCB from the component side and secure it with its washer and nut on the “top” or label side. Bend the tags so that they touch the PC stakes on the board and solder them in place. Trimpot VR2, for the audio amplifier output biasing, can also be installed at this stage, followed by the battery holder, on/off switch and headphone socket. The battery holder is held in place with self-tapping screws. The power switch and headphone socket are mounted directly on the board. Speaker mounting dio, the steel blade of the screwdriver will affect the resonance of the coil and you will get misleading results. You should use a set of plastic alignment tools (they’re quite cheap) and use one which has a blade that’s a neat fit in the slot of the slug. If you can’t purchase a suitable alignment tool, you can make one out of a piece of scrap plastic shaped at one end so that it is like a screwdriver blade and sized to neatly fit the slug slot. You can easily do this with a sharp utility knife and needle files. Many a plastic knitting needle has disappeared from mum’s sewing basket over the years to make alignment tools! When installing the ferrite rod antenna, secure the ferrite rod in place with cable ties but keep them loose for the moment, as you will need to adjust the coil position later during alignment. The coil on the ferrite rod has four very fine cotton-covered coloured siliconchip.com.au wires. Keep these the length that they are, ie, do not cut them short, since they are already pre-tinned. The circuit board connections for the antenna coil connections are labelled with the colours: clear (CLR), black (BLK), red (RED) and green (GRN). The clear wire is the one that is at the far end of the coil and is separate from the remaining three wires. The plastic dielectric tuning capacitor (or tuning “gang”) is normally supplied with two tiny M3 screws which are used to secure it to the PCB. After these are inserted and tightened, the three tags need to be bent at right angles to insert into the holes on the PCB. They are then soldered in place. You’ll need a hacksaw to cut the volume control potentiometer shaft to 17mm in length (from where the threaded boss starts). There is a small location spigot on the side of the pot, which is not needed, so it can be snapped off with a pair of pliers or cut Celebrating 30 Years The speaker is fastened directly to the PCB using four M3 screws and nuts, with short lengths of hookup wire between the loudspeaker and speaker PC stakes. Note that there are eight speaker mounting holes, two sets of four on two different circumferences. So select the correct holes for your particular loudspeaker and orient it so the terminals are nearest to CON2. Now check all your work very carefully and you will be ready for the next stage which is alignment. Aligning your radio The major difference between this project and any other that you may build is the need for alignment. Even if you have assembled the radio precisely as we have described so far, there is little chance that it will work satisfactorily when you first turn it on. This is because even tiny variations in component values and characteristics and even slightly different PCB track widths and fibreglass thickness can cause frequency shifts which throw the workings of the radio off. There are various adjustments to compensate for this, including the adjustment slugs in the IF transformers, which need to be “tweaked” to give the best gain and frequency response. You will also need to adjust the slug in the oscillator coil and the trimmer capacitors associated with the tuning gang to give the best tracking. The December 2017  69 Fig.3: this shows the locations of the antenna and oscillator trimmer adjustments on the tuning gang. resonant circuit of the oscillator (T2, VC3 and VC4) must track with the aerial resonant circuit (T1, VC1 and VC2) across the whole of the broadcast band. Otherwise, the set’s sensitivity will vary quite markedly as you tune it. This also helps to ensure that stations appear at their correct locations on the tuning dial. Before you start the alignment process, rotate trimpot VR2 fully anticlockwise. This will reduce the quiescent current in the output stage transistors, Q6 and Q7, to zero. Rotate the volume control pot and the tuning knob fully anticlockwise too. This done, connect a 9V battery or 9V DC power source (a 9V DC plugpack or 9V power supply – but make sure the centre pin is positive) and then measure voltages around the circuit. Connect the negative probe of your multimeter to the GND test point and then verify that the following voltages are correct: TP+ (8.88V), TP1(1.55V), TP2 (8.88V), TP3 (1.1V), TP4 (8.88V), TP5 (1.78V), TP6 (9V), TP7 (4.7V), TP8 (4.3V), TP9 (3.73V), TP10 (4.2V). In each case, the voltage should be within about 10% of the value noted above assuming that the supply is exactly 9V. If the voltages are quite different from the values listed above, then you should investigate why. For example, if your supply is actually putting out 9.5V then the readings which are supposed to be 8.88V could easily be 9.38V instead (and TP6 will be 9.5V). By the way, these voltages are ‘no signal’ voltages. That means little or no signal should be picked up by the input stage and the volume control is turned down so that there is no signal going through the amplifier stages. The presence of signals will alter these voltages, although not greatly. You can also measure the current drain now. This can be done by connecting your multimeter (selected for measuring a low current range) across the on/off switch between the centre and rear terminals at one side of the switch. Alternatively, connect the multimeter between the anode of diode D3 and the 9V battery positive terminal. With the switch set switched off, the current through the meter should be less than 10mA. We measured 3mA on our prototype. If you measure a lot more (more than 10mA) or a lot less (under 1mA), disconnect the multimeter and check the board carefully for assembly errors, solder bridges, etc. Aligning the IF stages involves injecting a 455kHz signal into the front end of the circuit. As mentioned, earlier, the DDS IF Alignment unit from September 2017 makes this easy. See Resistor Colour Codes Qty Value 4-Band Code (1%) 5-Band Code (1%)  1 1.2MΩ* brown red green brown brown red black yellow brown  1 1MΩ brown black green brown brown black black yellow brown  1 820kΩ grey red yellow brown grey red black orange brown  1 47kΩ yellow purple orange brown yellow purple black red brown  1 39kΩ orange white orange brown orange white black red brown  1 27kΩ red purple orange brown red purple black red brown  1 22kΩ red red orange brown red red black red brown  1 12kΩ brown red orange brown brown red black red brown  1 10kΩ brown black orange brown brown black red brown  1 4.7kΩ yellow purple red brown yellow purple black brown brown  2 3.3kΩ orange orange red brown orange orange black brown brown  1 2.2kΩ red red red brown red red black brown brown  2 1kΩ brown black red brown brown black black brown brown  1 470Ω yellow purple brown brown yellow purple black black brown  1 100Ω brown black brown brown brown black black black brown * 1.2MΩ 5% carbon can be used: its colour code will be brown red green gold 70 Silicon Chip Celebrating 30 Years Small Capacitor Codes    Qty 3 1 4 1 1      Value/Type 100nF ceramic 47nF polyester 22nF polyester 10nF polyester 4.7nF polyester EIA 104 473 223 103 472 IEC 100n 47n 22n 10n 4n7 the side panel on how to do this. The alternative is to connect an RF oscillator, set to 455kHz, through a 1nF ceramic capacitor to test point TP1. If you don’t have an RF oscillator, you could use an audio signal generator set to produce a square wave at 152kHz with an 800mV output level. Since a square wave produces odd order harmonics, it is the third harmonic (3 x 152kHz) from the square wave at 456kHz that will be your signal for the IF alignment. Connect your multimeter (set to read DC volts) between test point TP3 and ground. Set the RF generator to give a signal output of about 1mV RMS or the audio signal generator square wave to 800mV RMS. The idea is to now adjust each of the slugs in the IF transformers in turn for a minimum voltage on test point TP3. As you adjust the slugs, the gain of the IF stages improves and the signal fed to the detector diode (D1) increases. The detector diode rectifies the IF signal and so, as the signal increases, the negative voltage produced by the detector increases. Hence, the voltage at test point TP3 decreases. Note that after adjusting all the slugs, you may wish to go back through them again and check that they are all set at their optimum position. It’s sometimes possible to make improvements the second time around that were hard to see initially. Oscilloscope method If you have access to an oscilloscope, you can connect it to TP6 and observe the IF signal directly. Now, as you adjust the slugs, you will see the signal increase or decrease. Adjust the slugs for the best possible (ie, highest) signal amplitude. If you notice any clipping of the signal at TP6, just reduce the signal input from your RF oscillator. Tracking adjustments These adjustments ensure that the RF input circuit and the local oscillasiliconchip.com.au Scope1: voltage at the collector