WEBVTT NOTE Test Equipment and Soldering 1 00:00:00.000 --> 00:00:18.687 Meters: how each connects. Instrument, Voltmeter; Measures, electric potential; Connection, in parallel with the component. Instrument, Ammeter; Measures, electric current; Connection, in series with the component. 2 00:00:18.687 --> 00:00:36.938 Instrument, Ohmmeter; Measures, resistance; Connection, across an unpowered component. The connection follows from what is being measured. Voltage is a difference between two points, so the meter bridges them. 3 00:00:36.938 --> 00:00:50.211 Current flows through, so the meter must be in the path. A multimeter measures voltage and resistance, and current, on most, through separate terminals. 4 00:00:50.211 --> 00:01:09.160 How an ohmmeter works is worth knowing because it explains its hazards: by applying a small current and measuring the resulting voltage. The meter is a source, not a passive observer. Two ways to destroy a multimeter. 5 00:01:09.160 --> 00:01:14.274 Question. Which of the following can damage a multimeter? 6 00:01:15.174 --> 00:01:22.214 Answer. Attempting to measure voltage when using the resistance setting. 7 00:01:22.814 --> 00:01:37.059 On resistance, the meter supplies its own small current and expects to see only its own voltage. Applying an external supply drives current back into a circuit designed for microamps. 8 00:01:37.059 --> 00:01:45.233 The other classic is the mirror image: leaving the probes in the current terminals and measuring voltage. 9 00:01:45.233 --> 00:01:56.676 The current input is nearly a short circuit, so across a supply it becomes exactly that, and the internal fuse, if you are lucky, is what stops it. 10 00:01:56.676 --> 00:02:11.933 And when measuring in-circuit resistance: ensure the circuit is not powered. Both for the meter's sake and because a reading taken on a live circuit is meaningless anyway. A useful ohmmeter trick. 11 00:02:11.933 --> 00:02:20.933 Question. What reading indicates that an ohmmeter is connected across a large, discharged capacitor? 12 00:02:21.833 --> 00:02:25.833 Answer. Increasing resistance with time. 13 00:02:26.433 --> 00:02:38.491 The ohmmeter's small current charges the capacitor. As it charges, less current flows for the applied voltage, so the meter reads a rising resistance. 14 00:02:38.491 --> 00:02:46.691 Watching that climb is a quick, no-extra-equipment test that a large capacitor is not short-circuited. 15 00:02:46.691 --> 00:02:57.302 It is also a reminder that the ohmmeter is a source, the same reason you must never connect one to a charged capacitor. Dummy loads. 16 00:02:57.302 --> 00:03:02.117 Question. What is the primary purpose of a dummy load? 17 00:03:03.017 --> 00:03:08.937 Answer. To prevent transmitting signals over the air when making tests. 18 00:03:09.537 --> 00:03:14.937 Question. What does a typical RF dummy load consist of? 19 00:03:15.837 --> 00:03:22.157 Answer. A 50-ohm non-inductive resistor mounted on a heat sink. 20 00:03:22.757 --> 00:03:37.421 Every word of that description earns its place. 50 ohms so the transmitter sees a perfect match. Non-inductive so it stays 50 ohms at RF rather than becoming reactive. 21 00:03:37.421 --> 00:03:47.695 On a heat sink because every watt you transmit becomes heat, the power law again, and a 100 W test is a 100 W heater. 22 00:03:47.695 --> 00:03:59.637 Tuning up into a dummy load is the courteous default, and it is not a transmission, so no identification is required. Antenna analysers. 23 00:03:59.637 --> 00:04:08.397 Question. Which of the following is used to determine if an antenna is resonant at the desired operating frequency? 24 00:04:09.297 --> 00:04:12.857 Answer. An antenna analyzer. 25 00:04:13.457 --> 00:04:27.537 An analyser sweeps a range and shows impedance and SWR against frequency, so you see not just whether the antenna is resonant but where, and therefore which way to cut. 26 00:04:27.537 --> 00:04:43.377 An SWR meter with a transmitter tells you the value at one frequency and requires transmitting to get it. Reading SWR. 1:1 indicates a perfect impedance match between antenna and feed line. 27 00:04:43.377 --> 00:04:58.798 4:1 indicates an impedance mismatch, nothing more specific than that. A directional wattmeter can determine SWR, by measuring forward and reflected power separately and comparing them. 28 00:04:58.798 --> 00:05:16.984 Solid-state transmitters reduce output as SWR rises to protect the RF output amplifier transistors. Reflected power appears as voltage and current stress on the finals, and a transistor has no thermal margin to spare. 29 00:05:16.984 --> 00:05:29.388 Power lost in a feed line is converted into heat. It does not disappear and it does not radiate usefully; it warms the cable. Coaxial cable failure. 30 00:05:29.388 --> 00:05:35.228 Question. Which of the following causes failure of coaxial cables? 31 00:05:36.128 --> 00:05:39.290 Answer. Moisture contamination. 32 00:05:39.890 --> 00:05:52.050 Water wicks along the braid, ruins the dielectric, and raises loss permanently. It is why sealing outdoor connectors is not optional. 33 00:05:52.050 --> 00:05:59.118 Question. Why should the outer jacket of coaxial cable be resistant to ultraviolet light? 34 00:06:00.018 --> 00:06:07.115 Answer. Ultraviolet light can damage the jacket and allow water to enter the cable. 35 00:06:07.715 --> 00:06:24.511 The chain is UV → cracked jacket → water → dead cable. Cable sold for outdoor use has a UV-resistant jacket; cable sold for indoor use does not, and it fails in a few seasons on a roof. 36 00:06:24.511 --> 00:06:44.395 Foam-dielectric coax has less loss per foot than solid dielectric, because air is a better dielectric than polyethylene. The trade-off is that foam is more easily crushed and wicks water faster once breached. Soldering. 37 00:06:44.395 --> 00:06:52.755 Question. Which of the following types of solder should not be used for radio and electronic applications? 38 00:06:53.655 --> 00:06:57.015 Answer. Acid-core solder. 39 00:06:57.615 --> 00:07:08.695 Acid flux is for plumbing. In electronics it corrodes joints from the inside for years afterwards. Use rosin-core. 40 00:07:08.695 --> 00:07:15.775 Question. What is the characteristic appearance of a cold tin-lead solder joint? 41 00:07:16.675 --> 00:07:20.875 Answer. A rough or lumpy surface. 42 00:07:21.475 --> 00:07:34.804 A good joint is smooth, shiny, and concave where it wets the lead. A cold joint is dull, grainy, and blobby, solder that was melted but never properly wet the metal. 43 00:07:34.804 --> 00:07:43.933 It may conduct today and fail intermittently for months, which makes it one of the most annoying faults there is. 44 00:07:43.933 --> 00:07:59.686 The cause is almost always insufficient heat in the joint: heat the work with the iron and feed solder to the work, not to the iron. Check yourself. You want to measure the current a radio draws. 45 00:07:59.686 --> 00:08:15.035 How do you connect the meter? You connect an ohmmeter to a large capacitor and the reading climbs steadily. Faulty? Your coax is five years old on a roof and loss has crept up. Likely cause? 46 00:08:21.035 --> 00:08:36.646 In series, in the path the current takes, and with the probes in the current terminals, not the voltage ones. No, that is exactly what a good capacitor does as the meter's small current charges it. 47 00:08:36.646 --> 00:08:42.035 Moisture, most likely after UV damage to the jacket cracked it open.