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Dil\lG 'I'I If. AC LIVOL'I'M f.~fER B Because the AC Millivoltmeter can measure audio signals down into the microvolt region it must be carefully constructed to minimise crosstalk and feedback in its high gain circuitry. Special measures must also be taken to avoid common mode signals and earth loops. By LEO SIMPSON & BOB FLYNN As far as our new AC Millivoltmeter is concerned, building the high gain circuitry is only half the job. Correct installation and wiring inside the case is critical if the proper performance is to be obtained. Earthing and layout of the wiring inside the case is especially critical. Taking short cuts in this area will result in an instrument which gives unreliable or even nonsensical results. Do the job properly and you'll have a measuring instrument of which you can be proud. We had to build several pro- totypes of this instrument and try out many variations in the wiring before we could be sure that we had a layout which would give correct results under a wide range of measurement conditions. That is not to say that putting the AC Millivoltmeter together is going to be a difficult job; it isn't. But if you vary the wiring layout or the components you are asking for trouble. Let's mention some of the critical areas of the instrument before we go on to detail the construction: • Insulated BNC sockets for the signal input and CRO output; • Wiring and layout of the mode switch; • Termination of the various shielded cables; • Metal shields around the Input Attenuator S1 and Noise Switch S4; • Mains and power supply wiring; • Transformer orientation. Some component substitutions are permissible though and we'll list those as we progress through the article. Assembly procedure To simplify the description, we'll assume that you have purchased a complete kit of all parts, including pre-punched metalwork and a screen-printed or Dynamark front panel. We'll also assume that you have a meter movement with the correct scale. To date though, only Dick Smith Electronics have indicated that they will supply the complete kit. Other kitsellers may come to the party at a later date. If you're not buying a complete kit but have opted to build your own from scratch you'll need to refer to the parts list and to a panel in this article which tells where you can buy some of the special parts needed. Assuming that you have the complete kit laid out in front of you, we suggest you start by building the power supply into the case. Then you can move on to assembling the main board, wiring the switches and finally the wiring inside the case. Power supply Both BNC sockets on the AC Millivoltmeter are insulated from the case even though the entire "earthy" side of the circuit is connected to case. Note the two voltage scales and dB scales on the meter. 46 SILICON CHIP The power supply is a version of our Universal Power Supply Board presented in last month's issue. It uses a 30V centre-tapped transformer (Altronics M-2855) and a bridge rectifier. We understand that the kit supplied by Dick Smith Electronics will use a 2851 15V 0 0 0 INPUT SEE SEPARATE DIAGRAM FGft 11, 13, S4 AHB S5 EONNECTIONI Fig.1: the overall wiring for the AC Millivoltmeter. Note that no part of the circuit, except for the transformer core, is connected to the mains earth. And there is only one connection from the circuit to the case, at the solder lug on the top righthand corner of the board. SEPTEMBER 1988 47 This view of the power supply inside the rear panel of the case shows how the transformer is angled to minimise hum. Note the all-plastic mains switch and heatshrinlc sleeving on the contacts. THREE WAFERS, EACH SINGLE POLE 12 POSITION CLICKER PLATE SINGLE POLE, 12 POSITION WAFER CLICKER PLATE SHIELD PLATE I { \ S1 \._ S3 TWO WAFERS, EACH SINGLE POLE 12 POSITION CLICKER PLATE CLICKER PLATE SHIELD PLATE I \ TWO POLE., S POSITION WAFER \ S5 S4 LORLIN ELECTRONICS MINIATURE ROTARY SWITCHES TYPE RA DIMENSIONS IN MILLIMETRES Fig.2: this diagram shows the wafer arrangement on the four rotary switches. Note that all switches must have make-before-break (shorting) contacts. transformer (as made by Ferguson and others) with two half wave rectifiers. Our wiring diagram shows the former version. Watch the polarity of the diodes and electrolytic capacitors when wiring up the power supply board. Note that the pinouts for the positive and negative 3-terminal 48 SILICON CHIP regulators are not the same. Don't get confused and swap them over or install them the wrong way around; they won't like it. Use PC stakes or pins for the AC input and DC output connections to the board. It makes connections easier. The power supply board and transformer are installed inside the rear panel of the case. The board is mounted using screws and tapped spacers which may be of metal or an insulating material such as Nylon. The transformer must be isolated from the case. This is done using