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Studio Twin Fifty Stereo Amplifier, Pt.2 Last month, we introduced our new high performance stereo amplifier and described the circuit operation. This month, we present the construction and troubleshooting details. By LEO SIMPSON & BOB FLYNN Most of the construction of the new amplifier is quite straightforward. The work mainly involves mounting components on the three printed circuit board assemblies. These are for the power amplifiers, the phono preamplifier, and the tone control and headphone amplifier circuitry. There is relatively little wiring inside the chassis. This is because all the pots, the headphone socket and the pushbutton switches (but not the mains switch) are mounted directly 18 SILICON CHIP on the tone control board. Similarly, the rotary input selector switch is mounted on a small PC board which is then soldered at right angles to the phono preamp board. Mounting the switches and pots in this way eliminates the tedium of running separate leads to the terminals of these components. It also greatly reduces the possibility of wiring errors. We understand that at least two kitset suppliers, Altronics and Jaycar Electronics, will be selling kits for this project. Both the Altronics and Jaycar kits will come with a prepunched chassis and the front panel labelling will be silk-screened. Starting construction The first job is to assemble the phono preamplifier board. This board is coded SC01103921 and carries the parts for the phono preamplifier and the RCA input and output sockets. It also carries six sets of stereo tracks which run between the RCA sockets on the rear panel and the selector switch at the front of the board: An additional set of stereo tracks run from the Tape In sockets to three pins on the front of the PC board. Before mounting any of the parts, it is a good idea to carefully check the copper pattern on the underside of the board. You should especially check for shorts between the long parallel tracks to the selector switch. Don't just rely on a visual check .. • • PHONO CD TUNER VCR AUX1 M AUX 2 TAPE OUT TAPE INPUT Rt GND• TAPE IN L• Fig.5: the parts layout for the phono preamplifier board. Install the 23 PC stakes at the external wiring p~ints first, then install the remaining parts exactly as shown. The capacitors marked with an asterisk should be close tolerance types. here - switch your multimeter to a high Ohms range and use it to confirm that the tracks are isolated from each other. This test will quickly locate faults on any board that has not been correctly etched. You will need to go through a similar checking procedure with each of the other boards when you come to them. Fig.5 shows the parts layout for the phono preamplifier board. The first job is to install the 23 PC pins. Fourteen of these support the selector switch assembly and these should be installed from the copper side of the PC board; ie, so that the shoulder of each pin sits against its respective copper pad. Another three PC pins are required for the power supply connections near the centre of the board (+15V, 0V & -15V), while the six remaining pins are located at the left and right channel outputs and the tape inputs. No particular order need be followed when installing the remaining parts on the board but it's best to start with the smaller parts (resistors and wire links) first. Check the values of all resistors with your multimeter before installing them on the board- it's .all too easy to misread the colour codes. You'll also notice that four of the capacitors (two in each channel) are marked with an asterisk. These capacitors are in the feedback network of the phono preamplifier and, ideally, should be close tolerance (5% or better) types in order to obtain accurate RIAA equalisation. If you have a capacitance measuring function on your digital multimeter, you can specially select these capacitors. If you are buying a kit, the kitset suppliers may elect to supply 5% capacitors or may also hand select the capacitors. Take care with the orientation of the LM8 33 IC and the electrolytic capacitors. The two input inductors (L2) are made by Philips (type 4312 020 36760) and are soldered straight into the board. No fiddly winding is required. Construction of the phono preamp PC board can now be completed by soldering in the three 3 x 2-way RCA socket panels. One of these, adjacent to the LM833, is cut down to a 2 x 2way, so that a total of 16 RCA sockets is provided. Selector switch assembly Fig.6 shows the details of the selector switch board. Position the switch with the locating pin towards the top and push the body of the switch all the way down onto the board before soldering the terminals. The pads along the bottom edge of the switch board can now be soldered to the 14 PC pins on the phono preamplifier board. Tone control board Fig.6: the selector switch should be mounted on its board with the locating pin towards the top. Check the PC pattern carefully to ensure that none of the input pads are shorted together. The