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By Nicholas Vinen The Currawong Stereo 10W Valve Amplifier, Pt.2 In Pt.1 last month, we described the circuit and presentation of our new Currawong stereo valve amplifier. We now describe the PCB assembly and detail the timber plinth and chassis wiring along with detailed instructions on putting it all together. M OST OF THE parts for the Currawong are mounted on a single large PCB. This then slides into a slot near the top of a timber plinth, with the remaining components – primarily the two large power transformers – underneath the PCB and attached to the plywood or MDF base. The front panel carries the headphone socket, volume control, power switch and status LEDs. The input connectors, loudspeaker terminals and power socket are mount­ed on the rear panel which is recessed into a cut-out in the rear of the plinth. 82  Silicon Chip So let’s start putting the main PCB together, which is a significant part of the work involved in building the Currawong. PCB assembly Start the PCB assembly with reference to overlay diagram Fig.6. The board is coded 01111141 and measures 272 x 255mm. It’s 2mm thick, which makes it more rigid and stronger than typical 1.6mm laminate. Start by fitting the smaller resistors. The colour-coded stripes on small resistors aren’t always distinct so it’s best to check each value with a DMM. Use the lead off-cuts to make the two wire links (next to LK4 and LK5). Follow with the three 1N4007 diodes (D4-D6), in the top corners of the board, with the striped cathode ends towards the right or bottom of the PCB as shown. IC1 can go in next – there’s no need for a socket. Check that its pin 1 notch/dot is towards the left side of the board before soldering it. Then fit all the 1W resistors. Their colour codes are usually clear however it doesn’t hurt to measure them to be sure. None of these run hot so they siliconchip.com.au This view shows the completed PCB assembly for the prototype amplifier, with the front-panel fitted. It’s housed in a timber plinth and fitted with Perspex covers to protect the user from high voltages. can be mounted in contact with the PCB. You may find it difficult to get the specified 9.1kΩ 1W resistors, so 8.2kΩ resistors can be used instead with only a minor impact on performance (don’t use 10kΩ as this may prejudice overall stability). The two large 1N5408 power diodes are next, with both cathode stripes facing the bottom of the board. These will get a little warm so we recommend spacing them about 5mm off the board (eg, using a 5mm-wide strip of cardboard as a temporary spacer). The W04 bridge rectifier can also go in now; again it’s a good idea to space it off the board a little. Next, fit blue LEDs3-6. These have a dual purpose: to indicate the presence of HT and to illuminate the transformers. They also form part of the HT bleeder circuit so must not be left out (if you must omit them, use wire links in their place). Angle each one back so that it will shine on either T3 (LED5, LED6) or T4 (LED3, LED4) and make sure the longer anode siliconchip.com.au leads go through the holes closer to the righthand side of the board. Now you can mount all the TO-92 package small signal transistors, ie, Q2-Q9. Don’t get the three different types mixed up. Follow with the six fuse clips. Check that the fuse retention lugs are on the outside before soldering the clip in place, otherwise you will not be able to fit the fuses. Also make sure that they are pushed all the way down onto the PCB before soldering them in place. Next on the list are transistor Q1 and linear regulator REG1. These are both fitted with small heatsinks and it’s important that the heatsinks are isolated from the device tabs using WARNING! HIGH VOLTAGES High AC and DC voltages are present in this circuit. In particular, mains voltages (230VAC) are present on the IEC socket and the primary side of the mains transformers (including the wiring to the power switch). In addition, the transformer secondaries together provide a 114V AC output and the power supply produces an HT voltage in excess of 300V DC which is present on various parts of the amplifier circuit (including the output transformers). Do not touch any part of the amplifier or power supply circuitry when power is applied otherwise you could get a severe or even fatal electric shock. The blue LEDs in the circuit indicate when high voltages are present. If they are lit, the power supply and various parts on amplifier board are potentially dangerous. The completed amplifier must be fitted with Perspex covers as described in Pt.3 next month to ensure safety. December 2014  83 DC Out Left Input F3 5A SLOW 10k D2 COM 5408 25V 2200F NC COIL 4007 470 1W NO D6 LK6 100pF 470nF 630V F1 1A SLOW ~ – 100F Right 1M 120 W04 RLY1 114VAC In 1M 120 100pF +~ 6/12VAC 5408 BR1 12.6VAC F2 3A SLOW Left Output 1M 1W D1 8 COMMON Q9 470 BC547 REG1 LM/LT1084–ADJ 470F 400V TRANSFORMER 410T32 INSULATE TRANSFORMER TERMINALS WITH DOUBLE-SLEEVE HEATSHRINK TUBING (SEE TEXT) INSULATE TRANS TERMINALS WITH DO HEATSHRINK TUBIN 100F 50V LED5 7 4 2 47 1W 10k 1W 1M 1W 1M 1W 10k 1W V2 12AX7 4 1 8 9 V1 12AX7 4 3 680 1W 6 2 7 2 7 1 8 V6 12AX7 LK5 FEEDBAC 9 470nF 630V 220nF 630V 470nF 630V 1.5F 1.5F 1M 1M 100nF 150k 330 220 1W 39F 400V + + 220 1W 5 3 220nF 630V 470nF 630V CON6 470 220nF 630V 5 6 39F 400V 84  Silicon Chip 1M 1W 39F 400V 1M 1W 220nF 630V 47 1W 120k 1W 9 3 V7 6L6 6.8k 1W 1 8 4 10k 1W LK4 FEEDBACK 120k 1W 2 7 1 2 V4 6L6 