tapped insulating pillars, 10mm long. The mains cord earth is terminated to a solder lug which is secured by one of the transformer mounting screws. The mains earth does not connect to the case itself but does run over to the Earthing switch, S6. The mains switch must be rated at 250VAC and be of all-plastic construction, such as made by Swann. Do not use a miniature toggle switch for this task. (Note: the Swann switch was not specified in the parts list published last month). By installing the transformer on pillars and using an all-plastic switch, the mains wiring will be done to double-insulation standards. This is necessary because neither the circuitry nor the case is connected back to earth. This is done to ensure that earth loops do not occur when measurements are being performed with earthed equipment. Note that the transformer is oriented at an angle of about 30° to the base of the case. This is to minimise hum injection into the circuitry. The Active (brown) wire of the mains cord is connected directly to one terminal of the mains switch while the centre terminal of the switch is connected to one of the transformer primary wires, via a 2-way insulating terminal block. The other terminal of the insulating block terminates the neutral wire from the mains cord and the other primary wire from the transformer primary. Both terminals of the mains switch should be sleeved with heatshrink tubing to prevent any possibility of contact with the AC mains. When the power supply is complete you can turn it on and check the ± 15V outputs from the 3-terminal regulators. That done, tie the mains wires to the insulating terminal block together with a ALL WAFERS VIEWED FROM REAR WAFER NEAREST CLICKER PLATE WAFER REMOTE FROM CLICKER PLATE ® WAFER NEAREST CLICKER PLATE 0 Switches MIDDLE WAFER M WAFER REMOTE FROM CLICKER PLATE Fig.3: this diagram shows how the wafers for S1, S3, S4 and S5 are wired. All resistors on S1 and S4 are 1% metal film types. The letter coding for the various wires corresponds with points shown in the wiring diagram, Fig.1. small Nylon cable tie. This is to reduce the possibility of any of the mains wiring from coming adrift inside the case. As a final step in the power supply construction, cut and shape a suitable piece of Presspahn or Elephantide (Bakelised canvas or Kraft paper) to sit under the mains terminal block and to cover all the mains wiring. We have left this out of our prototype so that the mains wiring could be photographed. If you can't obtain Presspahn (it is hard to get) use a piece of plastic icecream container cut and glued to shape (use M.E.K. or A.C.C. glue). Main circuit board Most of the AC Millivoltmeter's circuitry, with the exception of components wired to switches, is mounted on the main circuit board. This measures 193 x 98mm. We circuits. At one end of the board there are two long wire links. These should be run with insulated single strand wire, to avoid the possibility of shorts. Note that the Set Level pot is a 16mm diameter PC mounting type. It has been specified because of the limited space on the front panel. Be careful with the orientation of the electrolytic capacitors and ICs. There are three 50µF non-polarised electros and these can be installed either way around. Note that all the ICs are oriented in the same direction. When the board is finished, carefully inspect it and then put it aside. have used miniature capacitors (monolithic and miniature metallised types) throughout the circuit in our prototype but the board has been designed so that it will also accept the more conventional greencaps (metallised polyester). Capacitor tolerances are not critical in the circuit; the ± 10% tolerance of greencaps is adequate. Resistors are another matter. We have specified 1 % tolerance metal film resistors throughout, partly to obtain precise values and partly for their low noise. Don't substitute carbon film types. PC pins or stakes should be used for all external connections to the board. Doing it without PC pins is not practical. We suggest you install the PC pins, wire links, resistors and diodes first, followed by the capacitors and then the integrated While the switching in the AC Millivoltmeter is fairly complex, there is not a great deal of wiring. It must be done in the right order though. All the resistors on the Input Attenuator switch St and those on the Noise switch S4 can be wired onto the switches before they are installed. S1 is a single wafer switch and all the attenuator resistors are installed around it as shown in the wiring diagram of Fig.3. You can also refer to the relevant photos. S3 is a triple-wafer switch with intermediate shield plates between the wafers. Do not even think about using a conventional single-wafer 3-pole switch in this application; the crosstalk between the sections of the switch will make the circuit unstable. You must use the triplewafer switch specified. S5, the Filter switch, is a 