The previous section dealt with faultfinding using nothing more than our own senses. Although a lot can be done this way, a point will be reached when this approach is not enough. This is where test equipment comes into its own. There exists a huge array of test equipment on the market and in order to keep things simple, I am restricting my list to the most common items of test equipment that we are likely to need for general faultfinding. I must stress that this list is my opinion. You may have other ideas.

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So what test equipment do we need?

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We really need to reduce the requirement to a list of priorities based on their usefulness. It would clearly be nonsense to invest in an expensive piece of test equipment if it were rarely if ever used. So, treading the tightrope and accepting that most peoples requirements will be different, I will now list my test equipment priorities. The list is divided into three groups, essential, highly desirable, and nice to have if you can afford it. The order of the equipment within those groups is purely arbitrary and is not intended to convey any order of priority.

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Essential.

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1. Test meter.

A test meter is your best friend in the battle against electronic faults, and believe me, it often is a battle. With the addition of simple components, their usefulness can be extended into the RF spectrum. They come in a huge variety of styles with costs ranging from cheap to very expensive. The first question we must answer is "what type do I need, analogue or digital?"

Analogue test meters, often referred to as "Multimeters" are probably typified by the 'AVO' range of professional test meters which have dominated the test meter market for many years. The 'Avo' however was designed for the professional market and cheaper variants are available. The main benefits of the analogue test meter, can be summarised as follows:

a. Relatively low cost.

Analogue meters can be bought cheaply. However, as with all things, read the specs, some cheap multimeters are completely useless for electronic testing.

b. Reasonably rugged.

Good analogue meters often have movements that can cope with a certain amount of shock. The most rugged of the analogue movements is the 'taut band' type which does not have conventional bearings and offers less friction with improved accuracy.

c. Reliable if looked after.

Mechanical meter movements do not take kindly to being dropped or abused.

d. A moving pointer indicates trends and changes very well.

When used for tuning and 'peaking', a pointer cannot be bettered. This is one of the big advantages of the analogue meter. Although some DVMs offer a bar graph display which simulates an analogue pointer, they rarely give the same 'feel' as a pointer.

Digital test meters, or DVMs (Digital Volt Meter) as they are often referred to, are nowadays the norm, having largely displaced the old analogue range of meters. The main benefits of the digital meter are:

a. Low cost

Due to modern integrated circuit technology, digital test meters have few internal parts and as a result are easy and cheap to produce.

b. High accuracy and resolution

Digital displays eliminate operator errors, such as parallax error and range reading errors. The most common range of affordable DVMs are those with 3½ or 4½ digits. This means that if the lowest voltage range is 200 mV, the 3½ digit meter will read to 199.9 mV, a resolution of 100µV, and the 4½ digit meter will read to 199.99 mV, a resolution of 10µV. Resolution is the ability to detect small changes. It should not be confused with accuracy.

It is by no means uncommon to see cheap DVMs with a basic voltage accuracy of around ±1%. For a 200mV range, this equates to a measurement error of 2mV. A reasonably good DVM will offer a basic accuracy of 0.1% or better. Hence, good resolution does not necessarily mean good accuracy.

c. Rugged and reliable

Due to the lack of a mechanical meter movement, DVMs are generally very rugged and will normally survive everyday knocks and bumps without a problem. As with all electronic equipment though, it should be treated with a little care. Don’t throw it around like a set of spanners. It may survive the shocks, but its calibration accuracy may well suffer.

d. Easy to read

A digital display presents the user with clear well defined digits, without the disadvantages such as parallax error of the analogue meter.

e. High Input Impedance

One of the prime benefits of the DVM is probably its very high input impedance, or, to put it another way, its lower tendency to load or influence the parameter being measured. Input impedances for most common DVMs is typically 10 MegOhms or more.

So which meter is best? Well there is no simple answer to that one, it really is a matter of personal preference. Buy the best you can afford. My own view, is that a digital meter generally has the upper hand, giving better measurement accuracy and usefulness per £. If a requirement for ‘real pointer' is encountered, then a simple analogue panel meter along with one or two resistors will often suffice.

