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This is how the kit appears as you take it out of the box. The meter movement and front panel are one assembly. Note the resistors wrapped around a strip of cardboard. By LEO SIMPSON Build this low--cost analog multimeter Are you in the market for a low cost analog multimeter? You are? Why not put together this kit multimeter? You'll save a little money and learn about multimeters in the process of assembling it. If you are on a tight budget, a low cost analog meter may be just the ticket until you can afford a digital meter with all the bells and whistles. And even if you don't have budgetry constraints [lucky you), a low cost meter such as this is often handy to have in your glovebox or car toolkit. Perhaps you already have a digital multimeter and don't consider that you need an analog meter. Well, there are times when an analog meter is more suitable than a digital type. For example, if you are aligning a radio [such as the AM Stereo Radio described this month) an analog meter is much more useful when trying to obtain a peak reading. As another example, the "Ohms" range of an analog meter is useful for judging whether a capacitor is good or not. You can do this by seeing how the meter's pointer flicks up the scale when the capacitor is connected. 4 SILICON CHIP So while analog meters have been largely superseded by their fancier digital cousins, they are still handy in certain applications and where sheer accuracy is not required. For most of the time, you just don't need high accuracy. Meter features The meter in question is a fairly basic unit with the more or less standard DC sensitivity of 20,000 ohms/volt on the DC ranges. What this means is that the meter will draw a maximum of 50 microamps from the circuit being measured for a full scale reading. When measuring voltages in high impedance circuits, you have to take this "loading" effect into account. The DC voltage ranges are 2.5, 10, 50, 250 and 1000 volts. The AC voltage ranges are 10, 50, 250 and 1000V while the Ohms ranges are 10k0, 100k0 and 1MO. DC accuracy is 4% of full scale deflection while The board is supplied partially assembled with a fuseholder, capacitor, trimpot, the current shunt and the insulated hookup wires. AC accuracy is 5% of full scale deflection. DC current ranges are 5mA, 50mA, 500mA and 10 amps. As well, there is a continuity range [with internal buzzer) and two battery check ranges for 1.5V cells and 9V batteries (eg, Eveready 216). These ranges pull a current of 20mA from the battery under test so they give a quite useful indication of battery condition. Overall size is 135 x 90 x 40mm thick so the unit is small without being in the tiddler class. Now to put it together. Construction When you open the kit, you will find a bunch of parts in a plastic bag. These will include the meter test leads, two penlite cells, a piezo buzzer and the trimpot for the zero ohms adjustment. The printed circuit is supplied with a number of pieces of coloured hookup wire already soldered to it, as is the heavy gauge brass [?) shunt wire for the 10 amp current range. The range switch is part of the printed board pattern, as you will see when you have a look at it. Also included in the parts collection is a strip of cardboard with all the resistors and diodes wrapped around it. These resistors are numbered and the value is listed. The same goes for the diodes. Assembly of the resistors and diodes is then simply a matter of installing them according to the "R" and "D" numbers on the PC board. This is what the board looks like when you have installed all the resistors. Don't bend the current shunt while installing resistors under it. We put the diodes in first, noting that D1 was a germanium 1N60 type while the others were silicon 1N4148s. The cathode ends [where the diode arrow symbol points to) of the diodes are indicated by a black stripe on one end of the diode bodies. Then we put the resistors in. Finding the position for Rl on the board is tricky - it's underneath one end of the current shunt wire. The spring contacts for the rotary switch are fitted as shown here. The piezo buzzer is secured with two small self-tapping screws. RESISTOR COLOUR CODES This is the back of the meter with the assembled board in place. The mounting arrangement for the Ohms adjustment pot is a little crude - it is connected to short pieces of tinned copper wire which are soldered to the board pattern. Resistors R4, R5 and R6 interfere with a yellow wire which is labelled as J5 on the board. Push it out of the way while you install these resistors. R7 is tricky too, because you have to manipulate it in under the current shunt wire to install it. Don't bend the current shunt wire out of the way though - if you do, you could disturb one of the solder connections. When you come to R12 there is another fork in the road. Right next to R12 is another resistor position but it is unmarked - leave it vacant. R13 is over near trimpot R21. To help you identify the resistors we have listed them with their colour codes. Note that there are some very odd values included, values that are specially made as precision multipliers for multimeters. Incidentally, why are the range resistors called multipliers? The answer is that they 'multiply' the basic voltage sensitivity of the meter movement itself. When all the resistors are in place, their leads can be soldered and then cut flush with the board surface. Now you can turn your attention to the meter itself. It comes assembled with the meter movement and the switch rotor. You have to add the springy metal contacts to the rotor. One of the photos shows how they fit. You can also fit the piezo buzzer with the two small self