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Building PART 2 – By LEO SIMPSON & PETER SMITH The SC 480 Putting together our new high performance amplifier modules Last month, we presented the circuit and performance details of the modules which effectively replace the very popular, but ancient, ETI480 amplifier. This month, we show how to assemble the PC board(s) and connect them for the very best performance. A s discussed last month, there is a choice of two modules; one with plastic power output transistors (Version 1) or with TO-3 metal encapsulated output transistors (Version 2). At this stage, it is likely that Version 2 will be available from all major kitset suppliers, while at least one supplier has indicated that they will be selling both versions. Both modules are straightforward to assemble but the transistor mounting details differ considerably between the versions. Therefore, we’ll begin by describing the assembly of parts common to both versions. Common assembly Before installing any components, check the PC board for defects such as shorted or open-circuit tracks or undrilled holes. Fix any defects before proceeding further. Referring to the overlay diagram in Fig.1 (for Version 1) or Fig.2 (for Version 2), install the wire links first, using tinned copper wire. Then you can install the resistors, followed by the three diodes. If not sure of the resistor values, use your multimeter to check each one’s value before soldering it in place and make sure that the diodes are installed the right way around. The four 0.22Ω 5W resistors and the 6.8kΩ 0.5W resistor should be mounted about 1mm proud of the PC board surface to improve heat dissipation. All six small transistors (Q1-Q6) can go in next. There are three different transistor types in this group (BC557, BC639 & BC640), so be particularly careful with placement. Install the four fuse clips and five 2-way terminal blocks next. Note that the small retaining lug on each clip must be positioned to the outer (fuse end) side, otherwise fuse installation will be impossible. These components must be seated firmly on the PC board surface before soldering. By now, your board should be starting to take shape! Continue the good work by installing all the capacitors, 54  Silicon Chip starting with the smallest devices and working up to the largest. The two 470µF 50V electrolytics are polarised components and must be installed the right way around. Note also that they’re oriented differently to one another. If you get either of them the wrong way around, they will be damaged when power is applied and may even explode. Just to confuse the issue, there are two other electrolytic capacitors on the board, 1µF and 47µF, and both are non-polarised and can go in either way around. By the way, some manufacturers call these bipolar (BP) and some call them non-polarised (NP). We intend to standardise on the term “NP”. Note that the 68pF ceramic capacitor, between collector and emitter of transistor Q5, must have a rating of 100V. Similarly, the 150nF (0.15µF) capacitor at the output of the amplifier should have a minimum rating of 250V. Continue construction by installing the multi-turn potentiometer (VR1), 6.8µH choke (L1) and “Polyswitch” thermistor (PTC1). We understand that some kits may not include the optional Polyswitch thermistor, so install a wire link in its place if necessary. Winding the choke If you’ve building your amp module from a kit, the 6.8µH choke may have been supplied pre-wound. If so, all you’ll need to do is scrape the enamel insulation off the wire ends, tin them and solder the part in place. Alternatively, it’s a relatively simple matter to wind the choke yourself. You’ll need a 13mm I.D. plastic former (bobbin) and about three metres of 1mm enamelled copper wire. Begin by bending the wire at right angles, about 10mm from one end. This will be the starting end. Slip it into the bobbin and position the end in one of the slots. Now wind www.siliconchip.com.au FEA P RO T U R E JEC T The two versions of the SC480 amplifier: on the left is what could be called the direct replacement of the ETI-480, with the same TO-3 output transistors (2N3055/2955). Performance, though, is streets ahead. On the right is the “plastic” version using TIP3055/2955 output transistors. These are easier to mount to a heatsink than the TO-3 variety. on 23.5 turns as evenly and tightly as possible. Pass the remaining wire length out through the opposite slot and cut off any excess, leaving about 10mm protruding. Wind