A word of warning for the digital enthusiasts. Digital meters are very good but they do have some 'features' that you should be aware of. The first is a tendency to tell lies or latch up when in the presence of high levels of R.F. This may be conducted up the test leads or alternatively, radiated RF may be picked up by the DVMs internal circuitry through its plastic case. So when next working on that transmitter P.A. with the covers removed, spare a thought for your DVM. It's a sensitive little thing. The second 'feature' is due to the internal digital circuitry of the DVM. This results in residual conducted noise on the test leads which can be coupled into any circuitry the DVM may be connected to. This is not normally a problem, but low level circuitry such as mic pre-amps can respond to and amplify this noise. The last 'feature' is sensitivity to high voltage or static. Generally speaking most DVMs or quite rugged in this respect. However, should you inadvertantly connect your DVM to the EHT inside your T.V. then don't expect it to survive. Internal flashover can completely trash the internal electronics.

2. A good soldering iron.

There is no substitute for a good soldering iron, and by 'good' I mean one that is mains isolated via a low voltage transformer and is also static safe i.e. there is a discharge path to ground for the soldering iron tip to prevent component damage due to build up of static electricity. Additionally, a soldering iron which will accept a range of different bit sizes, shapes and temperatures is preferred. A 40 to 50 watt rating is a good general purpose rating. A sound investment here will pay off in the years to come.

3. Safety Goggles.

When working on high voltages, high capacity batteries or capacitors, it is essential, especially when applying power for the first time. Anyone who has witnessed large capacitors exploding, will need no convincing of this. It is also good practice to wear safety goggles when soldering. Your eyesight is too precious to take risks with.

4. Test Leads

A range of test leads to suit the test equipment you use is another essential part of your arsenal. Apart from the obvious leads that go with your test meter, other leads which may be useful are:

a. Mains extension leads.

b. Single test leads of varying lengths with insulated 'croc' clips or 4mm plugs at each end.

c. A 1 MegOhm resistor with short leads terminated in insulated 'croc' clips. Very useful for discharging high voltage capacitors safely.

5. Magnifier (x10)

The requirement for a magnifier increases in direct proportion to your advancing years.

6. Worklight.

You can't fix it if you can't see it. An 'anglepoise' lamp or LED / low heat fluorescent are very useful aids for detail work.

7. A range of components.

A range of resistors and capacitors for component substitution.

8. A range of small hand tools

A certain minimum number of tools will be required such as cross head and flat bladed screwdrivers, cutters, pliers, tweezers, small spanners, nut spinners etc. Various tools are easily obtained at reasonable prices from many different sources. Hamfest is one. The exact choice of tools will be influenced by the type of work to be undertaken and the financial outlay. Again, the best advice has to be, "Buy the best you can afford that meet your requirements."

9. Dummy Load.

If you are carrying out tests on a transmitter of any sort, or even if you are not, you should already have one of these. Quite apart from not carrying out tests on air, any tuning and modulation adjustments, should be made into a dummy load. But then you knew that already didn't you?

10. ESR meter.

A relatively simple piece of test gear, but one that can save a lot of time when searching for 'dud' or faulty electrolytic capacitors. They are very simple to build and there are many designs and circuits to be found on the internet. They can also be purchased for reasonable sums.

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Highly Desirable.

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1. D.C. Power Supply.

A god send when testing battery powered equipment. Very useful when building and testing homebrew equipment and QRP projects.

A good basic specification for a DC power supply is one that can deliver a regulated output between 0 and 30 volts at a current of 1 to 2 amps. If it has a presettable current limit control then so much the better. A current limit can save much heartache and damage by limiting the power under fault conditions.

Two golden rules to remember when connecting any electronic equipment to a variable DC power supply:

i. Check the polarity is correct when connecting up..

ii. Set the correct output voltage before you switch on.

