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Stereo preamplifier with IR remote control Despite its circuit complexity, the Studio Remote Con­trol Preamplifier is easy to build. This month, we conclude with the full construction details. PART 3: By JOHN CLARKE The Studio Remote Control Preamplifier is housed in a 1-unit high black rack-mounting case. This is fitted with a screen-printed front panel which incorporates a smoke-coloured (neutral) Perspex window for the LED displays. A plastic film mask is fitted over the LED displays so that only the segments that have been lit are visible through the Perspex window. Inside the case, most of the parts are accommodated on two PC boards: a main board coded 01308931 (350 x 230mm) and a dis­play board coded 01308932 (243 x 25mm). The handheld transmitter is housed 56  Silicon Chip in a small plastic case which has a front panel label measuring 73 x 63mm. It uses two PC boards, one for the electronic circuitry and the other to provide the necessary contacts for the switch membrane. This switch membrane is acted upon by 15 plastic-chrome buttons which pro­ trude through the front panel. Before starting construction, check all three PC boards for breaks in the copper tracks or shorts between tracks. Any defects should be repaired immediately. Check that all holes have been correctly drilled also. Begin assembly of the main PC board (01308931) by install­ing all the PC stakes (at external wiring points), wire links and resistors – see Fig.7. Keep all the wire links straight to avoid shorts to neighbouring components. The 27Ω 5W resistor should be mounted about 1mm above the board to allow the air to circulate beneath it for cooling. The ICs can be installed next, taking care to ensure that they are all correctly oriented. In particular, note that IC17-21, IC23 and IC15 are oriented differently to the remaining ICs, so check these carefully. We do not recommend using sockets for any of the ICs except for IC14 (the microprocessor IC), as this could prejudice the audio performance. Once the ICs are in (do not plug IC14 in yet), install the diodes, regulators and capacitors, again taking care to ensure that all polarised components are correctly oriented. Note that the two 4700µF capacitors must be mounted on their sides as shown on Fig.7, so crystal. Do not mount LEDs 10-18 at this stage – that step comes later. Finally, tack solder the six 6mm standoffs to their mount­ing points on the underside of the PC board. This is best done by first bolting the standoffs to the board, so that they are held in the correct positions. Display board that they don’t foul the lid. Exercise caution when mounting the three 3-terminal regula­ tors, to make sure you don’t get them mixed up. Each regulator is fitted with a heatsink and fastened to the PC board with a screw and nut. Use the largest heatsink for REG1 and smear all mating surfaces with heatsink compound before bolting each assembly to the board. The relay, bass and treble pots, tone switch and headphone socket can now be mounted, followed by the 500kHz ceramic resona­tor and the 3.58MHz Below: a plastic film mask is fitted over the LED displays so that only the display segments are visible through the Perspex window. The photodiode sits behind a window in the mask to prevent reflections from the LED displays. Fig.8 shows the assembly details for the display board (01308932). Begin by installing the wire link, then install the PC stakes from the copper side of the PC board at the 1, 2 and 3 locations. This done, install the three 7-segment LED displays and the pushbutton switches. Be sure to orient each switch with the flat side of its body facing to the left and take care with the orien­ tation of the LED displays (ie, bevel towards top left). The infrared photodiode (IRD1) is mount­ed with its leads at full length and bent at right angles so that its front face sits vertically – see photo. LEDs 1-9 should only be inserted and not soldered at this stage, so that their height can be adjusted later. Be careful with their orientation, as LEDs 4-6 are dif­ferent to the others. You can easily identify their leads because the anode lead is always the longer of the two. Mating the boards The main board is butted to the back of the display board at right angles and the two soldered together via matching