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close this bookRadio and Electronics (DED Philippinen, 66 p.)
close this folder23. RADIO SERVICING
View the document23.1. IMPORTANCE AND SUBJECT OF FAULT FINDING
View the document23.2. FAULTS AND FAULT FINDING
View the document23.3. FAULT FINDING METHODS

23.1. IMPORTANCE AND SUBJECT OF FAULT FINDING

Repair of a radio is an economical activity (if done professionally). Therefore it is an activity during which somebody who is undertaking it has to keep in mind economical questions in order to be able to earn his living.

Therefore the question - which method of fault finding should be chosen in a special case - will be answered normally under its economical aspect.

During the overall activity of repairing a radio the section FAULT FINDING has an overwhelming importance. If a fault has been located it has lost its “horror”, because its remedy requires only some rather simple skills and some knowledge about components and materials.

If repair is done as a business, fault finding is a main factor of calculation of costs. The time consumed for it is a very big portion of the overall time spend for the repair. A quick location of faults is therefore reducing the costs and gives so the professionalist a big advantage in competition.

To be able to choose the fitting method of fault finding under special circumstances, it is necessary to have a general viey of all possible methods to track down faults. An important role play the measuring instruments.

Instruments, finding devices, signalgenerators and so on, are items which are very expensive and therefore must enter the calculation of costs as well.

Even if a beginner does not have all instruments available on the market he has a big advantage if he has a profound knowledge of all possible methods because he is only then able to apply the best method which will cause the smallest costs.

23.2. FAULTS AND FAULT FINDING

Repair of devices working on high frequencies is one of the most difficult problems which modern technology can force us to solve.

Almost every radioreceiver consists of a big number of components which seem on the first glance all OK.

Each of them could be the cause of the fault. They do not necessarily look different if they are in a condition of proper working or if they are really faulty.

With our normal senses we can recognize only very minor differences if at all. Burnt resistors, open circuits, spilt condensers, burnt coils and so on, are rather rarely to be found.

In most cases we can only observe effects of faults of which the fact that the loudspeaker is totally dead could be simplest one. Since during fault finding we cannot achieve any development by using our senses, measuring instruments must be applied in order to display the electrical condition in and around the components.

Only this enables us to draw conclusions. The stringing up of such conclusions is defining the method which has to observe all conditions which are important for the operation of the whole device.

THEORETICAL KNOWLEDGE IS ESSENTIAL

If fault finding is not to be done at random, theory is a must. Nobody is able to repair a device reasonably if he does not understand the function of each component and each block of components.

At this point we find the big difference between all other kind of electrical equipment and especially high-frequency equipment. Somebody can be a rather good mechanical engineer (or “fundi”) and for example repair a typewriter very well without knowing the laws governing leverage. Without knowing the laws and effects of electricity, of dc-currents, ac-currents, high- and low-frequencies and so on, nobody will be able to repair an electrical device especially not a radio.

If somebody is starting to repair such a device his first thought has to be to follow the idea which has been put into action with that device.

Looking to the big number and the different kinds of components. It is mostly possible to guess at least which kind of receiver is brought to you.

It would be possible to trace all the circuits contained in it, but this would be very tiresome and a very long procedure. It is obviously much better to collect a good number of circuit diagrams and to refer to the fitting one immediatedly.

But even if the fitting circuitdiagram is available it makes no sense if this is only a collection of symbols and values for the repairing person. He must be able to draw conclusions about the function of each component and he must be able to predict the function of each block within the whole arrangement.

Theoretical reflections must accompany the repair from start to the end.

OBSERVATIONS OF THE RADIO OWNER

Not at all each fault in a radio can be observed if you only operate it only for some minutes. And not at all the radio must be totally dead if it is faulty.

Moreover there are a lot of possible defects which can cause a radio not to operate properly but to operate anyhow. Especially in cases of these defects “in between” (totally dead or normal function) the owner and user of the radio can give us some helpful hints for the repair if we ask him in a clever manner.

It is an additional skill of a radio repair professionalist to talk with his costumers in a way which will enable him to get a good clue for his work. But he has to keep in mind: the costumer is a layman whose conclusions are very often wrong. His sorrow that the bill could be too high may lead him to “bend” the truth.

It is the skill of a professionalist to reach as nears possible to the truth. For example to find out if a fault occured all of a sudden or if there were observed some small defects already before. It is also possible to find out if something has been burning at the fault. You should not neglect this simple method of investigation with the costumer. It can help you to save time with some preliminary steps in fault finding and it will help the costumer to develop a trustful relation to the repairing person, a condition will pay off very soon in getting more costumers.

Every radio technician should act somehow like a good medical doctor.

STATISTICS OF FAULTS

As more experienced a radio technician is, as more he tends to do some preliminary checks before he starts the actual systematical fault finding. That is because experience taught him that there are a certain number of faults which appear much more often than others. Moreover there are certain blocks of radios which show-more often faults than others. If one takes this different probability of faults in account he can draw conclusions where the fault might be located most probably. These experiences can of course influence the method of fault finding. For this reason there was undertaken for quite a number of years a research concerning all kinds of radio receivers. After introduction of integrated circuits there was done a second research of the “fault behaviour” of this new type of receivers.