on a couple of turns of insulation tape to hold everything in place. Scrape the enamel insulation off the wire ends with a scalpel blade or similar and tin with solder. Don’t worry if the result is less than perfect. An extra turn or a small deviation in the wire will have little effect on the performance of your completed module! With a bit of minor adjustment, the assembly should slip neatly into position in its PC board holes. Press down firmly so that the bobbin contacts the PC board surface and solder in place. Well, that completes the assembly of the parts common to both versions of the amplifier. Now we’ll describe how to mate the power transistors to your chosen heatsink, starting with Version 1. Mounting the output transistors (Version 1) As depicted in the various photographs, the four plastic power transistors and three driver transistors all mount along one edge of the PC board. Transistor spacing is arranged so that the assembly will bolt up to many commonly available heatsinks. Our prototype is shown attached to an Altronics H-0552 heatsink. Other suitable types include the Dick Smith Electronics H-3406 and the Jaycar HH-8546. To begin, fit 10mm tapped spacers to the two remaining PC board holes using M3 x 6mm screws. Notice how this leaves the PC board sitting in a rather lop-sided attitude, with one side higher than the other. We slipped a second, temporary pair or 10mm spacers under our board to bring it back to the horizontal and therefore make the remaining assembly tasks easier. Next, fit the seven transistors (Q7-Q13) into their respective holes, but do not solder or cut their leads short just yet! www.siliconchip.com.au Butt the assembly up against your chosen heatsink and centre it roughly within the available space. If you’re using one of the heatsinks mentioned above, then you should be able to line up the transistor mounting holes with the spaces between the heatsinks fins. This makes life much easier; there’s no need to drill into fins or tap threads in blind holes! In the horizontal plane, the transistors should be mounted as close to the centre of the heatsink as practical. Obviously, this will be limited by available transistor lead length. Once you’re happy with the positioning, mark off and drill 3mm holes for each of the seven devices. After drilling, remove all burrs from around the holes so that the mounting surface is entirely smooth. That done, loosely attach the transistors to the heatsink using the mounting hardware shown in Fig.3. Check that the PC board is sitting horizontal and at right angles to the heatsink and tighten up the screws. Flip the assembly over, taking care not to disturb the PC board position, and solder all transistors in place. Finally, it’s a good idea to make sure that all transistor collectors are indeed isolated from the heatsink. To do this, set your meter to read Ohms and measure between the heatsink and the metal tab (collector) of each TO-218 device. Also, measure between the heatsink and centre lead (collector) of each TO-126 device. You should get a high resistance (open circuit) reading in all cases. Mounting the output transistors (Version 2) In most cases, kit suppliers will provide a predrilled heatsink or heatsink bracket with the Version 2 module. However, if you’re building yours from scratch, here’s how to get all the holes in the right places. To begin, position the PC board on the smooth side of the heatsink surface. If you’re using the recommended heatsink, then the board should be a good fit on the 35mm shelf. February 2003  55 Fig.1: component overlay and matching photo below of the plastic transistorequipped SC480 amplifier (Version 1). It’s very easy to build when you use these to help you. The complete circuit diagram and description were presented in last month’s SILICON CHIP. In this version, the transistors can bolt directly to just about any heatsink. It is definitely the easier form of construction. 