220nF 630V 3 22k 1W 4 5 3 470nF 63V 220k 1W 220nF 630V 47 1W 6 8 + 5 6 1M 1W 1 3 V3 6L6 6.8k 1W 5 100k 1W 2 8 6.8k 1W 4 7 6 47k 1W 1 330 5W 5 330 5W 6 8 A K 22k 1W 7 LED4 A K 270k 1W A 3.3k 1W LED6 K 1.25W 2.5W 5W 10W 15 COM 220k 1W 1.25W 2.5W 5W 10W 15W + LK3 + + + LK1 470F 400V 9.1k 1W 82 1W 100F 50V 100F 50V + 1M 1W 560 4004 470 120 C 100F OUTPUT TRANSFORM T4 + 14111110 refiilpmA evlaOVUW 0T1 oeretS "gnowarruC" TPU 330 5W Insulating washer & bush siliconchip.com.au IC1 Q4 STX0560 Right 100pF 10k 1W 470 1W 120 4007 8 COM COMMON 47k 1W Q1 KSC5603DTU 47k 1W RLY2 100F 50V D4 Q2 STX0560 100F 50V 1M INSULATE TRANSFORMER TERMINALS WITH DOUBLE-SLEEVE HEATSHRINK TUBING (SEE TEXT) Q3 STX0560 Insulating washer & bush OUTPUT TRANSFORMER T4 1M 5408 D5 1M 1W D1 NC F1 1A SLOW 470nF 630V COIL 1M 120 Right Output NO 114VAC In 4007 + + 1.25W 2.5W 5W 10W 15W COM 39F 400V A 7 1 4 2 V7 6L6 47 1W V8 6L6 47 1W 10k 1W 1M 1W 1M 1W 10k 1W 220nF 630V 1 8 9 9.1k 1W 2 7 V5 12AX7 1M 1W 220nF 630V 3 220nF 630V + siliconchip.com.au IC1 4093B 1M 100nF INSULATING BUSH Q1 PCB SILICONE WASHER M3 NUT 1k Q6 BC557 Q8 BC557 Q7 BC547 150k 1M M3 x 10mm SCREW 470nF 630V 470nF 630V 1.5F insulating washers and bushes – see Fig.7. In each case, start by bending the leads of the TO-220 device so they fit through the pads on the board, with the tab hole lined up with the PCB mounting hole. Note that Q1’s centre lead is bent closer to the package than the other two (due to the high voltage between the pins) but this is not the case for REG1. To mount each device, place an insulating bush in the tab hole, then feed an M3 x 10mm machine screw through from the top. Slip an insulating washer under the device, over the screw thread, then slide the heatsink on from underneath. Drop this assembly onto the PCB, ensuring all pins go through their respective holes, then use a shakeproof washer and M3 nut to hold it in place. In each case, ensure the heatsink and insulating washer are straight before tightening the nut fully, then solder the leads. That done, fit the two relays, making sure they are nice and flat on the PCB before soldering more than two pins. Now it is time to install the ceramic capacitors, followed by the MKT (metallised polyester) capacitors, in increasing order of height. Follow with the six pin headers for LK1-LK6. Try to solder these flat on the board and neatly aligned with the edges of the board for best appearance. You can then bend the leads of the FLAG HEATSINK Q5 BC547 39F 400V 9 4 6 22k 1W 1 8 V6 12AX7 120k 1W 2 5 LK5 FEEDBACK 220k 1W 3 7 6.8k 1W 1M 1W 4 39F 400V + 5 220nF 630V 470nF 63V 6 680 1W 3 3.3k 1W 3 100k 1W 2 5 6.8k 1W 4 8 47k 1W 1 330 5W 5 330 5W 8 6 270k 1W K 6 22k 1W A K + LED3 LED4 7 Fig.6: the parts layout on the top of the PCB. This diagram has been split and the righthand side partly duplicated where it meets the magazine gutter, for clarity. The text describes how the valve sockets are mounted; they must be mechanically secured before the pins are soldered. Note that D1, D2 and the 5W resistors should be stood off the board to allow air to circulate around them and don’t forget to insulate Q1 & REG1 from their heatsinks. The 100µF capacitor should be soldered above D6. + 100F 16V LK2 470 1k STAR WASHER Fig.7: the mounting details for transistor Q1 and regulator REG1. In each case, the device tab must be isolated from the heatsink using a silicone insulating washer and insulating bush. December 2014  85 Parts List Since publishing Pt.1 last month, we’ve made a few small changes to the chassis arrangement and this affects the list of parts required. Please note the following changes: Main board (1) Delete vertical RCA sockets for CON1 & CON2; add short stereo RCA-RCA lead (2) 2 x 8.2kΩ 1W resistors can be used instead of the 2 x 9.1kΩ 1W resistors listed Revised chassis parts 1 timber plinth with base (see text) 1 top cover cut from 3mm clear acrylic (details to come) 1 small tube acrylic glue 1 front panel, code 01111142, 249 x 30mm 1 rear panel, code 01111143, 248 x 53mm 1 160VA 37+37+15+15V toroidal transformer (Altronics MC5337) 1 80VA 12+12V toroidal transformer (Altronics M5112) 4 screw-on 50mm equipment feet (Jaycar HP0832) 1 15mm anodised aluminium knob to suit VR1 1 snap-in fused IEC mains male socket for 1.6mm panels (Altronics P8325) 2 M205 250VAC 1A slow-blow fuses (one spare) 1 red chassis-mount RCA/RCA socket 1 white chassis-mount RCA/RCA socket 2 red binding posts (Jaycar PT0453, Altronics P9252) four 5W resistors to fit through their mounting holes. There are two pairs of holes for each; we used the inner pair but it isn’t mandatory. As with the 1N5408 diodes, use a 5mm spacer to stand each resistor off the board. Keep them level and straight for a tidy result. The 630V polyester capacitors can then be fitted. The PCB is designed with multiple pads for each capacitor, to suit different lead spacings. If you have an odd one, you may need to bend its leads out however most should drop straight in. Refer to Fig.6 to see which type goes where. Now solder the smaller electrolytic capacitors in place, ie, the six 100µF types. In each case, ensure that the 86  Silicon Chip 2 black binding posts (Jaycar PT0461, Altronics P9254) 1 SPST ultra-mini rocker switch, 250VAC rated (Altronics S3202, Jaycar SK0975) 1 1m length 2-core mains flex 1 1m length 3-core mains flex 1 200mm length 3mm diameter black heatshrink tubing 1 1m length 5mm diameter clear heatshrink tubing 1 200mm length 20mm diameter black heatshrink