2-pole 3-position type and while we used one from Lorlin, you could save a few dollars by using one which is readily available · from kitset suppliers. However, our wiring diagram depicts the Lorlin switch types throughout. You should solder all the connecting wires to the Mode and Filter switches before they are installed. At this stage you should be ready to install some of the hardware in the case. This includes the two insulated BNC connectors, toggle switch S6, pushbutton switch S2 and then the main printed circuit board. The board is installed with SEPTEMBER 1988 49 This view shows most of the main board, along with Input Attenuator S1 and the Noise switch S4. Note the two wafers for S4 and the tagstrip for the 11,tF input coupling capacitor. four 5mm spacers and secured with screws and nuts. Now install the Mode and Filter switches and make all the connections from them to the printed board. Run a short length of shielded cable from the CRO output socket to the adjacent points on the board. Note that the shield is not connected at the socket end. Instead, the shield solder lug connection of the BNC socket runs over to the centre lug of the Earthing switch, S6. One side of this switch is then wired to the earthing lug on the transformer while the other terminal goes to the 0V connection on the power supply board. Switch shields S1 and S4, the Input Attenuator and Noise switches, are surrounded by a metal shield to prevent crosstalk from other parts of the Actual size artwork for the meter scale. 50 SILICON CHIP circuitry. The cutting and folding diagrams for these shields are shown in Fig.4. They can be made from thin aluminium or tinplate. The shields are held in place by the switches themselves. The input BNC socket has its centre pin directly connected to the lµF 200V blocking capacitor. The other end of the capacitor is supported on an isolated tagstrip (ie, it should not short to the shield). The outer shield solder lug of the BNC socket connects to the main input earth for the board, adjacent to the pushbutton, S2. The wire runs through a hole in the shield, right under the socket, then drops down in the corner, loops around the pushbutton switch and then terminates on the main earth point. This point also terminates the shield side of the short coax cable from the Input Attenuator, S1. Note that this shielded ea ble takes the shortest possible route, via a hole in the surrounding shield, adjacent to the BNC socket. The wire from the middle wafer of S3 (S3b) to S4b runs via a hole in the relevant shield. The wires from S4a to the printed board should be twisted together. A 1000 resistor runs from one side of the pushbutton switch to a point adjacent to the signal input on the earth. The side of the pushbutton is wired direct to the input point. Both the resistor and connecting wire should be kept as short and direct as possible. The meter movement may now be installed in the case and wired into circuit. Note the polarity markings ( + and - symbols) stamped on the meter. We found it necessary to reduce the basic sensitivity of our prototype meter by connecting a 2.2k0 resistor across it. This is visible in the photos but not shown on the wiring diagram. This won't be necessary with the final circuit. Finish the wiring by connecting the three supply wires from the power board to main board. These should be twisted together with the wire to the Earthing switch, S6. Set-up and calibration Do not apply power yet. First check with your multimeter that the earth pin of the 3-pin power plug connects to the transformer core but to no other part in the circuit. With the Earthing switch in the "Float" position, the shield section of the CRO output BNC socket should connect to the mains earth. With the Earthing switch in the "Case" position, the shield section of the CRO socket should connect to the case. Again using your multimeter (switched to a low "Ohms" range) check that the outer shield of the Input BNC socket connects to the case. Some readers may think that this means the socket does not have to be insulated but it does. Now apply power and check that the ± 15V supply rails connect to the DC inputs on the mains board. Then using the OV input as a reference point, check that + 15V is present at pin 7 of ICs 1, 2, 3, 4, 6 and 8 and at pin 8 of IC5 and IC8. Check also that - 15V is present at pin 4 of all eight ICs. Offset voltages The next step is to check the output voltage of each op amp. Pin 6 of ICl can be expected to be less than ,1 00 00 00 0 I ,1 ~ 0 l! Actual size artwork for the main board of the AC Millivoltmeter. ± 100mV but it will vary depending on the setting of the Input Attenuator. Similarly, pin 6 of IC3 and IC4 should be less than ± lOOmV, depending on the setting of the Noise switch, S4; with S4 at - 30dB and the Input Attenuator at lmV, pin 6 of IC3 can go to - 1.3V or more Pin 6 of IC2 can be expected to be less than ± 100mV when the Mode switch S3 is in the "Volts" setting but will go to as much as - 5V or more when S3 is in the SEPTEMBER1988 51 This top view shows the connections from the power supply board to the main board. Note the wire from the main board OV point to the Earthing switch for the CRO output socket. Calibration 205 10 20 I ~ 37 0 ~ ~ ~ "' N ... ~ "'"' :::, C !!! ~ BEND UP ."' 