56  Silicon Chip www.siliconchip.com.au Fig.2: similarly, Version 2 of the SC480 amplifier with TO-3 transistors. While more robust (and also very similar in appearance to the original ETI-480), mounting the TO-3 transistors does require more care and skill in drilling the heatsink. In the ETI-480, the transistors were attached to a bracket which then attached to the heatsink. While a practical alternative, the heat transferrance is not as effective. www.siliconchip.com.au February 2003  57 Fig.3b (below): the smaller (TO-126) devices mount in a similar way to the larger transistors but no insulating bush is required as there is no metal tab. The back of the transistor must still be insulated from the heatsink. Fig.3a (above): detail showing how the plastic (TO-218) transistors are mounted on the heatsink, with their insulating washers and bushes. Not shown here is the PC board because it does not fasten to the heatsink. Clamp in place and use a sharp scriber or pencil to mark through the holes of all seven transistor positions as well as the two heatsink alignment holes (see overlay diagram). Make sure that you’ve marked all 21 holes before unclamping the board. Gently centre-punch your marks before drilling. Start with a small drill size (about 1mm) and work up to the final size in several stages for best results. Note that a drill press is mandatory for this job. Drilling accurate holes in thick aluminium with a hand drill is almost impossible. The TO-3 transistor holes (four for each device) should be drilled to 6mm. All other holes should be drilled to 3mm. After drilling, remove all burrs from around the holes so that both top and bottom heatsink surfaces are completely smooth. Note that the holes sizes in the PC board do not match the heatsink hole sizes. This is as intended; do not enlarge the PC board holes! On the PC board, all transistor mounting holes as well as the two heatsink alignment holes should be precisely 3mm in diameter. In addition, the TO-3 transistor emitter and base lead holes should only be large enough to accept the respective device leads. OK, let’s put it all together. Figs.4a (top) and 4b (bottom): here’s how the transistors mount to the heatsink (or heatsink bracket) in Version 2. All of the nuts and bolts which hold the transistors on the heatsink also hold the heatsink on the PC board. 58  Silicon Chip Position the PC board beneath the heatsink shelf and insert the two M3 x 12mm alignment screws through from the top. Fit M3 washers and nuts (on the PC board side) and wind up but don’t fully tighten just yet. Now take a moment to admire your work. Looking through from the top, the PC board holes should be centred within the heatsink holes. If any deviate by more than about a millimetre from centre, then you may have problems. The heatsink holes are sized so that the TO-3 transistor mounting screws can pass right through without making contact. If there is any chance of the screws shorting to the heatsink, then you should consider insulating them with high-temperature tubing or similar. Assemble the transistors to the heatsink and PC board as shown in Fig.5, starting with the smaller (TO-126) devices. We've recommended nickel-plated brass screws www.siliconchip.com.au Parts List – SC480 Amplifier Module(s) Parts common to both versions Parts specific to Version 1 (plastic) Semiconductors 3 BC557 PNP transistors (Q1-Q3) 2 BC639 NPN transistors (Q4, Q5) 1 BC640 PNP transistor (Q6) 1 BD139 NPN transistor (Q7) 1 MJE340 NPN transistor (Q8) 1 MJE350 PNP transistor (Q9) 3 1N4148 small-signal diodes (D1-D3) Parts specific to Version 2 (TO-3) 1 6.8µH air-wound choke (L1) (see text) 1 Raychem RXE250 Polyswitch (IH=2.5A, IT=5A, 60V) (PTC1) 3 TO-126 silicone-impregnated insulating washers 4 M205 PC mount fuse clips 2 M205 3A slow blow fuses Capacitors 2 470µF 50V PC electrolytic 1 47µF 16V non-polarised (bi-polar) PC electrolytic 1 1µF 16V non-polarised (bi-polar) PC electrolytic 4 220nF (0.22µF) 63V metallised polyester (MKT) 1 150nF (0.15µF) 250V metallised polyester (MKT) (10 or 15mm lead pitch) 1 100nF (0.1µF) 63V metallised polyester (MKT) 1 12nF (.012µF) 63V metallised polyester (MKT) 1 330pF 50V ceramic disc (or plate) 1 68pF 100V ceramic disc (or plate) or polystyrene 1 10pF 50V ceramic disc (or plate) Resistors (0.25W, 1% unless specified) 2 22kΩ 1 18kΩ 1 15kΩ 2 4.7kΩ 1 2.2kΩ 1 1kΩ 1 680Ω 1 470Ω 8 100Ω 1 10Ω 1 6.8kΩ 0.5W 5% 1 6.8Ω 1W 5% 2 560Ω 5W 5% wire-wound (for calibration, see text) 4 0.22Ω 5W 5% wire-wound 1 200Ω 0.5W 25-turn trimpot (VR1) Connectors 5 2-way 5.08mm pitch 10A terminal blocks Miscellaneous 230mm (approx.) 