One of the most useful devices in the workbench of the repairman is the generator of RF signals. With this device, many things can be done more than simple tests and adjustments of radios. See in this article some of the many uses of the generator of sera/es, an instrument that can not be missing in your work table, if you intend to become a professional of repairs. A signal generator is a test and adjustment instrument that produces signals of high frequencies (RF or radiofrequency), and these signals may or may not be modulated in amplitude by a fixed audio signal.
The simplest models produce signals in a narrow band and have only a modulation frequency. The most refined models have different frequency bands and can be adjusted for different modulation percentages. In the repair of appliances, a model of shared characteristics and, therefore, of available cost can give good results, mainly if the technician knows how to take advantage of all its resources. Among the RF generators that are easily obtained, and give good results for their robustness, we have the B & K-Precision line. The most economical (but good) are the EZ00D and 2050 models. A typical model suitable for a repairer to start, and even for those who do not possess resources to acquire more complicated models, would have the following characteristics.
Four frequency bands that cover the following values:
Band 1 – 450 to 1,000 kHz;
Band 2 – 900 to 2,000 kHz;
Band 3 – 325 to 8MHz:
Band 4 – 7 to 16 MHz.
The instrument can also have an audio output that allows its use as an audio signal injector.
The signals are obtained in their output in two intensities (high and low). Also, in attenuator, it is possible to have exact control of its amplitude according to the sensitivity of the circuits tested.
See the reader that this device. In addition to having two critical frequencies that are 455 kHz and 10.7 MHz (the intermediate frequencies of AM and FM receivers), it also covers the full range of short waves most common in radio receivers.
What kind of jobs can be done with a device of this type?
Undoubtedly, the leading utility that is had with the generator of signals and that, therefore, it must be the most known by all technicians, is the adjustment of radios.
In the first case, the radio has accessible antenna and ground terminals, where the device is connected. In the second case, some turns of wire around the box allow the realization of inductive coupling, in the fact that there are no accessible antenna and ground terminals.
The first step is to recognize which are the coils and trimmers that must be adjusted in a receiver.
The coils of FI and the oscillating coil of a conventional radio are equal in appearance, barely identified by the colors of their cores.
The colors and the function are (AM radios):
Red – oscillator coil;
Yellow – 1st FI
Blanca – 2nd FI;
Negra – 3rd FI.
For FM receivers, identification can be made from the diagram.
The trimmers of the oscillator coil and the antenna are in the variable capacitor itself.
In the case of an AM / FM radio, we will have four trimmers in the variable capacitor, six coils of IF, and also the oscillators. In the case of the FM oscillator, it usually has no adjustment because it is wound in the air core.
Let’s see then how to use the signal generator to align a receiver.
Alignment of a receiver
- To adjust the FI coils, we must give preference to the inductive coupling of the signal generator.
- Set the signal generator for the intermediate frequency of the radio: 465 kHz or 455 kHz for medium and short wave radios (AM) and 10.7 MHz for FM radios. If the radio was AM / FM, adjust the AM part first.
- Tune the radio to a frequency where no stations are operating. The radio should be at a medium volume.
- Connect the signal generator and open its intensity control (attenuator) until you get a continuous whistle in the loudspeaker, as low as possible, but audible.
- The intensity control can be high or low as needed to pick up the signal.
- If you do not hear anything, it’s because 1st FI is very out of frequency.
- Slightly move the frequency selector of the signal generator (around 465kHz) until you hear the signal. If it is far from the expected frequency, see at the end of the text how to proceed.
- After doing all this, with the appropriate key, adjust the coils of the FI coils, starting with the 3rd to obtain the maximum intensity in the loudspeaker.
- Made the adjustment, reduce the intensity of the generator, and repeat all the adjustments.
- Do this as many times as you deem necessary to obtain the maximum sensitivity that is until you hear the signal on the loudspeaker with the lowest possible signal strength of the generator.
- In some types of receiver, when the best adjustment is obtained, a whistle (heterodyne) is heard at the change of stations. This problem can be avoided by slightly detuning only the second IF transformer, without apparent loss of sensitivity.
- In the second stage of the work, we move on to the alignment of the converting and tuning stage.
- We then adjust the signal generator to 600 kHz and tune the receiver to the same frequency setting. The signal strength fixed in the generator must be less than that used in the adjustment of the FI coils.
- Adjust the core of the oscillator coil (red) to obtain the maximum signal strength.
- Now tune the receiver around 1,500 kHz place the signal generator on the same frequency, with the signal of an intensity that does not saturate the receiver, that is, that does not produce the maximum volume, but is barely audible.
- Adjust the trimmer of the variable capacitor that corresponds to the oscillator coil, to obtain maximum signal strength (the trimmer of the variable capacitor that corresponds to the tuning) changes the frequency when we move it. In contrast, the trimmer of the antenna only adjusts the intensity of the signal).
- Then tune the generator around 1600 kHz and adjust the antenna trimmer for maximum sensitivity. Check if at low frequencies, around 550 kHz, good sensitivity is obtained. If this does not happen, run the antenna coil over the ferrite slowly and without touching it with your hands, until you get the best sensitivity.
- For FM receivers, the procedure is the same, except for the fact that we can only adjust the Falls. For the adjustment of the RF stages, the local stations can be used as a reference.
- We must also observe that the indicated procedure is one of the many existing ones and that they are valid in the adjustment of receivers. Readers who study manuals of various devices may find other methods that also produce satisfactory results.
Calibration of scales
- Experimental receivers can bring tuning frequency problems tuned to their editors. With the help of a signal generator, the elaboration of a scale is much more comfortable. In truth, without this instrument, only by knowing the frequencies of the stations that are heard can a reasonably good job of scale calibration be done.
- We then adjust the frequency until the signal can be heard in the speaker. The value that appears in the generator can then be annotated in the scale of the receiver itself.
- A standard procedure is to make the marking of points at regular intervals.
- For the medium wave band, the points normally used are: 550 – 600 – 650 – 700 800 – 1,000 – 1,200 – 1,400 – 1,600 kHz.
- If the reader wants, of course, he can do the dialing in the same way as the one adopted in the standard AM radios. The same is true for shortwave receivers.
- If the tuned band is verified on commercial radio and it is found that the lower stations are not captured and that there is a concentration at the other end, we can suspect that the core of the antenna coil is broken.
- The signal generator also serves to determine the sensitivity of receivers. The reader can easily compare the sensitivity of two receivers.
- Just place the receivers that you want to compare at a distance of 1 or 2 meters from the signal generator that will have the red output clamp connected to a piece of wire of about 30 em, used as an antenna.
- Tune the two receivers to the generator signal.
- Go reducing the intensity of the signal in the attenuator until one of the receivers does not “respond” anymore; this will undoubtedly be the least sensitive.