pads along the bottom of the board are soldered to the 14 matching PC pins on the phono preamplifier board. · Fig. 7 shows the parts layout on the tone control PC board (code SC01103923). Commence assembly by installip.g PC pins at the external wiring points , then install the wire links, resistors, capacitors and semiconductors. Check the orientation of polarised parts carefully when installing them on the board. These parts include the three !Cs, four diodes, four transistors and the electrolytic capacitors. The 6.8µF and 22µF capacitors are bipolar APRIL 1992 19 15mm tapped spacers and screws. Don't fit the dress panel to the chassis at this stage. It should be left in its protective wrapping for as long as possible, to protect it from scratches. When all the pot nuts are secured, use your multimeter to check that all the pot cases are electrically connected together, via the chassis. Ifnot, it might be necessary to remove the board from the chassis and then take a round file to lightly clean off any paint or oxide from around the pot mounting holes. The reason for making sure that the pots are properly earthed via the chassis is to keep hum and noise to the minimum possible. Power amplifier board This view shows the completed phono preamplifier board with the selector switch board soldered to it at bottom right. The input RCA sockets are soldered directly to the phono preamplifier board & consist of two 3 x 2-way panels plus a single 2 x 2-way panel (cut down from a 3 x 2-way panel). types and can be installed either way around. Note that a short length of shielded cable must be wired into the board, adjacent to IC3. The headphone socket, pots and pushbutton switches should be left till last. Be sure to push them all the way down onto the board but don't solder all the leads at this stage. Instead, tack solder diagonally opposite pins at either end of each component. 20 SILICON CHIP The tone control assembly can now be tested in the chassis to ensure that everything aligns properly. Adjust the alignment of the pots and switches as necessary before soldering the remaining pins. That completes the tone control board. It can now be mounted in the chassis and secured using the pot nuts and lockwashers. The rear of the tone control board is secured using two The power amplifier board is identical to the stereo module published in our February 1992 issue except for some minor details. The specified heatsink is a 3mm-thick aluminium angle extrusion, 60 x 60 x 290mm long, with a cutout in the horizontal section to accommodate the toroidal power transformer. The component diagram for the power amplifier board is shown in Fig.8. We suggest that you mount the PC pins, resistors, diodes and wire links Below: check the orientation of all polarised components carefully when installing them on the tone control board. The pots, switches & headphone socket must be pushed all the way down onto the board before being soldered. e GND• 1~i~\ L• .C!L). 1uF (Ll OUTPUT TO POWER AMPLIFIERS D.5W ~ ~ R• Fig.7 (above): follow this layout diagram when wiring up the tone control board. The circuit diagram published last month shows the pinouts for the transistors. Fig.8 at right shows the parts layout for the stereo power amplifier module. Note that transistors Q7-Q11 in each channel are bolted to the heatsink before being soldered to the PC board (see text). on the board first, followed by the capacitors. There are only two electrolytics on the board, apart from those in the power supply. Make sure they are mounted with correct polarity. Most of the remaining capacitors are MKT polyester capacitors which have a standard lead spacing (pitch) of 5mm. We recommend against greencaps as they won't fit. The fuses specified are M205 20mm-long types, as widely used in commercial stereo amplifiers. All the TO-92 transistors (Q1 -Q3, etc) are mounted with the flat side facing towards the front; ie, away from the heatsink. Similarly, the TO-126 transistors (Q4-Q6, etc) mount with APRIL 1992 21 When you have assembled the stereo power amplifier, go over the board carefully & check that the correct transistor has been used at each location & that it is correctly oriented. The board should initially be tested with the fuses removed & 5600 5W resistors soldered across the fuseclips. the metal side facing to the front. The exception is Q7 which mounts with its metal face to the heatsink (with a mica washer, of course). The 0.4 70 3-watt resistors are made by Philips and have been specified to save board space, being a lot more compact than the common SW cement "bathtub" types. Mount them so that they clear the board by about 34mm. The 6.8µH output inductors are each wound on a Philips 4322-02130330 or Neosid 60-601-72 coil former using 24.5 turns of 0.8mm enamelled copper wire. Clean and tin the ends of the inductor before installing them on the board. Output transistors The way to mount the output transistors is as follows. First, all the Darlingtons and the two BD139s were mounted on the aluminium heatsink. In