tubing 1 50mm length 50mm diameter black heatshrink tubing or large insulating boot (Jaycar PM4016) 1 1m length heavy duty red hook-up wire 1 1m length heavy duty black hookup wire 1 500mm length figure-8 speaker wire 1 12-way screw terminal strip (Jaycar HM3194, Altronics P2135A) 1 M4 x 10mm machine screw 2 M4 nuts and shakeproof washers 2 yellow 5.3mm ID eyelet crimp connectors 2 red 8.4mm ID eyelet crimp connectors 5 red 6.4mm insulated spade crimp connectors 4 solder lugs 1 5mm cable clamp (P-clamp) 12 black 4G x 12mm self-tapping screws 12 4G x 9mm self-tapping screws 1 4G x 6mm self-tapping screw 1 3mm ID flat washer 7 3mm ID spring washers 10 small Nylon cable ties longer (positive) lead goes in the pad closer to the front edge of the board, as shown on Fig.6. Valve sockets The valve sockets are secured to the board before soldering, so the solder joints aren’t under stress. The specified sockets have solder lugs and the board has been designed with slots to accommodate them. Start with the smaller 9-pin sockets. Feed M3 x 10mm machine screws through the top mounting holes and tighten Nylon nuts on the underside. Slip two Nylon washers over each screw thread, then pass the screws down through the mounting holes The 9-pin sockets are secured using M3 x 10mm machine screws, with a Nylon nut & two Nylon washers used as spacers at each mounting point. This mock-up shows the final mounting arrangement used for the 8-pin sockets (it differs slightly from that used on the prototype). These sockets are secured using M3 x 15mm screws and M3 x 6.3mm tapped Nylon spacers. on the board, guiding the solder lugs through the slots. If it won’t go in, check that you have the right orientation as it will only fit one way. You may need to put the solder lugs under a small amount of tension to get them to go through the slots, due to the way they are angled. But once they all line up it should slip into place and you can push the socket right down so it’s sitting on the Nylon washers. Use a shakeproof washer and M3 nut to secure the screw closest to the front (bottom) edge of the board. Fit a Nylon washer and nut to the other (this is necessary to avoid shorts to adjacent PCB tracks) – see Fig.8. Do both nuts up tightly, check that the socket is sitting level on the PCB and then solder the pins and repeat for the other three sockets. It isn’t necessary to trim the solder lugs after soldering. For the larger 8-pin sockets, the arrangement is similar but their mounting brackets are supplied separately. Take a bracket and feed M3 x 15mm machine screws through the top of the mounting yokes (see photos), then loosely screw M3 x 6.3mm Nylon tapped spacers on, just tightly enough siliconchip.com.au MOUNT CAPACITOR HORIZONTALLY Another view of the completed PCB assembly, this time taken from the rear. Check the board carefully after assembly to ensure that all polarised parts are correctly orientated. Note that the 2200μF capacitor indicated by the red arrow must be mounted horizontally (ie, on its side) as shown in Fig.6, not vertically as shown here. so that the screws stay in place. Now position this assembly over the PCB and adjust the spacing so that the two screws are equally far from the centre of the mounting bracket and they pass through the appropriate holes on the PCB. You can then remove the bracket and drop the socket in place. Some “jockeying” may be required but it should fit easily once you get all the pins lined up. These sockets can be installed with eight different orientations but only one is correct. The notch in the central hole must face towards the lefthand side of the PCB. If you solder one incorrectly, it will be difficult, if not impossible to remove (see Fig.6). With the socket pushed down onto the PCB and orientated correctly, slip the bracket on top and secure it in position with shakeproof washers and M3 nuts. Note though that for valves V4 and V8, the mounting screw closest to the front (bottom) edge of the board must be secured with a Nylon washer and Nylon nut instead. Do both mounting nuts and screws up tightly, then re-check the socket orientation before soldering the eight lugs. Repeat for the other three sockets. With all the sockets in place, fit the siliconchip.com.au five low-profile 39µF 400V snap-in capacitors, again with their positive terminals towards the front (bottom) of the board. These should be pushed all the way down before soldering. The 2200µF capacitor can now go in however it must be laid over towards transformer T3 or else the top cover will not fit later. There should be sufficient room for it to sit flat on its side on the PCB. Like the others, it is polarised and the negative stripe should face up. You can then fit the two large 400V capacitors between T3 and T4. Doublecheck their orientation before soldering the leads and they too should sit right down on the board; if they aren’t perfectly vertical, they may not later fit through their corresponding holes in the top cover. Output transformers The 15W 100V line transformers (T3 & T4) come fitted with a terminal block on top and stickers indicating the taps. We removed these as we felt it improved the appearance. The stickers can be peeled off and the glue residue gently cleaned off using an appropriate solvent. Methylated spirits or isopropyl alcohol are good choices are they are unlikely to damage the transformer but try not to soak it. The terminal blocks can be simply pulled off although they’re a tight