51 21 45 45 4 ~ 0. :::, C !!! f I: 52 :1·,~1- 44 54 SILICON CHIP "'"' "'"' "Noise" setting, and depending on the setting of the Noise switch S4. To check IC5a, the Filter switch S5 must be in the "20Hz-20kHz" setting and the output voltage at pin 1 can be expected to be within ± lOmV. The same applies for pin 7 of IC5 although the setting of S5 does not matter for this measurement. (The reason S5 must be in the "20Hz to 20kHz" setting to measure pin 1 of IC5 is that the input of IC5a, pin 3, is not biased for the other switch settings; ie, "Flat" and "A Wt"). Pin 6 of IC6 should be within ± lOmV, regardless of switch settings. The same applies to pin 6 of ICB. Pin 7 of IC7a can be expected to be around - 130mV while pin 1 of IC7a should ideally be less than ± lmV. If it is not, there is likely to be a slight deflection of the meter movement. If there is an offset of several millivolts, it may be necessary to adjust the 6.2k11 resistor at pin 3. We did not have to do this on the two prototypes we built. HOLES : A : 3.2mm DIA. B: 5mm DIA. C: 10mm DIA. O: 19mm DIA. DIMENSIONS IN MILLIMETRES Fig.4: these are the dimensional and bending details for the metal shields to go around the Input Attenuator and Noise switches. To calibrate the AC Millivoltmeter, you will need a lkHz sinewave oscillator with a known amplitude output, preferably 1V RMS or more. Set the Mode switch to "Volts", the Input Attenuator to 1V and the Filter switch to "A Wt". Feed a lkHz sinewave of 1 V RMS amplitude into the input and adjust trimpot VR4 for full scale deflecction of the meter. Then move the Filter switch to the "20Hz to 20kHz" setting and adjust VR3 for full scale deflection of the meter. Then move the Filter switch to the "Flat" setting and adjust VR2 for full scale deflection. That completes the calibration procedure. If you have wide range sinewave oscillator with a calibrated attenuator, you can now check the frequency response and accuracy of the attenuators associated with S1 and S4. To verify the Noise Floor of the instrument, first put the lid on the case and secure all the fastening screws. Short the BNC input socket with a shorting plug or a piece of wire held in place with a small Special Parts If you're not purchasing a complete kit from Dick Smith Electronics or one of the other suppliers, there are number of parts which may be not readily available from your local supplier. The case for the prototype came from the Australian Transistor Company in Melbourne; phone (03) 898 2933. The Lorlin switches came from C & K Electronics (Aust) Pty Ltd; phone (02) 635 0799. The front panel, meter scale and printed boards are available from ACS Radio Pty Ltd, phone (02) 587 3491 or Jemal Products, phone (09) 350 5555. In New Zealand, contact Marday Services, Auckland, phone 88 5730 . All the op amps should be readily available with the possible exception of the OP27. This can be obtained from Geoff Wood · Electronics, phone (02) 427 1676. Cl) "'ea (..) + + 0 cc (..) cc w w I~ I- ..J 0 -> - . .J ..J ea ...I ea • • "C crocodile clip. Any noisy transformers or computers should be as far away as possible. Set the Earthing switch to "Float". Set the Input Attenuator to lmV, the Mode switch to "Noise", the Set Level control fully anticlockwise and the Filter switch to "Flat". Wind down the Noise switch until a pointer deflection towards the top of the scale is obtained. This should occur on the - 50dB Noise range. Press the Noise pushbutton and the reading should drop down scale to about - 3. This corresponds to a reading of - 55dB with respect to lmV. While still holding the pushbutton down, move the Filter switch to the "20-Z0kHz" setting. The reading then drops by lldB. It should now be about - 4 on the - 60dB range. Finally, move the Filter switch to the "A Wt" setting. The reading should then drop to about - 7 on the - 60dB range, with the Noise pushbutton still depressed. Note that it is normal for the meter pointer to flick up scale when switching Noise ranges below - 30dB. ~ ea "C 0 N I I.LI Cl) c5 z ~· • ea "C I • ea "C ..,, 0 I 0 ~ "C 0 i Cl) cc + :!i:! • .lll: I.LI : ; : . :i cc .:i ::c 0 • ea• 0 It) I • N_ ea + cc ea "C 0 <C et "C 0 ID ...I I W> 1-L.L.I Cl) L.U ...I > M > 0 ... > 0 ... • • • ... • • e M > I::::, Cl.. 3: + =• • •> • • ... > >•e > ... ... > e E 0 0 E 0 M E z I.LI c,; 0 Cl) :E - • g;?• Cl) ...I • + !l = > M 0 I- ::, L Cl) "' "ci 0 > 0 M > 0 + Cl.. 3: + Cl)~ L.U f:3:E ~ ccccO Cl..c::,Z LL + _J This is the actual size front panel artwork. Artwork for the power supply board was shown on page 48 of the July 1988 issue. SEPTEMBER 1988 53