con­nector tracks. Before doing this however, push LEDs 10-18 through their holes in the display board, then bend their leads downwards about 3mm away from the LED bodies, ready for insertion into the main board. Now offer the front panel to the main board and insert the LED leads into their respective holes. This done, arrange the main board so that its underside is 1mm above the bottom edge of the display board, then lightly tack solder the boards together at a couple of mounting points. Check that the two boards are at right angles before soldering the remaining tracks together. Finally, LEDs 10-18 can be soldered to the main board. Position them so that they sit flush against the display board before soldering their leads, and align them in a straight line to make a good bar display. Transmitter assembly Fig.9 shows the wiring details for the two transmitter boards. Before installing any of the parts, check that the two boards fit inside the case. In particular, the 01308933 board should be fitted to the base to check that the clips hold the board correctly and that the plastic alignment pin passes through the hole in the centre of the board. If the board is too wide for the clips, carefully file it down to size. Once the board fits correctly inside the case, snip the top off the alignment CAPACITOR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ Value IEC Code EIA Code 0.22µF   220n   204 0.15µF   150n   154 0.1µF   100n   104 .068µF   68n  683 .015µF   15n  153 .01µF   10n  103 .0047µF   4n7  472 330pF   n330   331 100pF   n100   101 39pF   39p   39 22pF   22p   22 10pF   10p   10 November 1993  57 10k 1 BASS 22k 22k 1 1k 100W 10pF 100W 1 10uF 1 IC14 MC68HC705C8P IC2 4051 47k 47k REG3 7915 330  330  330  330  330  330  330  330  1 IC20 4511 4.7M IC102 4051 47k 330  330  330  330  330  330  330  REG2 7815 1 10uF 10uF 0.1 X1 D3 10uF 47k 47k 330  100W 10uF 10uF 39pF 47k 47k 0.1 330  1 ZD2 47k 47k 330  330  330k 1k 1 1 330  39pF 10uF D13 D14 10k 1uF D12 10k D11 330  330  330  330  330  330  330  1 IC19 4511 100  0.1 330  1 0.1 100  IC108 5534 10pF 10pF IC15 AD7112CN ZD1 IC104 5534 IC8 5534 1 0.1 330  0.1 100pF 1 1k 47k 100pF 330  IC17 ULN2003 0.1 K A K A K A K A K A K A K A K A K A 330  IC21 4511 4.7k 330k 330k 1 LED10-18 1 10pF 10pF 1 IC18 ULN2003 1 IC6 5534 0.1 IC116 OP27 1 0.1 1k 1 0.1 IC16 OP27 0.1 IC4 5534 S5 IC106 5534 100  10pF 100k IC3 4053 4.7k 1k 470uF .01 1 D15 10k 330W 4700uF 0.1 4700uF 1 D9 27  5W 0.15 10uF 330  58  Silicon Chip D5-D8 15V 15V CT 0.22 1 0.1 IC23 MV601 IC9 ULN2003 4.7k 0.1 1 100k REG1 7805 10k 10k 10k 10k 10k 10k 10uF 330  0.1 IC11 4051 IC12 4013 10uF 10uF .0047 1 1 D16 22uF 120  10k D4 .01 10uF 120  10uF IC10 4042 IC13 4011 330  10k 10k 100k 100pF X2 100pF 47  47uF 6.8uF 1 1 3 IC22 SL486 2 .015 Fig.7: parts layout for the main PC board. Make sure that all polarised parts are correctly oriented & note that a socket is used for IC14. We recommend replacing IC6 & IC106 with an OP27GP or LM627 and removing the 10pF capacitors between pins 5 & 8. 22k .01 330pF 0.1 10pF .01 R OUT GND GND IC107 5534 10k W 100W 22k 22k 22k 1.5k 330pF 10pF 1k 10uF 10pF GND VCR R GND CD R GND TUNER R GND AUX1 R GND AUX2 R 4.7k 1k 1.5k Q1 10k 10pF GND VCR L GND CD L GND TUNER L GND AUX1 L GND AUX2 L Q101 1k TREBLE 4.7k 10k 33  4.7k 4.7k 330k 47k 100  47k 100  GND OUT R 22pF D101 47k 100uF BP 100  100 TAPE 4x.0047 33  IC105 5534 100k IC101 5534 16k .068 .0047 100  0.1 33  33  22pF D1 PHONES Q102 1 Q2 1 1 390 1M 1M GND IN L TAPE GND IN R GND OUT L D2 100pF .015 200k 100uF BP D102 IC5 5534 L101 10pF 10pF 47uF BP 150  0.1 10k IC7 5534 GND PHONO L 100k 390  10k 10pF 47k 10k 16k 200k 100uF BP 100  6.8uF BP 0.1 100uF BP 6.8uF BP RELAY 10k 1 0.1 .015 D10 10k 100  100pF 100k 10pF 100k PHONO R IC1 5534 L1 .068 .0047 GND 10k L OUT 47uF BP 150  LED1 LED4 LED7 K A A K K A LED2 K A LED5 A K LED8 K A K A LED3 A K LED6 K 2 S4 A S2 1 3 A S3 A K IRD1 DISP1 DISP2 LEDS10-18 K DISP3 LED9 Fig.8: parts layout for the display PC board. The infrared photodiode (IRD1) is mounted with its leads at full length & bent at right angles so that its front face sits vertically – see photo. pin with a pair of sidecutters so that it is flush with the top of the board. This will allow the IC to sit over the alignment pin. The switch matrix board (code 01308934) can now be tested in the lid of the case. It