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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