|Radio and Electronics (DED Philippinen, 66 p.)|
|23. RADIO SERVICING|
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.
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.
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.