The results of these two researches are displayed in fig. 217.


fig. 217

The overall number of receivers with integrated circuits having been faulty was remarkably less than the number of only transistorized receivers.

It is nor yet clear if this was due to the fact that integrated circuits equipped receivers have been fairly new in service while normal transistor receivers had already a rather long service time.

In both statistics it is obvious that most of the faults occure in the powersupply section of radios.

Mechanically originated faults like they are caused very often in the push-bot-ton-gear for changing bands or stations are also rather often found. While the faults in HF- and IF-blocks are found very seldom.

Interesting is too, that the number of faults in the AF-sections of radios which integrated circuits is remarkably lower than in radios equipped with normal transistorized circuits.

From these statistics you can draw the following CONCLUSION:

IF YOUR HAVE NO IDEA WHERE TO START WITH FAULT FINDING IT IS MOST PROMISING TO START TO CHECK FIRST THE POWER SUPPLY AND THEN THE AF-SECTION.

THE PRE-CHECK

If you have no clue at all where to start which fault finding you should undertake some simple checks first of all. Aim of this PRE-CHECK is to find out simple faults before you start the actually time consuming fault finding procedure:

1. Switch on the radio, turn the volume control to full. Now check if you hear some noise at the instant of switching. If not: CHECK THE POWER-SUPPLY!

2. While you do the first step (described above) it is adviceable to check by touching the power-transistors, or the power amplifier IC if it is getting hot. If this happens SWITCH OFF IMMEDIATEDLY - THERE MUST BE A SHORT CIRCUIT WITHIN THE POWER-AMPLIFIER.

3. Now you should have a close look to the circuit boards. You should look for any components which show destruction (burnt resistors, spilt capacitors, loose wires or interrupted conducting paths on the printed circuit).

4. Now touch the antenna socket with the antenna plug. You should hear some noise - if not - YOU CAN BE SURE, THAT THERE IS NO OR NOT ENOUGH AMPLIFICATION OF THE INPUT SIGNAL. To localize this fault,....

5. ... Switch the receiver to “sound channel” and touch the audio input with your finger.

You should hear some noise now.

if this is the case:

CONCENTRATE ON IF-STAGES AND MIXERSTAGES

if this is not the case:

CONCENTRATE ON THE AF-AMPLIFIERS.

After this PRE CHECK you can start with the systematical fault finding as it will described now.

23.3. FAULT FINDING METHODS

Repair is mainly ... “setting the device back to the condition of normal operation, because we presume the device brought to us for repair has worked before properly”. We can presume as well most of the preconditions - necessary for correct operation - are still given. Only a few - if not a single - components are defect. Such a defect causes:

- a lag of voltage at a number of terminals
- lag of current through some wires-
- change of resistance of the component.

Measurement of voltage, current or resistance can therefore serve as a reference to locate the actual fault. When you are searching for those effects of faults the various measured values are compared with the values expected (or given in the data sheet).

We call the methods:

VOLTAGEANALYSING
CURRENTANALYSING and
RESISTANCEANALYSING.

The voltageanalysing is the most often used one, because it can be carried out without any mechanical change of the device (voltage can be measured in parallel)

VOLTAGEANALYSING

Here we mostly start to measure the voltages at the power-supply and then the voltages at the supplying terminals of the blocks.

If we find a voltage missing or a strange value we have to consider what could be the reason for this effect.

WHICH MEASURING INSTRUMENTS SHOULD BE USED FOR VOLTAGEANALYSING?

In practice the measuring instrument necessary for fault finding plays an important role.

Very often it determines the measurements which can be undertaken, because it will determine the misreading in a certain case.

The most often used instrument in radio servicing was the moving coil instrument with an internal resistance of at least 50 kOhms.


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If anyhow possible, it should have a protection against overload and and it should be mechanically strong in order to be able it with you for outdoor repairs.

A disadvantage of these moving-coil instruments is the sometimes rather low internal resistance especially within the low voltage ranges (below 1 V).

For more complicated and sophisticated measurements there are available VALVE-VOLTMETERS or TRANSISTORIZED VOLTMETERS.

These are measuring instruments which are fed by a dc-amplifier of a very high input impedance and a output impedance matched to a moving coil instrument. Another type of instruments which is available since a few years are the so called DIGITAL-VOLTMETERS, which have dc-amplifier and an electronic circuit which shows the value of the voltage as a sequence of figures. The advantage of this type in comparison with other types is, they are easier to read small differences in values, but it is a problem to read it if the values are continuing to change.


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CURRENT ANALYSING

Real current measuring is undertaken only in very special cases, because it is necessary to open the circuit, and this means actually to dissolder a terminal of a component. This is time consuming and endangering the circuit, because you could break a connection.