fit and you may need to use pliers. The metal tab used to hold it in place is then bent down. The next job is to cut, solder and insulate four or five wires to the winding tap lugs. The 10W winding is not connected on the PCB so you don’t have to wire it up but we did anyway, because we thought it would look better. We used blue wire, to match the blue transformer insulation although different colours are shown on Fig.6 for clarity. Cut each wire to a length of about 70mm, then strip about 3mm of insulation from one end and 6mm from the other. Feed the longer section of exposed wire through one of the solder lugs and double it over, then solder it in place. Try not to heat the joint more than necessary or add too much solder. The output transformer terminals all operate at 308V DC and they must all be fully insulated with two layers of heatshrink sleeving to ensure safety. It’s just a matter of slipping a 15mm-length of 3mm-diameter blue heatshrink tubing over each terminal December 2014  87 Right Output Right Input + − 114V AC In − + − CON2 (or solder RCA cable direct to PCB) CON4 Left Input 6/12VAC 2x6.3V AC CON1 (or solder RCA cable direct to PCB) CON8 CON7 − "Currawong" Stereo 10W Valv C 2014 7 6 7 6 8 5 8 5 5 1 4 4 5 6 8 1 9 Insulate! 4 5 6 3 7 2 2 3 Insulate! Insulate! Insulate! 4 4 2 3 Insulate! 3 1 1 4 2 3 7 6 8 7 2 6 7 2 8 1 5 3 8 1 9 9 CON10 POWER LED1 RA GA 88  Silicon Chip VR1 LE A VOLUME siliconchip.com.au Left Output DC Out CON9 CON3 ve Amplifier 01111141 ED2 7 6 8 5 1 4 2 3 This view shows the parts in position on the underside of the PCB. Note the Nylon nuts and washers used to secure the valve sockets at various locations, as indicated by the red arrows. Note also that this is a prototype PCB and the short wire links on two of the 12AX7 valve sockets have been eliminated from the final version shown in Fig.8 Insulate! 4 5 6 3 7 2 8 1 9 PHONES CON5 k L G R Sw siliconchip.com.au and shrinking it down, then adding a second layer. Make sure each terminal is fully insulated, including the 10W tap, even if you aren’t soldering a wire to it. If necessary, use neutral cure silicone to ensure that there is no gap in the insulation where each terminal goes into the transformer. Twist the bare strands together at the other end of each wire and tin them in Fig.8: the parts layout on the underside of the PCB. A motorised pot is shown here for VR1 but a regular 16mm dual-gang log pot can be used instead if you don’t want remote volume control. The RA/GA markings for LED1 indicate the position of the red LED anode and green LED anode respectively. Note the orientation of CON3, CON4, CON7 & CON8 and be sure to use Nylon nuts and washers at the indicated “insulate” positions when securing the valve sockets. preparation for mounting. Do the same with the three pre-existing wires, after trimming them so that they will reach their PCB pads with a little slack. You can place the transformer temporarily on the board to check this. Don’t cut the leads too short. Once all the wires have been prepared, fit the transformers to the board using M4 x 10mm machine screws, shakeproof washers and nuts. The front side (facing the bottom of the board) should have five or six connections, while the rear of the transformer has two. Make sure they are nice and square with the rear edge of the board, centred on their mounting positions and firmly secured. It’s then just a matter of soldering the eight wires to the PCB. The topmost of the five front wires goes to the leftmost pad, the next one down to the second-from-left and so on. December 2014  89 294 (NOT DRAWN TO SCALE) 277 2 9 S ID 6 19 S ID CUTOUT 232 x 40mm E 9 E 6 9 BOTTOM 20 277 28 250 9 52 13 ALL DIMENSIONS IN MILLIMETRES REAR PANEL 13 9 232 22 10 9 9 89 22 250 39 10 2 9 2 19 40 89 13 FRONT PANEL 10 (MDF BASE, INSET) 294 SIDE PANELS: 277mm LENGTHS OF 89 x 19mm DAR PINE WITH 45° INWARD CHAMFER AT EACH END, 2mm SLOT 9mm DEEP ALONG INSIDE, 13mm FROM TOP EDGE. ALSO 10mm DEEP x 9mm WIDE REBATE ALONG INSIDE LOWER EDGE REAR PANEL: 294mm LENGTH OF 89 x 19mm DAR PINE WITH 45° INWARD CHAMFER AT EACH END, 2mm SLOT 9mm DEEP ALONG INSIDE, 13mm FROM TOP EDGE. ALSO 10mm DEEP x9mm WIDE REBATE ALONG INSIDE LOWER EDGE, OTHER REBATE & CUTOUT AS SHOWN FRONT PANEL: 294mm LENGTH OF 89 x 19mm DAR PINE WITH 45° INWARD CHAMFER AT EACH END, 10mm DEEP x9mm WIDE REBATE ALONG INSIDE LOWER EDGE, SLOTS, REBATES & CUTOUTS AS SHOWN BOTTOM: 276 x 259 x 9mm MDF RECESSED INTO BASE Fig.9: the Currawong plinth details. It’s made from four lengths of 89 x 19mm DAR pine arranged in a rectangle with a 9mm MDF or plywood base. The slot cut into the sides and rear accommodates the 2mm-thick PCB while the cut-outs and rebates at the front and rear are for the two panels which the controls and connectors pass through. Skip the pad labelled “10W” if you only soldered four wires. These can be tack-soldered initially from the top (without melting the wire insulation), then pushed through the board and soldered from the bottom afterwards. Connect the three remaining wires as shown on Fig.6, then tie the bundles of four or five blue wires together using blue cable ties. Underside components The remaining parts are fitted to the other side of the board, as shown on Fig.8. Start with the two shielded cables which run down the middle. First, cut them to length and remove about 15mm of the outer insulation from either end, then twist the exposed shield braid wires together and strip about 5mm of the inner insulation