each case, they are mounted using the specified mounting kit consisting of a mica washer and plastic insulating bush for the screw. Heatsink compound is applied sparingly to both sides of the mica washer before it is set between the transistor and heatsink. The details are shown in the diagram of Fig.9. With all the transistors mounted on the heatsink, set your multimeter to a low ohms range and check that the transistor collectors are isolated (ie, 22 SILICON CHIP infinity resistance) from the heatsink. That done, set the heatsink upside down on your workbench and fit the PC board over the transistor leads. You may need to adjust some of the transistors so that their leads line up with the board holes. Tack solder a couple of Darlington transistor leads at each end so that the top board surface is about 8mm from the bottom edge of the heatsink. You will also need to slightly crank the leads of the BD139s (Q7) to line them up with their respective PC INSULATING MICA WASHER w,~jl SCREW I llllllD(3 -......._HEATSINK t T0220 DEVICE Fig.9: transistors Q7-Q11 are each isolated from the heatsink using a mica washer & insulating bush. Smear the transistor tabs & mica washers with heatsink compound before bolting the assemblies together & use your multimeter to check each transistor as it is mounted to ensure correct isolation. board holes. When you are satisfied with the lining up of the board, solder all the transistor leads to the board pattern. You will also need to make some wire connections underneath the board, before it is installed in the chassis. These are shown in the diagram of Fig.10. First, run a length of hookup wire between the +38.SV power supply output and the +38.SV input to the left channel. Second, run two wires from the GND points to the main board earth point between the two 4700µF power supply capacitors. Twist the wires together as shown in the diagram. The next step is to install the phono preamplifier board which has the selector switch board and RCA socket panels soldered to it. Before doing that though, fit the earth binding post (near the phono inputs) and connect a 380mm length of green hook-up wire to it. The other end will connect to the chassis earth point near the transformer. Now fit the two tapped spacers, one 10mm long and one 15mm long to the phono board. The 10mm spacer will mount over the horizontal section of the heatsink for the power board: Now place the phono preamp board into the chassis and secure it to the rear panel by means of 12 small selftapping screws into the RCA socket panels. Do not fit screws into the underside of the tapped spacers at this stage. The power amplifier board and its Fig.10: these power supply connections must be run on the underside of the power amplifier PC board before it is installed in the chassis. Use insulated hook-up wire & twist the leads to the GND connections together as shown. J,: .·. +38.5V L +38.5V TO LEFT CHANNEL CONNECTION MADE ON UNDERSIDE OF BOARD LF.FT ANO RIGHT CHANNEL EARTHS MADE TO JUNCTION OF FILTER CAPACITORS ON UNDERSIDE OF BOARD heatsink may now be slid into the chassis so that the horizontal portion of the heatsink slides under the phono board. Fit two 6mm long hollow spacers under the board and secure it in place with screws and nuts. Chassis wiring Take great care with the mains wiring -your personal safety depends on it. Fig.11 shows the chassis wiring details. The mains cord enters through a hole in the rear panel and is securely clamped using a cord-grip grommet. Strip back the outer sheath of the mains cord by about 35mm before connecting the Active (brown) and Neutral (blue) leads to the mains terminal block. The Earth lead (green/ yellow) is soldered to one of the adjacent solder lugs. The second solder lug terminates the earth lead which is run along the rear panel from the binding post terminal adjacent to the phono input sockets. Don't alter the earth wiring you may get a hum loop if you do. The primary leads of the transformer are connected to the mains terminal block, while the 25V sec- Below: this view, together with the chassis wiring diagram (Fig.11) on the following page, shows how everything fits inside the case. Orient the power transformer so that its leads face towards the front panel this is important to achieve minimum hum). APRIL 1992 23 RESISTOR COLOUR CODES 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 No. 2 2 6 12 2 2 2 4 8 4 2 2 10 4 4 2 12 2 4 2 Value 4-Band Code (1%) 5-Band Code (1%) 1MO brown black green brown r;ed black yellow brown brown black yellow brown red red orange brown brown grey orange brown brown blue orange brown brown green orange brown brown black orange brown yellow violet red brown orange white red brown brown grey red brown brown green red brown brown black red brown blue grey brown brown brown grey brown brown brown green brown brown brown black brown brown blue grey black brown brown green black brown blue grey gold brown brown black black yellow brown red black black orange brown brown black black orange brown red red black