0.7mm tinned copper wire (for links) and nuts (not steel) for securing the TO-3 devices. This gives a measurable, albeit small, performance improvement. Tighten up all screws but don't solder the transistors leads just yet. Set your meter to read Ohms and measure between the heatsink and the metal can (collector) of each TO-3 device. Also, measure between the heatsink and centre lead (collector) of each TO-126 device. Obviously, you should get a high resistance (open circuit) reading in all cases. If everything checks out, then solder all transistor leads to complete the assembly. Of special note here is that the TO-3 transistor mounting screws must be tightened up before soldering the base and emitter leads. If this is done in reverse order, then stress will eventually crack the solder joints and perhaps even delaminate the PC board copper. www.siliconchip.com.au 1 PC board coded 01201031, 119.4mm x 90.2mm 4 TO-218 silicone-impregnated insulating washer sets (with bushes) 1 Heatsink (1.2°C/W or lower) (eg. Altronics H0552) 2 TIP3055 NPN power transistors (Q10, Q12) 2 TIP2955 PNP power transistors (Q11, Q13) 2 M3 x 10mm tapped spacers 2 M3 x 6mm pan head screws 7 M3 x 15mm (or 16mm) pan head screws 7 M3 nuts 7 M3 flat washers 7 M3 star washers 1 PC board coded 01201032, 149.8mm x 118.7mm 4 TO-3 silicone-impregnated insulating washers 1 Heatsink with 35mm flange (eg. Jaycar H-8550) 2 2N3055 NPN power transistors (Q10, Q12) 2 MJ2955 PNP power transistors (Q11, Q13) 2 M3 x 12mm pan head screws 11 M3 x 15mm (or 16mm) pan head screws (nickelplated brass) 13 M3 nuts (nickel-plated brass) 16 M3 star washers (stainless steel) 8 M3 flat washers Power Supply 1 4 2 1 2 4 4 PC board coded 01201033, 90.2mm x 54.6mm 1N5404 3A power diodes (D1-D4) 4700µF 50V PC electrolytic capacitors 56V/2.4A centre-tapped mains transformer 3-way 5.08mm pitch terminal blocks M3 x 10mm tapped spacers M3 x 6mm pan head screws Parts for optional preamp supply section 2 2 4 1 15V 1W Zener diodes (ZD1, ZD2) 100µF 16V PC electrolytic capacitors 2.2kΩ 1W 5% resistors 3-way 5.08mm pitch terminal block Heatsink brackets As mentioned previously, some Version 2 kits will be supplied with pre-drilled heatsink brackets (as was the ETI-480). The assembly details listed above should still apply, with the main difference being that once assembled, the bracket must be bolted to your heatsink of choice. When attaching a bracket to a heatsink, make sure that you have good mating across the entire facing surfaces, and use heatsink compound to maximise heat transfer. Power supply assembly All secondary-side power supply components, apart from the transformer, are contained on a single small PC board, coded 01201033. Referring to the power supply circuit in Fig.5 and overlay diagram in Fig.6, you can see that the supply incorporates February 2003  59 Fig.5: the power supply for the SC480 is simple enough but adequate for the job. As explained in the text, if you don’t need a ±15V preamp supply, all components within the dotted box can be left out. Fig.6: the power supply PC board overlay, again with a matching photo below. This supply suits a single module – if you want to build a stereo amplifier, you will need two supplies (and either two transformers or one rated at around 5A <at> 56V AC). a ±40V section and an optional, regulated ±15V section. The ±15V supply is handy for those assembling a complete system and can be used to power a preamplifier module, for example. If you have no need for the low-voltage supply, then there is no need to install the associated components. For added flexibility, we’ve designed the PC board so that you can cut off the low-voltage end if you so desire. Begin assembly by fitting an M3 x 10mm tapped spacer to each corner of the PC board. Follow with the four diodes (D1-D4) and two 3-way terminal blocks, making sure that they’re seated firmly on the PC board surface before soldering. Be very careful with the orientation of all of these components, as mistakes here can result in major fireworks at power up. If required, all components in the optional ±15V section should be installed next. Note the zener diodes (ZD1, ZD2) go in different ways around. Last of all, install the two 4700µF 50V filter capacitors. Wiring Housing and wiring of the amplifier modules is totally up to you. However, we’ve outlined a Insulated TO-126 packages Transistors Q7-Q9 are manufactured in plastic TO-126 packages that usually include a small rectangular metal area on the rear. This area is electrically connected to the collector and thus must be isolated from the heatsink with an insulating washer (see Figs.4 & 5). However, some packages that we’ve seen do not have this metal area – they’re plastic (epoxy, actually) on both sides. This ‘isolated’ type package should be mounted without an insulating washer. Simply smear with a small amount of heatsink compound and bolt directly to the heatsink. 