away. Now twist together and tin these inner conductors and also tin the twisted end of the shield. Solder the shield wires into the larger of the two holes at either end of the board and the inner wires into the smaller pads – see Fig.8. Make sure there’s sufficient solder on the shield braid so that it’s rigid and can’t move and short to any adjacent pads. Also try to keep the wire reasonably taut along the bottom of the board. Once they’ve been soldered at each end, fit the six cable ties (three per wire) using the slots provided. Next, fit the four pluggable terminal blocks. Make sure these go in the right 90  Silicon Chip way around, with the curved sections towards the back edge of the board (ie, towards the nearest edge). The headphone socket can be mounted next and must be pushed all the way down onto the PCB. This can be followed by dual potentiometer VR1, after cutting its shaft to 15mm long. You can cut the potentiometer shaft using a hacksaw and then file off any burrs. If you’ve opted to have remote volume control, solder the two mounting lugs for the motor in addition to the six for the pot itself. Now for the two remaining LEDs: blue LED2 goes on the left (with the board right-side up) near the headphone socket while bi-colour LED goes on the right. Use a DMM set on diode test mode to figure out which of the bi-colour LED leads is the red anode – the LED will light red when the red lead from the multimeter is connected to this pin (in our case, the longer of the two leads). This lead goes towards the righthand edge of the board. Bend the LED’s leads at right angles 7mm from its lens and fit the LED so that the lens is centred 10mm below the top of the PCB (ie, 8mm from the bottom). The other LED is fitted in the same manner, with its longer (anode) lead also towards the righthand edge of the PCB. Input wiring Note that what the board is designed to accept vertical RCA sockets for the input signals, we decided it was easier to solder a stereo RCA cable directly to the board, which plugs straight into the RCA/RCA sockets on the rear panel. This provides more clearance on the underside of the board for the transformers. So we suggest you get a short stereo RCA lead, chop off a ~500mm length, strip it back and solder it to the left and right input pads, with the shield braid to the terminals marked “-” and the inner conductor to “+”. Now fit the three fuses; 1A for F1, 3A for F2 and 5A for F3 (all slowblow). The main board assembly is now complete. Building the plinth The base of the plinth is a sheet of 5-ply or 9mm MDF cut to 276 x 259mm while the rest is made from a single length of 89 x 19mm dressed allround (DAR) pine, cut to two lengths of 277mm for the sides and two lengths of 294mm for the front and back. Fig.9 shows the plinth details. A plunge router is required to cut the rebates while a mitre or drop saw is used to make 45° cuts so that the four pieces of DAR pine can be assembled in a similar manner to a picture frame. A drop saw is used to cut the 2mmwide slots but make sure that all the slots will later line up correctly. We used wood glue to hold it all together, along with 6G x 20mm wood screws to additionally secure the base. siliconchip.com.au Altronics Kit Altronics had advised that they will be selling a short-form kit (main PCB, all electronic parts and power supply parts) for the Currawong Stereo 10W Valve Amplifier (Cat. K5528). Fig.10: the Currawong wiring diagram. Besides the connectors on the rear panel and the power switch at the front, the only additional components in the case are the two toroidal power transformers (T1 & T2) and the terminal strip which is used to connect their secondaries to the main board. Be sure to pay close attention to the insulation and anchoring of all mains wiring and note that the IEC socket must be covered with heatshrink tubing (see photo). siliconchip.com.au December 2014  91 REAR PANEL M4 x 10mm MACHINE SCREW STAR LOCK WASHERS INSULATED CRIMP EYLET 2 x M4 NUTS INSULATED CRIMP EYLET Fig.11: the Earth leads are secured to the rear panel via insulated crimp eyelets as shown here. The second nut serves as a lock-nut, so that the assembly cannot come loose. Make sure that the leads are securely crimped. Once assembly is complete, check that the PCB will slide all the way back so that the front is flush with the front panel rebate. After assembly, we smoothed the plinth using sandpaper, stained it with “Jarrah” oil-based stain and finished it with a clear polyurethane lacquer. Putting it all together Start by fitting feet to the plinth. These should be placed just inside each corner and attached using 9mm 4GA self-tapping screws. Drill a ~5mm deep 2mm diameter pilot hole for each foot before putting the screw in. Next, fit the rear panel. This is held in place with a self-tapping screw in each corner and a few extras along the edge, primarily next to the mains input socket. You don’t need to put screws through every single mounting hole provided. Again, drill 2mm pilot holes for each screw; due to the limited amount of space, you may need to use a pin vice. Now fit the connectors to the rear panel. Fig.10 shows how the connectors are fitted. The IEC socket goes in with fuse towards the edge. It will snap into place and should not be able to move much once it’s in. The RCA connectors are supplied with two insulating washers; we kept the one on the inside but didn’t bother with the one on the outside as the solder mask on the panel acts