red brown brown grey black red brown brown blue black red brown brown green black red brown brown black black red brown yellow violet black brown brown orange white black brown brown brown grey black brown brown brown green black brown brown brown black black brown brown blue grey black black brown brown grey black black brown brown green black black brown brown black black black brown blue grey black black gold brown brown green black black gold brown blue grey black silver brown 200kn 100kn 22kO 18k0 16kn 15kO 10kO 4.7kn 3.9kO 1.8kn 1.5k0 1kn 6800 1800 1500 1000 680 150 6.80 ondary and centre-tap leads are soldered to three PC pins on the power amplifier board. Be careful not to transpose the centre-tap lead with one of the 25V secondary leads. To make it easier to solder the secondary wires to the amplifier board, we shortened the leads back and then added short extensions oflight duty hook-up wire, soldered and sleeved for secure connections. Be sure to use mains-rated 240VAC cable for the connections to the power switch. We used heatshrink tubing to cover the switch lugs and then covered the whole switch body with a larger piece of heatshrink tubing. The other end of the leads from the mains switch go to the terminal block, along with a .0lµF 250VAC "antithump" capacitor. Make sure that this capacitor is rated at 250VAC. Testing the board CAPACITOR CODES (10%) 0 0 0 0 0 0 0 0 0 0 0 0 0 24 Value IEC Code 1µF 1u0 330n 220n 150n 100n 0.33µF 0.22µF 0.15µF 0.1µF 820pF 470pF 330pF 150pF 68pF 33pF 2.2pF 820p 470p 330p 150p 68p 33p 2p2 SILICON CHIP EIA Code 105K 334K 224K 154K 104K 821K 471K 331K 151K 68K 33K 2.2K When the AC wiring is complete, do not add the shielded signal cables. The next process is to power up each board in turn and check that it is operating correctly. First, we do the power amplifier board, since it is the most involved. Before making any connection to the power board, check all your work very carefully against the wiring diagrams of Fig.8 and Fig.11. This done, remove the four 5A fuses and solder a 5600 5 watt resistor across each of the on-board fuseholders. These are current limiting resistors which reduce the likelihood of any damage to the output transistors in case you have done something silly like swapped a TIP142 for a TIP147. Next, solder two 2700 1W resistors across the low-voltage supply outputs - ie, between +15V & GND and between -15V & GND. These protect the regulators from excess input voltage until the tone control and preamp board supply lines are connected. Now apply power and check the DC outputs at the lefthand side of the board. The outputs marked ~38.5V and -38.5V can be expected to be around +45V and -45V because they are only lightly loaded. However, the ±15V regulated outputs should be close to their nominal values (note: the left channel amplifier is powered up during this test procedure) . Now connect the positive and negative supply leads to right amplifier Fig.11 (right): the chassis wiring details. Exercise care when installing the mains wiring & sleeve all exposed terminals on the fuseholder & switch with heatshrink tubing to prevent accidental contact. The leads between the mains switch & terminal block should be twisted together & should also be sleeved in heatshrink tubing. REAR PANEL SPEAKERS RIGHT LEFT GNO + + GND \.__r ACTIVE (BROWN) L_ COROGRIP -GROMMET X \~ LEFT OUTPUT PREAMPLIFIER PCB 6-- 4--•15V GND 5--15V POWER AMPLIFIER BOARD RIGHT SPEAKER~ J\ '-· '~:~' GND• GND + SOURCE ,,~. TONE CONTROL PCB ~ S6 FRONT PANEL APRIL 1992 25 running all the shielded cable and the speaker leads, as shown in Fig.11. You will also need to fit the extension shaft to the selector switch. This extension shaft passes through a plastic bush fitted to the front panel. Troubleshooting If the above measurements are not OK, the most likely causes are broken This close-up view shows the plastic bushing that's used to support the selector switch extension shaft where it passes through the front panel. channel. Set VRl fully anticlockwise in both channels - this gives the minim um setting for . quiescent current through the output transistors. Apply power again and measure the positive and negative supply rails. They should be within a few volts of ±38.5V. Now measure the other voltages on the circuit. They should all be within ±10% of the nominal values, depending also on whether your 240VAC mains supply is high or low (it is above 240VAC more often then not). The voltage at the output should be within ±30mV of 0V. Now switch your multimeter back to the Z00VDC range and connect it across one of the 560Q 5W resistors. Now adjust trimpot VRl for a reading of 22.4V. This gives a total quiescent current of 40mA. Do the same in the other channel. After five minutes or so, check the quiescent currents again and readjust VRl if necessary to get the correct voltage across the 560Q resistor in each channel. (If you are doing power tests on the amplifier and the heatsink becomes very hot, you can expect the quiescent current to at least double. When it