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Technician Class (Expires Jul 1, 2018)
Subelement T7

Station equipment: common transmitter and receiver problems; antenna measurements; troubleshooting; basic repair and testing

Section T7D

Basic repair and testing: soldering; using basic test instruments; connecting a voltmeter, ammeter, or ohmmeter

Which instrument would you use to measure electric potential or electromotive force?

• An ammeter
A voltmeter
• A wavemeter
• An ohmmeter

Electric potential and electromotive force are alternate names for voltage. Remember that voltage is present even if no current is flowing; a battery may be a 9 volt battery even though it is not connected to anything. Thus, it is referred to as electric potential, or electromotive force.

And, as you may guess, voltage is measured with a voltmeter.

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Tags: instruments electromotive force (voltage) arrl chapter 3 arrl module 4

What is the correct way to connect a voltmeter to a circuit?

• In series with the circuit
In parallel with the circuit
• In quadrature with the circuit
• In phase with the circuit

A voltmeter is used to measure potential difference across two points in an electrical circuit.

The voltmeter is connected in parallel at the points that you wish to measure voltage drop between, which usually means connecting it on each side of a given load (which has resistance and thus voltage drop).

A theoretically ideal voltmeter has infinite resistance; an "ideal" voltmeter thus has no effect on a circuit if connected in parallel but in series would entirely prevent the flow of electricity.

Ammeters are connected in series to the circuit element you want to measure. (Series means that the circuit is changed to go through the ammeter rather than merely connecting it to existing points without otherwise changing the circuit)

All elements that are in series with each other have the same current.

A theoretical ideal Ammeter has no resistance and thus no voltage drop, so adding the ammeter in the circuit should not change the current the circuit.

Be careful with this question, make sure you know Voltmeter from Ammeter!!!!!

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Tags: instruments electromotive force (voltage) arrl chapter 3 arrl module 4

How is an ammeter usually connected to a circuit?

In series with the circuit
• In parallel with the circuit
• In quadrature with the circuit
• In phase with the circuit

An ammeter measures current flow. Unlike voltage, which is potential, current represents actual movement of energy and work done through the system. Thus, in order to measure how much of it is flowing, the current must flow through the ammeter. That means that an ammeter must be connected in series with the circuit in order to get an accurate reading and in some cases avoid blowing the fuse in the meter.

Do not connect your ammeter in series with a power source with no load or there will be nothing to limit the current flow and you will blow the fuse in your meter! An ammeter (or a multimeter with the probes connected in the amp measuring ports) ideally has no resistance and thus no load, similar to a piece of wire. Connecting it in the wrong manner is just like causing a short circuit with a piece of wire.

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Tags: instruments electrical current arrl chapter 3 arrl module 4

Which instrument is used to measure electric current?

• An ohmmeter
• A wavemeter
• A voltmeter
An ammeter

Electric current is measured in amps and is measured with an ammeter. The other meters listed here are:

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Tags: instruments electrical current arrl chapter 3 arrl module 4

What instrument is used to measure resistance?

• An oscilloscope
• A spectrum analyzer
• A noise bridge
An ohmmeter

Resistance is measured in units of Ohms and is measured with an ohmmeter. The other meters listed here are:

***Just a technical note here: This question is a bit ambiguous since there are those of us who can/do/have measure resistance with our oscilloscopes in a pinch. May not be the easiest way but it is possible. Perhaps think of the ohmmeter as the quickest way to measure resistance not the only way.

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Tags: instruments resistance arrl chapter 3 arrl module 4

Which of the following might damage a multimeter?

• Measuring a voltage too small for the chosen scale
• Leaving the meter in the milliamps position overnight
Attempting to measure voltage when using the resistance setting
• Not allowing it to warm up properly

The resistance setting on a multimeter behaves as an ohmmeter to measure resistance. In general, an ohmmeter measures resistance by applying current through the resistor and measuring the resulting voltage drop.

If you connect an ohmmeter to a voltage source, the resulting external current applied by the voltage source could seriously damage the components of your ohmmeter (in this case, your multimeter in ohm mode).

As good practice, always check the multimeters setting to ensure it is in the correct mode and range for the expected measurement. If in doubt, start at the highest voltage range and lower until you get an in-range reading. Also, NEVER connect a meter in the Current mode to a voltage circuit as it will short it out and damage the meter, even if this is only blowing the protective fuse inside.

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Tags: instruments electromotive force (voltage) resistance arrl chapter 3 arrl module 4

Which of the following measurements are commonly made using a multimeter?