as an insulator anyway. Do them up nice and tight; the profile of the mounting holes will stop them from rotating. Similarly, the binding post mounting holes prevent them from rotating and should result in the wire hole through the metal shaft being aligned vertically. These should also be mounted 92  Silicon Chip securely, taking advantage of the supplied spring washers. It’s possible that some binding posts may have their wire hole misaligned even though the shafts are keyed, so check before fitting them. If any are misaligned, you may be able to disassemble the binding post and reassemble it correctly. By the way, the wire holes on the specified binding posts are quite small. You don’t need to use especially thick speaker wire with this amplifier due to the limited output power and low damping factor, but it would be possible to enlarge the mounting holes and fit bigger binding posts if necessary. Alternatively, use banana plugs, which plug into the end of the specified posts. With the posts in place, prepare the two internal speaker leads. Cut some figure-8 cable to ~200mm lengths, strip about 6mm of insulation from both ends and split the two halves apart slightly at either end. Solder the wires at one end to the smaller eyelets of some solder lugs. Put these wires aside so they can be fitted later. Power transformers The transformers should be located as shown in the wiring diagram (Fig.10). Leave enough room between the transformers and rear panel so that you can later reach behind the main PCB as it’s being slid in and plug the various connectors into the underside (this requires more clearance than is available above the transformers). Note that T1 at left is the larger of the two (160VA). We suggest a gap of no less than 50mm between T1 and the rear of the case. In practice, this means positioning the transformer mounting bolts so that they are approximately 115mm from the back edge of the plinth (ie, about 96mm from the inside rear edge). Mount the transformers using the supplied plastic mounting washers, metal plates and washers via 6mm holes drilled in the bottom of the plinth but do the nuts up loosely at this stage. Note that these mounting holes are the only ones drilled right through the base; all other screws used are selftappers which don’t penetrate fully. Now position the terminal block, as shown in Fig.10. Use three 12mm self-tapping screws to hold it in place, one in the middle and one at each end. Again, it’s a good idea to drill 2mm pilot holes first. For each pair of transformer primary wires (ie, blue and brown), cut a length of 5mm diameter clear heatshrink tubing to cover the entire length except for about 10mm at the end. Adjust the wires so that they run parallel and so that they end side-by-side, then shrink the tubing down. Bend the wires so they run as shown on the wiring diagram and terminate them in the terminal block. Once they’re firmly screwed in placed, fit a cable tie around the lot. The two grey wires from T1 aren’t needed, so bend the bare ends over in a U-shape and then insulate with some 5mm diameter heatshrink tubing. Now, twist the six sets of transformer secondary wires together (red/black and white/orange). This will help to minimise the hum and buzz fields rad­ iated by keeping the magnetic loops small. You can twist the grey wires in with their associated secondaries as we did, or leave them separate. Now it’s just a matter of bending the bundles of secondary wires down to reach the terminal block and screwing them in as shown in Fig.10. Be careful when doing the terminals up, since the solid copper wires are quite thin and are loose within their insulating sleeves. This makes it easy to think you’ve secured it in the terminal block when you haven’t so tug gently on each one to make sure it won’t come loose. Now make up two pairs of twisted red/black heavy-duty wires around 200mm in length and attach them to the near side of the terminal block, as shown in the wiring diagram. Screw the other ends into the plug portions of the pluggable terminal blocks as shown. Note the two extra short wires required for the 5-way plug; fit these now too. Once all the wires are in place, measure the resistance between the red/black pairs in the two terminal block plugs (for CON7 & CON8). You should get a low reading (<10Ω). Any higher than that suggests at least one wire is not making good contact in the terminal block, so go over them again. Earth wiring Before making any connections to the IEC socket, it’s a good idea to cover the exposed metal strip as this operates at 230VAC. We also shrunk a length of 50mm-diameter heatshrink tubing around the rear of the connector (Jaycar Cat. WH5582) – see photo. Two Earth wires are required. Start siliconchip.com.au HEATSHRINK SLEEVING OVER IEC CONNECTOR HEATSHRINK SLEEVING This is the view inside the plinth with all the wiring completed. Be sure to cover the IEC socket with heatshrink tubing as shown. by stripping the yellow/green striped wire out of a length of mains flex, then remove the insulation from one end and crimp securely into a 6.4mm insulated female spade connector. Plug this into the IEC mains input socket and route the wire to the rear panel Earth lug hole at the lefthand side. Note that if you are using a plastic boot to insulate the mains socket, you will have to feed the Earth wire through that before plugging it in. Cut the wire so that it reaches 150200mm beyond this Earth lug hole, then mark the point where