60  Silicon Chip www.siliconchip.com.au few important points below that Table 1: Resistor Colour Codes will help you to get the most from your project. No. Value 4-Band Code (1%) 5-Band Code (1%) Fig.7 depicts the most important    2 22kΩ red red orange brown red red black red brown elements of any wiring layout.    1 18kΩ brown grey orange brown brown grey black red brown Note in particular the position    1 15kΩ brown green orange brown brown green black red brown of the +40V & -40V wiring as it    1 6.8kΩ blue grey red brown blue grey black brown brown passes under the PC board. Posi   2 4.7kΩ yellow violet red brown yellow violet black brown brown tioning the wires exactly as we’ve    1 2.2kΩ red red red brown red red black brown brown shown helps to cancel the fields    1 1kΩ brown black red brown brown black black brown brown resulting from currents flowing    1 680Ω blue grey brown brown blue grey black black brown in the PC board tracks. This pro   1 470Ω yellow violet brown brown yellow violet black black brown duces the lowest possible signal  8 100Ω brown black brown brown brown black black black brown distortion.    1 10Ω brown black black brown brown black black gold brown To reduce radiated noise and    1 6.8Ω blue grey gold brown blue grey black silver brown to improve overall appearance, tightly twist the power supply and Earthing is one of the most important and perhaps conspeaker wires as shown. Use a medium to heavy-gauge tentious issues in system design. As we’ve shown in Fig.7, multi-strand wire for both of these connections. Connections to the modules are terminated in 2-way all ground (0V) lines must be returned to a single earth terminal blocks. Only one position (hole) of each of the point. If you’re building a stereo system, then its ground terminal blocks for the +40V, -40V and 0V (GND) connec- (0V) line must also be connected to this point. By the way, you’ll need two power supply modules for tions should be used. a stereo setup, as a single module is just not up to the task. Do not connect anything to the remaining position. Fig.7: here’s how to wire up your completed amplifier module and power supply boards. We’ve shown Version 2 here but wiring to Version 1 is identical. The mains wiring at right would be typical of most installations – but ensure that all exposed terminals are fully shrouded/insulated. www.siliconchip.com.au February 2003  61 You’ll also need two transformers, although it is possible to upgrade to a single, larger, unit of appropriate capacity. Setup & testing No doubt you’re keen to try out your new amp. But hold on a minute; there are a couple of checks and adjustments that must be made first! With nothing connected to the output terminals of the power supply module, apply mains power and measure both positive and negative rails. You readings should be Fig.8: PC board pattern for Version 1, coded 01201031. This board is the smaller of the two, measuring 119 x 90mm compared to the Version 2’s 150 x 119mm. Fig.9: PC board pattern for the power supply (01201033). Fig.10: PC board pattern for Version 2, coded 01201032. The “empty” sections of the board on the left and right side (shown dotted) were removed for aesthetic reasons (as our photos also show) but we imagine most kit and board suppliers will leave these sections in place, making it a rect-angular (and therefore cheaper) board. Performance-wise, it doesn’t matter either way. 62  Silicon Chip www.siliconchip.com.au close to the 40V mark, depending on With your multimeter set to read 50V Table 2: Capacitor Codes mains fluctuations. or more, measure across one of the A word or two of caution: avoid resistors and rotate VR1 clockwise Value IEC Code EIA Code contact with the ±40V DC supply until you get a reading of 28V. This  220nF  220n   224 rails. Although you may get less gives a total quiescent current of  150nF  150n   154 than a “tingle” from the positive or 50mA.  100nF  100n   104 negative rail, getting yourself across Give the amplifier about 5 minutes  12nF   12n   123 both of them simultaneously could to warm up, then readjust if necessary.  