• SWR and RF power
• Signal strength and noise
• Impedance and reactance
Voltage and resistance

A multimeter is thus named because it combines several measurement tools in one device. Most common multimeters these days provide an ohmmeter, an ammeter, and a voltmeter.

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Tags: instruments electromotive force (voltage) resistance arrl chapter 3 arrl module 4

Which of the following types of solder is best for radio and electronic use?

• Acid-core solder
• Silver solder
Rosin-core solder
• Aluminum solder

Rosin-core solder is a particular type of Flux-core solder that makes use of rosin flux. The advantage to rosin-core solder is that as you solder, the rosin in the core of the solder helps eliminate oxidation from the metals being soldered.

Another type of flux core solder that is commonly available is acid core solder which you do not want to use on electronics! This type of flux is for soldering heavy things like copper pipes. The acid can easily destroy thin metal such as circuit board traces.

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Tags: electronics electrical components arrl chapter 4 arrl module 10

What is the characteristic appearance of a cold solder joint?

• Dark black spots
• A bright or shiny surface
A grainy or dull surface
• A greenish tint

A "cold" solder joint results when not all metal surfaces are properly fluxed, brought up to the needed temperature, or if the solder is disturbed during the "plastic phase" as it cools and hardens

When this happens, the surface of the solder joint will appear dull or grainy instead of bright and shiny. Inspect every solder joint to ensure that the solder flowed smoothly over all conductors being joined.

A small crack in the solder around a large conductor is an indication that it was not heated sufficiently. Remember not to "play" with the solder, just heat up the whole connection until the solder flows freely filling all gaps, then remove the soldering iron and let it cool undisturbed.

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Tags: troubleshooting electronics arrl chapter 4 arrl module 10

What is probably happening when an ohmmeter, connected across an unpowered circuit, initially indicates a low resistance and then shows increasing resistance with time?

• The ohmmeter is defective
The circuit contains a large capacitor
• The circuit contains a large inductor
• The circuit is a relaxation oscillator

Capacitors oppose a change in voltage; when you start with an unpowered circuit, the voltage is 0. When you apply a current, to keep the voltage at 0 the capacitor starts charging up and passes the current freely (giving you a low resistance). As the capacitor charges there is more power stored inside it causing a voltage drop until eventually the capacitor is full and no longer allows any current through at all, until the voltage source is removed at which point the capacitor will try to discharge to keep the voltage at the new "equilibrium" level, that being the top level it finally balanced out at.

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Tags: resistance instruments arrl chapter 3 arrl module 6

Which of the following precautions should be taken when measuring circuit resistance with an ohmmeter?

• Ensure that the applied voltages are correct
Ensure that the circuit is not powered
• Ensure that the circuit is grounded
• Ensure that the circuit is operating at the correct frequency

An ohmmeter measures resistance, not voltage. Since resistance is basically a function of the voltage drop across a given portion of a circuit to the current applied across that same portion of the circuit (R=E/I), the ohmmeter applies a small current to the load and then measures the voltage drop across it. If the circuit is powered, not only will that affect the voltage readings that the ohmmeter will be taking but will likely be so much higher than the ohmmeter is using that it could (probably will) damage the circuitry in the ohmmeter.

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Tags: troubleshooting instruments resistance dc power arrl chapter 3 arrl module 4

Which of the following precautions should be taken when measuring high voltages with a voltmeter?

• Ensure that the voltmeter has very low impedance
Ensure that the voltmeter and leads are rated for use at the voltages to be measured
• Ensure that the circuit is grounded through the voltmeter
• Ensure that the voltmeter is set to the correct frequency

What they're getting at here, is YOUR SAFETY!

If you're measuring high voltage, you need to make sure the meter probes, leads and insides of the meter can take that high voltage without conducting that voltage to you or burning up. Both of those are undesirable features for a high voltage voltmeter.

Voltmeters have a very high impedance - indeed, the standard for voltmeters used to be 20,000 Ohms/Volt. You want to make sure the voltmeter doesn't affect the circuit that you're measuring, so it has to have a very high impedance. So the choice of low impedance is just wrong.

You always hear that things have to be grounded well. But you don't want to be grounding the high voltage through your meter! You might want to measure the difference in voltage between ground and some circuit, but the intention is not to ground the circuit. If you did that, the difference would be zero, and that measure would only obtain for a short while, the length of time it took to blow up your circuit or the meter.

Voltmeters might be measuring RF energy, but you usually want a voltmeter that will measure a wide range of RF frequencies, not a particular frequency. So setting it to the "correct frequency" is a bit of nonsense.

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Tags: arrl chapter 3 arrl module 4