mounts with the copper side towards the keys and is oriented so that the wire entry points are to­ wards the front (see photo). Check that the PC board fits between the integral guides and is located correctly by the four align­ment pins. File down the sides of the board if it does not fit comfortably. When everything is correct, begin the assembly of the components board by installing the IC, the links and resistors. This done, install the two 100pF ceramic capacitors, the 0.1µF capacitor, the 500kHz resonator and the battery clip leads. The 220µF capacitor must be mounted on its side, so that it will fit into the case. Transistor Q1 is mounted with its leads bent at right angles and is bolted to the PC board using a screw and nut. The two LEDs are mounted without shortening their leads so that they can be bent to sit on the plastic cup rests at the front of the case. Assembly of the switch matrix board (code 01308934) simply involves installing the seven wire links. Once these are in, connect the 8-way 100mm-long rainbow cable to positions 1-8 and the 3-way 100mm-long rainbow cable to positions 9-11. The other ends of the rainbow cables are connected to matching positions on the components board. The next step is to attach the front panel label to the case lid. This done, cut out the rectangular switch holes with a sharp knife and clean up the edges with a small file. The 15 chrome buttons are now installed from inside the lid and the membrane placed in position over these buttons. The contact pads on the switch matrix board should be cleaned before it is installed in the case. Use some steel wool (not the soap pads) for this This close-up view shows how the infrared photodiode is mounted. Make sure that the bevelled edge of the photodiode is at upper right. The metal cases of the two tone control pots are earthed by connecting them together as shown here & running a lead back to the EXT EARTH socket on the rear panel. job. Polish each copper switch pad area, then apply a smear of heat­sink compound over the polished areas. This step will prevent the copper from tarnishing, which in turn would lead to intermittent operation of the remote con­trol. Note that this treatment should also be applied to tin-plated boards. Now for the final assembly. Attach the switch matrix board to the case lid with six small self-tapping screws, then clip the components board into the bottom half of the case and bend the LED leads so that the LEDs sit on the plastic cup rests. Finally, feed the battery clip leads through to the battery compartment, then clip the case together and secure it with the self-tapping screw supplied. Chassis assembly Work can now begin on the preamplifier chassis. Assuming that all the chassis holes have been pre-drilled, you can secure the side and rear panels to the baseplate but leave the front panel off at this stage. The four rubber November 1993  59 LED2 LED1 TO BATTERY A K A 11 10 9 K 220uF Q1 2. 2  8 7 6 5 4 3 2 1 100pF 10  100pF 9 10 11 10k IC1 MV500 X1 1 2 3 4 5 6 7 8 0.1 Fig.9: this is the parts layout for the transmitter PC board & its companion switch matrix board. The two are linked together via 8-way & 3-way lengths of rainbow cable. Note that the two infrared LEDs (LED1 & LED2) are installed with their leads at full length. feet should be attached to the underside of the baseplate at this stage, to prevent scratches both to the chassis and to the bench top. The 18 RCA sockets on the rear panel must be insulated from the chassis, either by using insulated sockets or by using non-insulated sockets which mount on an insulated sub­panel. The earth terminal associated with these sockets (EXT EARTH) must also be iso- lated from the chassis. Once these parts have been mount­ ed, install the fuseholder, mains cord and cord grip grommet on the rear panel, then mount the power transformer, earth solder lugs, mains terminal block and power switch. Note that it will be necessary to scrape away the paint (or anodising) from the area surrounding the mounting hole for the earth lugs, in order to ensure a good chassis con­tact. Use a screw, nut and star washer to secure the earth solder lugs, then use a multimeter to confirm that they are indeed connected to chassis (the meter should read zero ohms). Check also that the rear and side panels are electrically connected to