it passes that hole. Using sharp side-cutters, carefully remove about 25mm of insulation at the marked point without damaging the copper conductors. This can be done by making a series of nicks around the wire at either end of the 25mm section, to separate that piece of insulation from the rest, then slitting down the isolated section and peeling it away. siliconchip.com.au Double over the exposed copper wire, squeeze it together using pliers and then crimp it into one of the yellow 5mm inside diameter eyelet connectors. Bare the copper at the far end of the wire; this goes into the centre terminal of the 3-way pluggable terminal block. Now for the second Earth wire. This needs to reach from the top of one transformer mounting bolt, to the rear panel Earth point and then to the other transformer mounting bolt. Cut it to length, mark the location of the rear Earth panel point, strip the insulation at each end and crimp an 8mm inside diameter red eyelet connector at either end. Strip away the insulation in the middle as before and crimp it into the other yellow eyelet. Now attach both yellow eyelets to the rear panel Earth point as shown in Fig.11. To do this, feed an M4 x 10mm machine screw in from the rear and place a shakeproof washer over the thread, followed by the eyelet connec- tor from the IEC socket, then another shakeproof washer, then the second eyelet and an M4 nut. Do this nut up tight, then do up another nut on top, so it can’t possibly shake loose. You can now remove the transformer mounting bolt nuts one at a time and fit the red eyelet connectors under the flat washers. When refitting the nuts, do them up firmly but not so tight as to risk crushing the transformer windings. Switch wiring Prepare the power switch by cutting a length of 2-core figure-8 mains flex to around 500mm, then strip away the outer insulation for about 200mm, exposing the blue and brown wires. Cut the blue wire short, to 40mm, then strip the end and cover it with 3mmdiameter black heatshrink tubing back to the sheath. Crimp on a 6.4mm red insulated spade connector. Cut the brown wire to the required length as shown in Fig.10 and strip the December 2014  93 The rear panel carries the IEC socket, the speaker terminals, the audio input sockets and the earth screw. Do not operate the unit without the Perspex covers in place (see Pt.3 next month). insulation at the end. Run this though a section of clear heatshrink along with the blue wire you cut off earlier and crimp a 6.4mm insulated red spade lug on the IEC socket end of the blue wire, as shown in the wiring diagram. Now plug the two spade connectors into the rear of the IEC socket (blue wire to Neutral, heatshrink-covered wire to Active). If using a boot, feed them through first. Lay the cable along the bottom of the case and screw it into the terminal strip as shown. Fit the P-clamp in the position indicated using a 6mm self-tapping screw and washer after drilling a small pilot hole. Preparing the front panel If you’re going to fit the optional remote volume control, you will need to drill a hole in the front panel for the IR receiver. This should be vertically aligned with the power indicator LED (at right) and 28mm to the left. Drill the hole to at least 5mm. Note that if using a SILICON CHIP front panel, there may be a hole position indicated on the rear but this may not be correct as we changed it while building our prototype. The mains switch can now be fitted. It should click into place but make sure it has the correct orientation, so 94  Silicon Chip that it’s switched down to connect the two terminals. If your switch doesn’t have an “on” marking on the front, use a multimeter to check which way around it should go. Now strip the sheath at the loose end of the mains twin flex back by about 30mm, strip the insulation from the two inner wires and crimp the two remaining 6.4mm insulated spade connectors onto these. Slip a couple of lengths of 20mm diameter black heatshrink tubing over this cable and then plug the two spade connectors onto the power switch lugs securely. That done, slide one length of the heatshrink tubing right over the rear of the switch body and shrink it down, then slip the other length on top and shrink that too. The rear of this switch must be thoroughly insulated (as explained above) since it is connected to mains Active and is near the front panel controls and other circuitry. Now fit the two speaker wires prepared earlier to the binding posts. This is simply done by securing the solder lugs between the two supplied nuts on each binding post shaft. Do this with the correct polarity as shown in Fig.10. You can finish all the wiring by fitting some cable ties. In addition to the one fitted to the transformer secondary wires earlier, use several others to tie the transformer secondary wires in bundles close to the terminal block so that none of them can come adrift. Also fit some cable ties to the mains and Earth wiring to hold it in place. Checking the wiring Removing the board after it’s fitted is a bit fiddly so it’s best to do as much checking as we can now. First, use a DMM set in Ohms mode to measure the resistance between the Earth pin on the IEC socket and each of the Active and Neutral pins. There should be no continuity at all (the meter should show “OL” or similar). Check also that there is no connection between any of the secondary winding connection points on the terminal block and any of the Earth, Neutral or Active pins on