tapping screws provided. Next, fit the Ohms adjustment pot to the printed 6 SILICON CHIP Resistor Value 5-Band Code (2% tolerance) R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 R18 R19 violet violet silver red violet red black silver red violet red black gold red violet white red brown red yellow red black silver red yellow red black brown red yellow grey violet red red yellow grey yellow red red brown green black orange red grey black black orange red yellow black black yellow red brown green black green red grey green blue red red orange green yellow orange red brown violet blue yellow red blue blue black yellow red grey orange black silver red orange black yellow red red yellow brown black black red 0.770 7.20 720 7.92k0 420 4200 48.7k0 48.4k0 150k0 800k0 4MO 15MO 85.6k0 354k0 1.76MO 6.6MO 830 30.4k0 4100 board. It is attached by short pieces of tinned copper wire to both end connections and to two of the middle connections, as shown in the relevant photo. The method is fairly crude but is identical to that in the fully assembled examples of this multimeter. Fit the thumbwheel knob to the Ohms pot using the small machine screw provided and then temporarily fit the board to the meter housing. Check that the thumbwheel protrudes through the meter front panel by the right amount, so that it can be easily operated. continued on page 107 And here's what it looks like when finished - a handy little meter with a useful combination of ranges. The two battery check ranges are particularly useful. the header connector. For further information contact All Electronic Components, 118-122 Lonsdale Street, Melbourne, Vic 3000. Phone (03) 662 3506. Fax (03) 663 3822. Get a plastic minigrip tool ·, .:..,_'\-. _ __ :-.,· ' If you are sick of trying to manipulate teensy components into protoboards or inacessible spots on printed boards, try these neat locking pliers with plastic insulated handles. There are two models, with straight or curved serrated jaws. A pair of tweezers with curved serrated jaws is also available. The locking pliers will sell for around $11.00 plus sales tax while the tweezers will sell for around $6.00 plus tax. For further information, contact Scope Laboratories, PO Box 63, Niddrie, Vic 3042. Phone (03) 338 1566. Pocket microscope for PCB inspection How are you on examining PCB solder joints under poor lighting? Ever notice that when the lighting is improved it is still very hard to examine a closely spaced board? What you need is a little microscope with its own built-in illumination. Now you can have it with this handy pocket microscope from Scope laboratories. This lightweight (only 80 grams) instrument has a 4-lens optical system giving 30 times magnification and it has its own built-in light source. It can be easily focussed above or below the surface on which it is placed too, which could be a bonus when inspecting components. The pocket microscope will be available shortly at a price expected to be around $41 plus sales tax where applicable. It should Build a multimeter: ctd from page 6 If that checks out, you can solder in the battery connections. As supplied these are rightangle brackets of what looks to be tinplate. These should be soldered in and bent as shown in the relevant photo. That done, fit the board to the meter housing and secure it with the self tapping screws provided. Now connect the red and blue wires to the meter movement (red to + and blue to - ). The black wire from the buzzer connects to the centre connection and the red wire goes to the terminal marked "to BZ +" next to R6. Mailbag: ctd from page 3 AC 240 volts and DC via batteries or they could be AC or DC 240 volt mains and DC via internal batteries. Not all sets were the AC-DC death traps that John refers to. To quote two brands, AW A sets were equipped with a transformer and were safe to handle while the Astor sets could easily have a live chassis. As a serviceman I made very sure that the chassis was "cold". (9). Sets that worked on both AC or AC/DC and batteries ran their valve That completes the wmng and assembly. All that remains to be done is to screw the back on to the meter and then use it to measure a few known voltages. Alternatively, you can make several AC and DC measurements and compare them to the same measurements made on a digital multimeter. If necessary, tweak trimpot R21 to give the best accuracy. Where to buy it The multimeter is available in kit form from all Jaycar Electronics stores at $34.95 (Cat No KC-5058). It is also available from Jaycar in built-up form at $39.95 (Cat No QM-1022). ~ filaments in series and were run off 9 volts DC at 50mA instead of 1.5 volts at 300mA for 5 valve sets. If the filaments had been run in parallel, the current drain would have been excessive on mains and in some circumstances hum on the filament line would have been difficult to eliminate. (10). Some valve portables, when used on mains, had a battery rejuvenating charging system that improved battery life a bit. I trust these points are of some interest to other readers who like the old valve radios. R. Champness Benalla, Vic Niddrie, Vic 3042. Phone (03) 338 1566. Fax (03) 338 5675. MEMORY CHIPS Prices at October 16, 1989 Type 256 Description Price ex tax 150ns XTs $4.00 120ns ATs $4.25 1 OOns 12MHz $5.00 80ns $6.25 ?Ons 20MHz $7.50 100ns 20pin $17.80 100ns $17.00 80ns $17.80 9x100 SimmLP $190.00 9x100 Simm HP $160.00 9 x 80 Fast Simm LP $210.00 Bx100 MacSimmLP $180.00 8x100 Mac2HP $155.00 Sales Tax 20%. Overnight Delivery 4x256 1Mb 1Mb - Credit Cards Welcome prove invaluable for both servicing and for quality control work. For further information, contact Scope Laboratories, PO Box 63, PELHAM Tel (02) 427 0011 Fax (02) 428 5460 NOVEMBER 1989 107