330pF  330p   330 be a shocking experience! You’ll probably find that the voltage  68pF   68p    68 If you have installed the optional across the other resistor differs by a  10pF   10p    10 ±15V section of the power supply volt or two; this is normal. module, then the large filter caps will Finally, measure across the output discharge to a safe level quite quickly at power off. (speaker) terminals. The reading should be within ±30mV However, without the low-voltage section, the ±40V rails of the 0V rail. will decay at a slower rate, so it’s wise to check the rails with your meter before touching anything. Problems? OK, the next step is to set the amplifier’s quiescent curIf you can’t adjust VR1 for the correct reading or the rent level. To do this, remove both fuses from the board output voltage is more than ±30mV, then there is a fault and solder a 560Ω 5W resistor across each fuse clip pair. on the board. You may find it easier to tack solder the resistors on the We’ve provided voltage readings for various points rear (copper) side of the PC board. in the circuit (see Fig.10, part 1 [SILICON CHIP January The purpose of the resistors, by the way, is to limit fault 2003]) that may help you to track down the problem. current should there be a problem with the board, as well Your readings should fall within ±10% of our listed as to simplify current measurement. values. Note that nothing should be connected to the input or Once everything checks out OK, switch off power, output terminals until these checks are complete. remove the resistors and plug in the fuses. You’re ready Set VR1 fully anticlockwise and then apply power. SC to rock! Book Review: Motor Home Electrics & Caravans Too!, by Collyn Rivers. Self-published 2002. Spiral bound, 296 x 210mm, 102 pages. ISBN 0 9578965 14. $42.50 Anyone who has been an electronics enthusiast over the past 30 years or so will be well aware of Collyn Rivers. He was the founding editor of “Electronics Today International” (ETI) magazine which went on to have a number of very successful editions in the UK and elsewhere, as well as being very successful in Australia. With that sort of background you can expect that he has put together a comprehensive but succinct guide to caravan electrics. The subject is not rocket science but there are a lot of myths and wrong information afield and Collyn has done a good job of setting the record straight. There are 30 chapters in all, plus four appendices. Collyn starts out by defining the essence of the problem – storing and extracting the maximum amount of energy from lead acid batteries, while ensuring that they give as many years of use as possible. The problems are the same whether you are running a solar-powered home in a remote location, a motor home, caravan or boat. Collyn also goes on to discuss alternators, voltage regulators, solar panels, wind generators and regulators. The book also defines how much power typical appliances require – far more than most people expect – and devotes a chapter to 240V inverters. A short (one-page) chapter is devoted to lighting and compact fluorescents and 12V halogen lamps are the recommended types. And while I found that I agreed with all the rest of the book, I disagree with a number of points in this chapter. First, it states that 12V halogens are intended to run on AC – ideally at 11.8V. Supposedly their life is reduced by 50 – 80% if run on DC. This flies in the face of evidence to contrary. Car headlamps have always run on 12V DC and their life is excellent – usually lasting for the life of the vehicle. Secondly, the book recommends the use of a 12V-to-240V inverter www.siliconchip.com.au to run 12V transformer-driven halogen lamps. “Their 11.8V output extends globe life and reduces consumption, making up for the converter’s 5-10% loss”. Well, not in my book! Typical inverters are around 70% (or less) efficient and typical 12V halogen light transformers are notorious for running hot and would be unlikely to run at more than 75% efficiency. Multiply the two efficiencies together and the result is that almost 50% of your battery power is lost if you take this inverter/transformer drive approach. In my opinion, you’re better off sticking with battery power to run 12V halogens. However, the chapter is redeemed by a recommendation against halogens and plumping instead for warm white compact fluorescents. Here again, for mobile home use, I would suggest the much brighter “cool white”. Other very useful chapters are devoted to refrigerators, water pumps, electric toilets, TV antennas, communications, low voltage and mains voltage wiring. Perhaps the most useful is the chapter entitled “Suggested solutions” which gives good information on real setups. All told, this is a very practical and informative book, written in an informal style which is very easy to read. To purchase the book, contact the publisher, Collyn Rivers, PO Box 3634, Broome, WA 6725. Phone 08 9192 5961. Email: collynr<at>bigpond.com.au (L.D.S.) February 2003  63