the IEC socket. Then take a quick look over the wiring and make sure nothing is touching or shorting to anything it shouldn’t be. Move all the loose wiring (terminal plugs, etc) out of the way, then plug in an IEC mains lead. Check that the power switch insulation is intact, then plug in and switch on. Check the AC voltage across each pair of red and siliconchip.com.au You Must Use A Ratchet-Driven Crimping Tool One essential item that’s required to build this amplifier is a ratchetdriven crimping tool, necessary for crimping the fully-insulated quickconnect terminals to the leads. Suitable crimping tools include the Altronics Cat. T1552, and the Jaycar TH1829. These all feature double-jaws so that the bared wire end and the lead insu­lation are crimped in a single action. Don’t even think of using one of the cheap (non-ratchet) crimpers that are typically supplied in automotive crimp kits. They are not up to the job for a project like this, as the amount of pressure that’s applied to the crimp connectors will vary all over the place. This will result in unreliable and unsafe connections, especially at the mains switch and IEC socket terminals. By contrast, a ratchet-driven crimp­ing tool applies a preset amount of pressure to ensure consistent, reliable connections. black wires connected to the terminal block plugs. Use caution when doing this as the transformer secondaries can put out over 120VAC – don’t touch the plugs while the power is on! It’s easier to probe the terminal block where the red and black wires are terminated. You should get close to 13VAC across the right-most output pair (going to the 5-way plug). Now, if the transformer phasing is correct, the other pair (going to the 3-way terminal) will read over 110VAC; possibly over 120VAC with no load. If you get a reading closer to 90VAC then you will need to switch off and swap around the black and red wires from the 80VA toroid. Power it back up and check that the voltage is now correct. If either reading is much lower than specified, there is probably a bad connection to the terminal block, so you will have to switch off and re-check all the connections. But assuming the voltages are OK, remove the IEC mains cord and proceed to final assembly. Mounting the board If you’re fitting the remote volume control add-on (to be described next month), make sure that the remote board is attached to the main board and that the motor is plugged in. Then slide the board into the case carefully and slowly, checking that the connectors on the underside don’t catch on any wires. Push it back about two-thirds of the way, with the attached RCA leads folded over the top, then plug them into the internal RCA sockets on the rear panel and push them all the way home. It’s a good idea now to check that there is good continuity between the siliconchip.com.au inner and outer contacts of the rear panel RCA sockets and the input wire solder termination points on the top of the PCB. They should all read low resistance. Now for the tricky bit. It’s necessary to plug the four terminal blocks into the underside of the board but you have to slide it almost all the way back for there to be enough clearance underneath to do so. Thus, you need to reach around the back edge of the board and push them up into place. And watch out because unfortunately, these pluggable connectors are open on the sides so it’s possible to plug them in offset from the correct position! Start with the 3-way and 5-way connectors in the middle of the board as these will have the best clearance and you won’t have to push the board back as far to plug them in. Note that the screw housing projection of each plug faces the front of the case. Once they’re in, you can check that the 3-way connector is fitted correctly by confirming good continuity between one of the valve socket mounting screws and the IEC socket Earth pin. Similarly, the 5-way connector is plugged in correctly when there is a very low resistance between the pins at either end, which you can probe on the top of the board. The procedure for the two speaker terminal plugs is the same but you will probably have to push the board back even further to make room for them to fit. Check for good continuity between each “+” speaker output pin on the top of the board and the red binding post. Assuming that’s all OK, push the PCB all the way back. You may find it hesitates when it reaches the rear panel This metal strip on the IEC socket operates at 230VAC and should be insulated using silicone sealant. but you should be able to “finagle” it in. Recheck the isolation between the Earth and Active/Neutral pins on the mains socket, and the Active/Neutral pins and the eight supply pads on the main board (ie, immediately behind the fuses), just to make sure that pushing the board in hasn’t disturbed any of the wiring. Now place the front panel over the pot shaft and gently push it back, guiding the two LEDs through their respective holes. Loosely fit the pot and headphone socket nuts, then you can drill 2mm pilot holes for the two lower mounting holes in the corners of the panel and attach it using two black self-tapping screws. Finish off by tightening the two nuts and attaching the knob. That’s all we have space for in this article. Next month we’ll go over powering it up and checking it out. Then we’ll fit the clear top cover, to make the whole thing safe to operate. We’ll also describe the optional remote SC control add-on board. December 2014  95