of Q1 with the set tuned to around 700kHz 700kHz + 455kHz = 1.155MHz). You can see that the oscillator waveform is a clean sinewave with an amplitude of around 350mV RMS. tor cover the correct range of frequencies so that you can tune over the entire broadcast band. Ideally, you need an RF signal generator to do this task. If you don’t have access to one, you will have to rely on tuning stations at the top and bottom of the band. In Australia, the broadcast band is specified as 531-1602kHz, so to be sure you are covering this band, it is normal to make a radio tune over a slightly wide range, eg, 525-1620kHz. If you are in an area where there are “out of band” AM stations, such as narrowcasting community stations up to about 1711kHz, you need to make the receiver tune slightly higher again. (See www.acma.gov.au/theACMA/ narrowband-area-service-licensing). Let’s first proceed on the basis that you have an RF signal generator. If you don’t have an RF signal generator, see the section entitled “Setting the tuning range without an RF generator”. With signal applied to TP1 via a 1nF Scope2: now a test signal has been coupled into the ferrite rod. The test signal was modulated onto a 720kHz carrier. You can see the effect of signal modulation in the thickening of the trace away from the centre. capacitor, set the generator to 525kHz and rotate the tuning knob fully anticlockwise. This sets the plates of the tuning gang “in mesh” which is the maximum capacitance condition, for the low-frequency end of the band. Now adjust the slug in the oscillator coil for maximum loudness of the signal via the speaker, or (if you are using an oscilloscope) for maximum signal amplitude at TP6. Next, rotate the tuning knob so that it is fully clockwise. Set your RF signal generator to 1620kHz. Tune the adjustment screw on the back of the tuning gang labelled “oscillator trimmer” (see Fig.3) for maximum signal amplitude, as before. Rotate the tuning knob fully anticlockwise and redo the oscillator coil slug adjustment again at 525kHz. This done, go back to the top of the band at 1620kHz and adjust the oscillator trimmer again. The adjustments need to be done a number of times as the top adjustment affects the bottom adjustment and vice versa. You have now adjusted the oscillator range so that the broadcast band can be tuned in and this also ensures that the stations are tuned in at the locations indicated on the dial. As a point of interest, the oscillator will now be tuned over the range 9802075kHz. That’s 525kHz plus the IF of 455kHz to 1620kHz plus 455kHz. Now you need to adjust the ferrite rod coil and antenna trimmer (on the back of the tuning gang) to maximise sensitivity by ensuring the aerial circuit is resonant at the tuned frequency. Set the tuning knob fully anticlockwise and set the RF signal generator to 525kHz, then move the coil along on the ferrite rod until the signal amplitude is at a peak. You may have to (carefully!) heat up the coil with a hot air gun to melt the wax between the coil and ferrite rod, before the coil can be moved. Setting the tuning range without an RF generator In the accompanying procedure for setting oscillator and antenna tracking, we assumed that you had access to an RF signal generator. For many constructors, this won’t be the case and they will have to rely on broadcast signals at the top and bottom of the broadcast band. However, this poses something of a ‘chicken & egg’ situation. How do you do the tracking adjustments if you cannot receive the signals? In most cases, you should be able to receive signals at or near the bottom of the broadcast band especially at night (typically high power ABC radio stations). For example, in Sydney, you can tune in to ABC Radio National at 576kHz. However, picking up a signal at the top end of the band might not be anywhere as easy. The highest frequency nationwide AM radio station is In Sydney, the highest commercial AM station is at 1269kHz (2SM). Above that, there are only community and narrowcast radio stations which may not be strong enough to use for siliconchip.com.au this purpose in all areas of the city. But there is a solution if you have another AM Radio since every superhet has a local oscillator and for an AM broadcast receiver, this oscillator will usually be 455kHz above the tuned frequency. Therefore, you can use the local oscillator in your other AM radio to set the tracking adjustments at the top of the band. The method to follow is this: place the ferrite rod of the Super-7 AM Radio near the antenna rod other AM radio. This rod will usually be at the top of the case. Rotate the tuning dial of the Super-7 AM Radio