Therefore direct current analysing is used only in a few special cases:

- if it is urgently necessary (because voltage measurement does not help anymore)

- or if it is extremly simple to undertake it (for example at a fuse, a contact of a switch or special plugged in links).

But you can very often measure the current INDIRECTLY by measuring the voltage across a resistor which is passed by this current. If you know what is the resistance of this resistor you can determine the current by Ohm's Law.

TOTAL CURRENT MEASUREMENT

By measuring the total current of a receiver it is also possible to draw some conclusions especially about the condition of the power-amplifier.


fig. 219

Fig. 219. shows how the current behaves in different cases.

But keep in mind to do this measurement at least in the beginning with the instrument set to the biggest current measuring range available (as smaller the measuring range as bigger the internal resistance and therefore the influence of the measuring instrument).

STATIC CURRENT MEASUREMENT

Another often used measuring method is to measure the current flowing in transistors of the power-amplifier while there is no sound produced.

Specially for this purpose we find very often links which can either be removed by hand or which can be easily dissoldered. If there is found a too high value it is very likely that one of the power-transistors is blown.

RESISTANCE ANALYSING

RESISTANCE ANALYSING HAS TO BE DONE WHILE THE DEVICE IS SWITCHED OFF ONLY (otherwise you can easily spoil the measuring instrument).

In case of valve radios it is possible to measure most of the resistances while everything is left like it was. In transistorradios, we must dissolder very often at least one terminal of the component whose resistor we are planning to measure.

SIGNALINJECTION AND SIGNALTRACING

All the fault finding methods described above are aiming at to find out if the normal operation conditions are given at a certain spot or not.

A very different approach is to undertake “spot-checks”.

This is done in order to find out:

- either up to which stage is the receiver still working properly.
- or from which stage on is the receiver still working.

The first method is done by injecting a signal at the input (aerial) and finding out from which stage to find out from which stage on the receiver is “dead” (signaltracing).

The second method is done by injecting a signal first very near to the output - for example at the input of the AF-stage - then a step backwards - for example at the input of the detector - and so on up to the aerial (signalinjecting). Of course this can only be carried out while the device is switched on and set to a reasonable volume.

It should be stressed that both of those methods will not allow to find out the faulty component but only the faulty block in the receiver. These methods are very fast. But if you want to apply them really professionally the equipment necessary is rather expensive.

THE FINGERTEST

ONLY ADVICEABLE IN TRANSISTORRADIOS!

The cheapest and easiest accessable “instrument” for signal injection is your finger. Your body is collecting electric and magnetic fields of your environment. These “signals” can be heard if you inject it to the AF-section of the radioreceiver. At a normal transistorradio we should hear a humming sound, if we touch for example the hot end of the volume control potentiometer. It is obvious, that this is very coarse and limitted method.

SIGNALINJECTORS

We know mainly three types of signalinjectors which can be used in radio servicing practice.

The cheapest possibility is a so called.

MULTIVIBRATOR.

This is an electric device which produces a flat-topped signal of any frequency. The trick - used here - is that easy flat-topped signal includes a wide frequency range of sinusoidal signals. So the multivibrator produces a “distortion” over the whole range of frequencies processed in a radio. Therefore we can inject its signal at every point of the receiver and it will have effect anyway. The problem with this kind of signalinjector is that we do not know which frequency is causing now the effect heard at the speaker.

If we want really to inject defined signals we have to use signal generators.

THE AUDIO FREQUENCY GENERATOR.

It has normally a frequency range of 10 Hz to 100 kHz. you can easily conclude, that you can use this generator only for the AF-stage and the demodulator.

THE RADIO FREQUENCY GENERATOR

If you want to inject defined signals to the IF-stages or to the aerial you need this type, which is normally able to produce frequencies between 100 kHz up to 300 MHz. It is equipped with a modulator section which can modulate the output-signal either with a fixed frequency (for example 1 kHz) or with an externyl produced audio signal.


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ANALYSING OF EFFECTS OF INJECTED SIGNALS

If we inject a signal we do it always by having a special effect in mind, which should be shown by the receivers output if the stage under research is in order. The term “shown” is already misleading, because without a measuring instrument we cannot see anything but only hear the effect.

Our ears are able to adapt to a very wide range of sound volume and therefore we ourselves are very poor “measuring instruments”.

In case of a fault for which we need a very exact reading of the output signal we need a reliable instrument for this signal.

We can either use a so-called DUMMY LOAD (a fitting reistor which represents the speaker for example) and a fitting moving coil instrument, or - much better - an OSCILLOSCOPE which is found more and more often in radio workshops nowadays. If we have access to a two channel oscilloscope we are extremly lucky because in this case we can measure input and output-signal at the same time, and we can compare it on the screen of the “scope”.

Then we can observe: - the frequencies - the shape - and the amplitudes of both signals. A situation which is “the dream” of a lot of radio technicians these days. For this reason here - at the end of this script - shall be given a short introduction to the use of an oscilloscope.


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