cools down though, the quiescent current should drop back to around 40mA). Measure the voltage across each 0.47Q 3W emitter resistor. They should all be about 9-lOmV, which means that each Darlington transistor is getting its rightful share of the quiescent current. If all is well, switch off, remove the 560Q 5W resistors and the 270Q 1W resistors, and fit the 5A fuses. Now connect the ±15V supply wires to the tone control board and check the voltages on it. This done, connect the supply wires to the phono preamp and check the voltages on it also. If all these checks are OK, you can complete the wiring of the amplifier by tracks or solder bridges between IC pins. For example, if you have the correct supply voltages on an IC but its output is close to +15V or -15V, it is most likely that there is a break in the feedback network or to the inputs to that IC. You can follow this up by measuring the voltage at the input pins of the ICs. Again, these should all be very close to 0V. If not, check for breaks in the copper track, poor solder joints, and that the IC is not in the wrong way around. Note: if you've put the IC in the right way around, it is most unlikely that any malfunction will be due to a faulty IC. What happens if one of the power amplifiers is not working? If the other channel is working correctly, then you have an ideal cross-check. Check the voltages in the good channel and then in the bad channel and you can usually get a fair idea of what the problem is. It is unlikely that you will get the same fault in both channels, unless you have made the same assembly mistake in both! And now we'll give you a few clues which may help you solve any problems. First of all, let's assume that most of the amplifier voltages are correct but that you have zero quiescent current. Look for a short across VRl or Q7. If you have lots of current through the 560Q resistors and cannot control it with VRl, look for an open circuit in the 680Q base resistor .. l i i ' -s,,,..,,_~:"· _ _· . - 'i . t'l 26 SILICON CHIP The rear panel carries the RCA input and output sockets, an earthing terminal, the spring loaded loudspeaker terminals and the mains cord grommet . The power amplifier board is fitted into the chassis by sliding the bottom of the heatsink bracket under the phono preamplifier PC board. A cutout in the bracket provides the necessary clearance for the power transformer. to Q7 or a defect in that transistor. What if the output of the amplifier is fully latched up at either +38V or -38V? The most likely cause of this is a defect in the first or second differential pair of transistors, or something silly like the wrong transistor, say a BF469 where a BF470 should be. Sol_der bridges between tracks can also cause this fault. The above are the more common problems with build-it-yourself amplifiers. Most times though, you can expect the unit to work perfectly at switch on. Listening tests No, we're not going to listen to music - yet. The idea of the next few checks is to make sure that everything is really working as it should. You'll need a pair of headphones. Plug them into the headphone socket, turn on the power and listen. With the Volume control at mini- mum setting you shouldn't be able to hear anything. If you now select the phono input and wind up the Volume to maximum, you will hear some hiss and a small amount of hum. That is normal. If you now switch to the other inputs (CD, Tuner, etc), the noise should drop to extremely low levels (we doubt you'll be able to hear anything, even in a very quiet room). If you now plug a pair of shorting plugs into the phono inputs, the noise on the phono inputs should drop to much lower levels. If it does, pull the shorting plugs, wind up the Volume control and check the action of the Bass and Treble controls. The Treble control should boost and cut the hiss and the bass control should boost and cut the low frequency phono noise. The Balance control should shift the noise from left to right and the Mono/Stereo switch should also work. When in the mono setting, the noise should appear from right in the centre of your head. Switching to Tape Monitor should kill the noise and the Tone Defeat switch should kill any boost and cut effect of the tone controls. Now if all of these things don't check out, it is pretty easy to localise the fault to a particular section. For example, let's suppose that .t here is no noise on the phono inputs, but very faint noise on the CD and other inputs. OK, that means a fault in the phono preamplifier but provided the voltage measurements here were correct, then you probably have an open circuit between the phono LM833 output(s) and the selector switch. Remember too that if you have a fault in.one channel, you can usually track it down by comparing the operation of the circuitry in the good channel with that in the malfunctioning channel. The front panel can now be mounted but be careful - one scratch and you'll ruin the appearance of the whole project. Fit the lid to the case and the job is finished. SC APRIL 1992 27