fully clockwise to tune to the top of the band. Tune your other AM radio to 1165kHz or as close to this as you can. This will set its local oscillator to 1620kHz. That’s the top of the band on the Super-7 AM Radio’s dial. As you do so, you should be able to hear faint heterodyne whistles from the speaker of the AM radio. Now proceed to peak the antenna and oscillator circuits as described in the article. Celebrating 30 Years December 2017  71 Scope3: waveform across the speaker with VR1 at its minimum setting and a ~1kHz modulated RF test signal inductively coupled into the antenna. The zero crossing artefacts are quite severe with no quiescent current. Now set the RF generator to 1620kHz and turn the adjustment screw on the back of the tuning gang labelled “antenna trimmer” (see Fig.3) until you peak the incoming signal again. You should now repeat these adjustments for the optimum response. When this is done, the ferrite rod coil should be fixed in place by re-melting the wax and allowing it to set. That completes the alignment of the radio. Quiescent current All that remains to be done is to set the quiescent current in the audio power amplifier by means of trimpot VR2. The best way to adjust the quiescent current is to feed a sinewave modulated signal into the front end of the radio from an RF signal generator. Connect an oscilloscope to the output at test point TP10 and adjust the volume control for a signal amplitude across the speaker of about 2-3V peakto-peak. At this stage, VR2 should still Scope4: the audio output sounded very raspy when capturing Scope3. We then rotated VR1 clockwise until the sound became much cleaner and took the screen grab shown here. The signal looks much more like a sinewave. be fully anticlockwise. If you now have a look at the signal on the scope screen, you will see the classic sinewave with crossover distortion with notches in the waveform at the crossover point (see Scope3). Now rotate VR2 slowly clockwise and you should see the crossover nicks disappear from the waveform and, at the same time, the sound should become cleaner. Rotating VR2 to reduce the crossover distortion will not increase the current drain by much (typically no more than a milliamp) but it will make a big difference to the sound quality. No ’scope? If you don’t have an oscilloscope, you can apply a signal at 1kHz from an audio generator (100mV is suitable) to the centre of VR1, with VR1 set to mid position. This will apply audio directly to the amplifier. Adjust VR2 for minimum distortion either by listening to the sound (it should become “pure” with adjustment) or by monitoring on an oscilloscope. By the way, you should measure the current drain of the radio while you are adjusting the quiescent current with trimpot VR2. Typically, the current drain of the radio at 9V should be less than 10mA when the volume control is at minimum setting (ie, no signal through the audio amplifier stages). With the volume control well advanced, to make the radio quite loud, the current drain may be 40mA or more. Don’t rotate VR2 any more than necessary as this will increase dissipation in the output transistors and will flatten the battery faster when listening. If in doubt, back it off a bit (rotate it anti-clockwise) until you hear an increase in distortion, then rotate it a tiny bit clockwise until that distortion is gone and you are near the ideal setting. Note that using the radio with high Here’s the completed Super-7 AM Receiver sitting on the four screws which secure it to the front panel. Don’t fit nuts over the PCB yet: it needs to be free to move as you slot in the right-hand end panel, which itself slips over the power switch and headphone socket. 72 Silicon Chip Celebrating 30 Years siliconchip.com.au volume will flatten the battery much more quickly than at low volume . . . The acrylic case Because it is self-contained (ie, fully on one PCB) the Super-7 AM radio would be quite happy working without a case. But if you want a really professional finish, you’ll want to put it into the purpose-designed acrylic case. Its appearance is not unlike the mantel radios of yesterday . . .only it is shiny black! The case measures 327 x 155 x 58mm (w x h x d) and the front, sides, top and bottom are made from a very smart high-gloss black. The back panel is transparent so everyone can admire your handywork! Provision is made in the left end panel for the on-off switch, a DC power plug and the 6.5mm headphone socket. On the front panel, attractive slots are milled for sound output immediately in front of the speaker and at the right end there’s a matching 105mm hole for the clear acrylic tuning “dial” which reveals the screen-printed PCB underneath with its major radio stations. While you can easily move the tuning dial with your fingers, we gilded the lily somewhat by gluing a large knob to the centre of the dial (a knob makes it easier to find elusive stations!) – whether you add a knob is entirely up to you. Immediately underneath and to the left of the tuning dial is the single “volume” control The case simply slots together and everything is held in place by four 50mm long pillars which go from front to back – more on these shortly. We’ve also made provision on the bottom front of the case for a pair of rubber feet which can angle the whole receiver back slightly. Again, this is entirely optional. Putting the case together Remove the nuts from the volume control pot and headphone socket, if fitted. It doesn’t matter if the clear acrylic “dial” is fitted to the tuning capacitor; it can be done now or later. Start with the front panel. Insert four M3 x 15mm screws through the four holes near the edges and put a washer and nut on each to hold them in place. Now slide the receiver PCB down over these screws, obviously oriented so the siliconchip.com.au speaker sits behind the slots and the dial markings behind the 105mm hole. Slide the left end panel into its slots on the front panel, at the same time engaging the on/off switch shaft and the 6.5mm headphone socket. You will probably have to lift the PCB on this end to allow this. When in position, refit the nut onto the headphone socket – this will hold the end panel in place. Now you can slide the bottom, top and right end panels into place, with their tabs fitted into the slots on the front panel and each other. Threaded standoffs It’s not easy (impossible?) to buy a threaded standoff long enough (45mm+) to hold the rear panel onto the front panel. If you can find (or make!) a 45mm M3 threaded standoff, more power to you! We made ours with a combination of 15mm and a 25mm M3 threaded standoffs, M3 studs to join them into single 40mm lengths, plus a few M3 nuts and washers to end up with the 50mm length required. The “stud” which joins the 15 and 25mm lengths was simply a short (15mm) M3 screw with its head cut off with a hacksaw. (You will probably need to run a nut over the cut-off section to reform the thread). Two M3 nuts were used between the two standoffs as spacers. Fig.4 shows this a little more clearly. The overall length of the standoff, top of PCB to bottom of rear panel, is 50mm. Given that nuts vary all over the place in height, simply choose the number of nuts and/or washers to make your standoffs 50mm long. We made four of these. The bottom BACK PANEL Fig.4: you need four 50mm M3 threaded ~10mm standoffs – but M3 SCREW just try to buy them! We made ours from 15mm and 25mm standoffs, joined with an M3 “stud” M3 NUTS made from + WASHERS (SPACE AS REQUIRED TO a headless ADJUST LENGTH) 15mm M3 screw. Nuts and washers ~15mm M3 SCREW were used to pack it PCB out to 50mm long. FRONT PANEL 25mm M3 TAPPED STANDOFF ends screw onto the M3 screws which pass through the case front panel (with a nut) and then the PCB. The top ends fasten to the four M3 screws which hold the rear panel in place. SC Using the DDS Superhet Alignment Unit ( Sept 17 ) The DDS IF Alignment unit makes aligning the Super-7 quite straightforward. While its IF alignment mode is handy for verifying the alignment is correct, the AM modulated signal generator is actually the mode we used the most during alignment. The DDS module allows you to generate the 455kHz, 525kHz and 1620kHz test signals with or without modulation. Simply enter the required frequency and select sinewave mode. We simply produced a maximum (or near maximum) amplitude signal and fed it to a small wire loop which we placed near the ferrite rod. However, you could also use the onboard attenuator to produce a lower level signal suitable for direct injection via a 1nF capacitor, as per the main text. Note that we found proper alignment much easier with the aid of a scope since this allows you to see how cleanly the modulated test signal is being demodulated and you can tweak the alignment to give not only the strongest but also least distorted signal output. Once you’ve completed the alignment procedure as stated in the main text, you can then set the generator frequency and switch to IF alignment mode to verify that the IF bandwidth peaks around 455kHz and has the correct ~10kHz bandwidth to the -3dB points, as shown in the screen photo below. ~15mm M3 STUD (15mm M3 SCREW WITH HEAD REMOVED) 50mm 15mm M3 TAPPED STANDOFF Celebrating 30 Years M3 NUTS + WASHERS AS REQUIRED December 2017  73