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2. Oscilloscope

No matter how good your other test equipment may be, there are times when there is no substitute for an oscilloscope. It is an important instrument in the way that it provides a window into the electrons world and allows us to view events that are normally invisible, even with a good test meter. A good general purpose oscilloscope is one with a minimum bandwidth of 20 MHz, 2 Y channels and a minimum Y sensitivity of 10 mV per div. This will enable a vast array of different problems to be diagnosed and can be picked up fairly cheaply if you shop around on the second hand market. Beware of old valve based oscilloscopes. They have a tendency to drift as they age, and are generally very heavy. Although a good valved oscilloscope can be found, they do not take kindly to being left for months in a damp shack or shed. They need to be kept dry and used regularly. For most of us, there is nothing to be gained in investing in a high speed oscilloscope with such niceties as delayed timebase and digital storage. These enhanced features do not come cheaply and cannot generally be justified unless you are spending significant time troubleshooting RF or high speed circuitry. Note: In recent years with the advent of solid state (smart) oscilloscopes, prices have been steadily coming down and the secondhand market is now almost awash with some quite well specd' equipment at very affordable prices.

Don't forget, you will also need a couple of good scope probes. Test leads terminated in 'croc' clips are useless other than for d.c. and low frequency audio applications. Good probes have low, well defined input capacitance and can be adjusted to compensate for the input capacitance. X10 probes will allow a larger signal to be input and also have the additional benefits of higher impedance and lower capacitance. Unfortunately, good probes are not cheap, so shop around.

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3. Frequency counter.

A good frequency counter can be very useful, especially for homebrew transmitters and VFOs. Luckily, there are plenty of cheap counters on the market. There are also several suppliers of portable battery powered frequency counters which are good value for money.

A word of warning: The vast majority of frequency counters have sensitive low level inputs. They do not take kindly to having 50 watts of RF forced down them. If in doubt, use an attenuator, coupling loop or signal sampler.

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4. Audio signal generator.

It is much more preferable when carrying out checks on microphone pre-amps, compressors and modulators, to be able to input a well defined sinusoidal waveform of known amplitude and frequency. It enables checks to be carried out for sensitivity, frequency response, clipping and distortion. So much better than blowing or whistling into a microphone! Again, a plentiful source of these are often found at Hamfests and car boot sales.

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5. RF signal generator.

Very useful for troubleshooting receivers. The essential requirements are a stable signal of known frequency. A calibrated output level is also very useful. Unfortunately, good signal generators usually command a good price and most of those seen on the secondhand market tend to be old valved types. These are often perfectly adequate, but check the stability if you can before you part with your cash. In recent years, more modern transistorised signal generators and also Direct Digital Synthesis based generators  have appeared on the market with digital readouts. These are often good buys if you can find one to suit your pocket.

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6. Component tester.

Not so long ago, the cost of component testers / analysers was prohibitive. However, in recent years some very good component testers have become available from the Far East at very reasonable prices. They typically have a LCD display and a multi posion test socket where the component under test can be connected without worrying about polarity. These testers will typically identify the type of component, the relative major parameter/s and the relevant terminals. They will cope with Resistors, Capacitors, Inductors, Diodes, and Transistors including MosFets, junction Fet. Well worth the investment.

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Nice To Have If You Can Afford It.

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1 Spectrum analyser

A marvellous tool for anyone involved in a lot of RF work or RF filters. Traditionally these have been very expensive pieces of equipment, but gradually more are being seen on the second hand market. If you can afford it buy it and use it.

A word of warning: As with frequency counters, most spectrum analysers are designed for low level signals at their inputs. Do not connect the output of your transmitter directly to the spectrum analyser input. It's a sure way to damage the unit beyond economical repair.

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2. Antenna analyser.

Another very useful piece of equipment if you are regularly tinkering with antennas and trying to get them to work. It is possible to build your own impedance/SWR bridge if you are keen.

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3. Modulation/Deviation meter.

For VHF and UHF comms work, an indispensable aid. Setting transmitter deviation without one of these is not an easy task. They do tend to be expensive and are only really justified if regular use is to be made of them.

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Well the list could probably go on, but the point I am making is, only collect the test equipment that you will really need and use. The more exotic items can often be borrowed. Test equipment is no substitute for knowledge. Click the NEXT button  for a list of a few common component types and failure modes.