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.