earth by measuring the resistance between the chas­sis earth terminal and each panel. If the panels are insulated from the baseplate, you may have to remove some of the anodising from around their mounting holes. The next step is to fit the film mask to the LED displays using double-sided tape. Arrange the mask so that only the LED displays are visible through the windows and position IRD1 so that it is centred behind its allocated window – see photo. When this has been done, fit the front panel to the chassis and push the PC board assembly towards it, so that the various switches and the two tone control pots protrude through their respective holes. Assuming everything fits, the PC board can now be secured in position using machine screws and nuts. In some cases, however, it may be necessary to either file or shim the board standoffs so that all the components pass through their front panel holes without fouling. RESISTOR COLOUR CODES ❏ No. ❏   1 ❏   2 ❏   4 ❏   2 ❏   7 ❏ 14 ❏   6 ❏   2 ❏ 23 ❏   7 ❏   2 ❏   8 ❏ 35 ❏   2 ❏   2 ❏ 10 ❏   1 ❏   4 ❏  1 ❏   1 60  Silicon Chip Value 4.7MΩ 1MΩ 330kΩ 200kΩ 100kΩ 47kΩ 22kΩ 16kΩ 10kΩ 4.7kΩ 1.5kΩ 1kΩ 330Ω 150Ω 120Ω 100Ω 47Ω 33Ω 10Ω 2.2Ω 4-Band Code (1%) yellow violet green brown brown black green brown orange orange yellow brown red black yellow brown brown black yellow brown yellow violet orange brown red red orange brown brown blue orange brown brown black orange brown yellow violet red brown brown green red brown brown black red brown orange orange brown brown brown green brown brown brown red brown brown brown black brown brown yellow violet black brown orange orange black brown brown black black brown red red gold brown 5-Band Code (1%) yellow violet black yellow brown brown black black yellow brown orange orange black orange brown red black black orange brown brown black black orange brown yellow violet black red brown red red black red brown brown blue black red brown brown black black red brown yellow violet black brown brown brown green black brown brown brown black black brown brown orange orange black black brown brown green black black brown brown red black black brown brown black black black brown yellow violet black gold brown orange orange black gold brown brown black black gold brown red red black silver brown These photos show how the rubber membrane fits in position over the switch buttons in the transmitter case. The copper contact areas on the switch matrix board are smeared with heatsink compound to prevent corrosion & ensure reliable operation. Once the board is finally secured, LEDs 1-9 can be pushed into the front panel holes and their leads soldered and trimmed. The bass and treble control knobs can also be fitted. Chassis wiring All that remains now is to complete the chassis wiring – see Fig.10. By far the most tedious part of the job involves the wiring between the RCA input sockets and the main PC board. This wiring must all be run using shielded cable, to prevent hum pickup and minimise crosstalk between channels. Note that the metal cases of the bass and treble control pots are earthed by running a lead back to the EXT EARTH terminal on the rear panel. This measure prevents the preamplifier from picking up hum whenever the tone controls are touched. It will be necessary to scrape away the anodising from the bodies of the pots in order to make good solder joints. Be sure to use 250VAC-rated cable for the wiring to the mains switch, fuseholder and the transformer primary. The Active (brown) and Neutral (blue) leads from the mains cord are connect­ed to the terminal block, while the green/yellow earth wire is soldered directly to one of the earth solder lugs. Insulated sleeving such as heat­ shrink tubing should be used to cover the bare terminals of the fuseholder and the mains switch, to prevent accidental contact. Note that a 0.0047µF capacitor is soldered directly across the mains switch and this should also be covered in heatshrink tubing. The wiring can now be completed by connecting the secondary terminals of the transformer to the PC board. This done, use cable ties to secure the internal wiring at various points. This is particularly important for the mains wiring, since it prevents any leads from coming adrift and shorting to the chassis. Testing Before applying power to the circuit, check your wiring carefully and re- check the PC board against the overlay diagram. Now apply power and check the supply rails at the output of each regulator. You should get +5V from REG1, +15V from REG2 and -15V from REG3. In addition, check that there is about 7.5V across each of the zener diodes (ZD1 & ZD2). If you don’t get the correct readings, switch off and correct the problem before switching on again. Assuming that everything is OK, check that +5V is present at pins 1, 3, 37 & 40 of IC14’s socket. If this checks out, switch off and install IC14, then The switch matrix board is secured to the lid of the case using six small selftapping screws, while the components board clips into position. Arrange the LEDs so that they sit in the small cups at the front of the case. November 1993  61 17 1 16 PHONES LEFT RIGHT PHONO 1 EXT EARTH TAPE IN TAPE OUT VCR VR1 2 3 S5 5 4 3 AUX 1 AUX 2 6 9 8 11 10 13 12 5 6 8 15 14 17 DISP3 TUNER 7 10 12 16 14 DISP2 CD LED10-18 4 7 DISP1 9 11 OUTPUT 13 1 15 2 3 IRD1 S3 S2 S4 LED7-9 1 2 3 2 LED4-6 FUSE LED1-3 CORD CLAMP GROMMET A BRN POWER TRANSFORMER N BLU MAINS TERM. EARTH GRN/YEL SOLDER LUGS 62  Silicon Chip .0047 240VAC 15V 15V 240V S1 Fig.10: be sure to use mains-rated cable for all connections to the mains terminal block, fuseholder & power switch. VR2 Use cable ties to secure the wiring at various locations, as shown in this photograph. Either insulated RCA sockets can be used or you can use noninsulated types mounted on an insulated sub-panel, as shown here. apply power again. The Attenua­tion display should now show a reading of 48.0dB and the 0-9dB balance LEDs should all be lit. The CD LED should also be lit. Now press the Mute switch on the preamplifier and check that all the balance LEDs except the 0dB LED extinguish. If this works, pressing the Up and Down switches should now alter the Attenuation display in 1.5dB steps. Note that pressing the Up switch will decrease the attenuation reading, while the pressing the Down switch will increase the attenuation reading. The remote control handpiece can now be tested for correct operation. Check that the ACK (acknowledge) LED on the receiver lights when one of the remote control switches is pressed and that the appropriate LED lights when each of the Source switches (Phono, CD, Tuner, VCR, Aux1 & Aux2) is pressed. Finally, Below: the rear panel carries nine pairs of RCA sockets for the input & output connections, an earth terminal & the fuseholder. check that the Up, Down and Mute switches operate the displays correct­ ly. Note that the balance adjustment is only available when the preamplifier is unmuted. Note also that the balance display can show two LEDs lit at the same time. For balance settings of 0dB, 3dB, 6dB, 9dB and infinity, only one LED is lit but for in-between settings, two LEDs will be lit. For example, both the 0dB and 3dB LEDs will be lit for the 1.5dB setting. The Tape Monitor, Source, Mono and Stereo selection re­ quires some explanation. Initially, when power is applied, the selection is Source Stereo. You can then select Source Mono by pressing the lefthand Source switch and Source Stereo again by pressing the righthand Source Switch. The Tape Monitor selection can be either “Tape Mon Mono” (by pressing the lefthand Tape Mon switch) or “Tape Mon Stereo” (by pressing the righthand Tape Mon switch). Connecting it up plifier connects between the signal sources (Phono, CD, Tuner, VCR, Aux1, Aux2 & Tape) and the power amplifier. In fact, it’s no different from any other stereo pream­plifier in this respect. When all the connections have been made, switch on and check that you can listen to each source. Check that the volume, balance and tone controls function correctly and that plugging in a set of headphones switches out the loudspeakers. The sound from the headphones should be clean and there should be virtually no background noise. Finally, check that there are no loud clicks and plops from the loudspeakers when the power to the preamplifier is switched on and off. A faint clicking sound as the volume level is changed SC is normal. Where To Buy Kits Readers are advised that kits for this project are not expected to be available until late November 1993 at the earliest. Kits will be available from Altronics, Dick Smith Electronics and Jaycar. The Studio Remote Control Pream- November 1993  63