One of the nostalgic areas of SMR is retro themes. She has taken a worthy place in the development of our site. And now, no, no, and there are crafts-alterations of the times of my youth - for example, after and from the "Young Technician" appeared
The famous "Meridians" of the Kyiv Radio Plant produced in the 70s - early 80s ... One of the last - "Meridian - 210"- the model is definitely retro. After all, more than 30 years have passed since the beginning of its production. Brought from Ukraine, well-preserved externally and fully functional radio receiver of the 2nd class.
After removing the back cover for the purpose of prevention, the receiver pleasantly impressed with its well-thought-out layout of blocks, a large (presumably powerful) magnetic system of a one-watt speaker, framed by the walls of a voluminous wooden case, giving an unforgettable "Germanic retro sound", good assembly and disassembly manufacturability provided by the designers both for the factory line and in case of repair during operation.
True, they introduced their “know-how” at the factory, saved on the radio components of the voltage drop indicator unit - on the PSU board (A9) there are unsoldered places left for the missing necessary elements ... (and we scold the “yellow” assembly and are surprised that in the UPS there is a PC or receivers-"soap dishes" lack many elements in the places of printed circuit boards intended for them ... It's old, and the disease seems to be characteristic of the socialist economy ...).
As usual - cleaning from dust (surprisingly, it turned out to be negligible), replacing electrolytes made in 1979 with fresh and modern ones, cleaning contacts and lubricating the links of the “telescope” ... and, almost professionally, interest in the possibility of changing the VHF band to FM.
Let's agree on terminology. This has already been briefly explained in the referenced articles on the restructuring of the Oceans VHF units:
VHF range (or VHF-1), this is an old, still Soviet GOST¢ a, range for FM stations in the frequency range 65.8 ... 73 MHz. It was in the old receivers that it was used.
The VHF-2 and VHF-3 bands are allocated in accordance with the international Radio Regulations and occupy frequencies of 87.5 - 108 MHz. Now we have this site (wrong!) called FM band(use of the abbreviation FM from the words Frequency Modulation not quite correct, translated as "frequency modulation" - FM). So the abbreviation FM will be FM, and it would be more logical to call the band "FM band" ...
Thus, the designation FM means the possibility of receiving stations with frequency modulation in the VHF band. But “Western” FM has settled down…
In that FM-band VHF-2 occupies a section of 87.5 - 100.0, and VHF-3 - 100 - 108 MHz.
The full FM band (without division) is used for broadcasting in USA, as well as in Ukraine– starting from 88 MHz. In some countries, this range is divided into "their" sections: 87.5 - 104 MHz (Western Europe) and 70 - 90 MHz (Japan).
IN Russia in the same range up to 100 MHz are the 4th and 5th television channels, and in many (not all) cities broadcasting is carried out at frequencies only above 100 MHz.
Let's agree that in the text of the article, we will call the old VHF band "VHF"(meaning the corresponding frequencies), and the mentioned FM band - "FM", with "their" frequencies.
Block VHF radio "Meridian-210"» placed in an aluminum screen-box and does not fall under the generally accepted designation of unified blocks, such as VHF-2-03E. Although the radio components in his circuit are the same as in many other blocks. The main ones are: the K237XA5 microcircuit and three KVS111B varicap matrices. True, blocks with this microcircuit without varicaps (with CPI) or with other types of varicaps (not a matrix), or a matrix, but using transistors, not microcircuits, are found, but this combination is apparently typical only for Meridians.
The quality factor of the circuit with the included varicap matrix does not allow to fully capture the frequencies of the entire FM band (VHF-2 + VHF-3 = 87.5 - 108 MHz). But I would like to - in my city as many as three stations broadcast on VHF-2 (Retro-FM, Avtoradio and Russian Radio). Thus, it was decided to divide the F M-band into two standard ones, by introducing an additional FM-band (VHF-2) into the radio receiver.
To transfer the reception frequency from the VHF band to FM (VHF-3) 100 - 108 MHz), it is necessary to increase the frequency of the GPA circuit above 108 MHz by the IF frequency = 10.7 MHz. Taking into account tuning over the range, its frequencies will be 110.7 - 118.7 MHz.
To receive VHF-2 (87.5 - 100.0 MHz), for design reasons, it was decided to reduce the high frequency of the previously tuned GPA to its tuning frequency of 98.2 - 110.7 MHz (VHF-2). This is easy to do - increase the capacitance of the capacitors included in the GPA circuit.
Fig.1
A switch will be required to connect an additional capacitor, provided that the appearance of the receiver is not disturbed by the introduction of yet another front panel (FP) control.
The way out was the separation of the switching groups of the P2K 2S1.1 switch, which include the setting indicator (“IND” button). This is the lowest switch on the receiver's PC, next to it are the power buttons APCG And VHF, on the right are preset buttons and knobs. That is, on the PP, a functionally complete “sector of VHF settings” is logically obtained, which undoubtedly has certain advantages when tuning the receiver to an FM station.
The only thing we lose in this case is the ability to use the tuning indicator in one of the FM bands. But this is not so important - the tuning indicator circuit is quite gluttonous (made using incandescent lamps of the MN type), and on all other ranges (LW, SV, all HF, VHF-2) the indicator works normally.
The switching control element (activating an additional VHF-2) is an economical low-capacity reed relay of the RES-55A type with a trip current of 33 mA and a voltage of 12.6 V (passport 0602, winding resistance is about 377 Ohm), while the threshold voltage of operation is about 7, 0 V. It is optimal to use RES-49 (passport 0201, winding resistance of about 270 Ohms, the smallest sizes!) With a trip current of 22 mA and a voltage of 12 V (or other similar relays suitable for parameters and dimensions for 9-12 V, but there will be other, relatively more or less economical parameters for the current consumption of the receiver).
Now, how to change the frequency setting elements in the VHF radio unit "Meridian-210"? In the diagram (Fig. 1), the ratings of the capacitors that should be installed (there is only one new one) or replaced are highlighted in red. The connection of the relay is shown - it fits quite freely in the VHF unit (see photo).
The 4L3 local oscillator coil is reduced by 2-2.5 turns, the UHF 4L2 circuit coil is reduced by 1 turn. Given the broadband input circuit 4L1, its elements do not change, you just need to configure it correctly (more on that below).
Soldering “new” capacitors and unwinding the turns of the coils can be done without removing the block board from the screen, but by cutting off the old capacitor (or the upper output of the coil) and soldering the outputs of the new capacitor to its remaining legs (or the output of the remaining part of the rewound coil). This method is convenient, as it allows you to select the frequency-setting elements "in place" (the number of turns, the value of the capacitors). In addition, the location of the elements on the board of VHF structures has a very significant effect on the frequency-determining circuits ...
The following photo shows the location of the printed circuit board of block A2, where, according to the diagram (Fig. 1), in the area of \u200b\u200bthe 2S1.1 "IND" switch, the conclusions of the switch and the conductive tracks are cut and switched.
The setup is simple. First, set the frequency of the GPA. For this, it is convenient to use receiver with dssh(type "Degen"). On the VHF band, in the depressed position of the button 2S1.1 "IND", i.e. the additional range of VHF-2 is turned off, by rotating the core of the 4L3 coil, they find the station of the FM band (higher-lower on the scale) and set the limits of the range. In the experiment, the brass core of the GPA 4L3 coil was replaced with a ferrite one, perhaps, after all, the winding of 2.5 turns is a lot and it was possible not to change the core. Therefore, when choosing the number of turns during the tuning process, you should not immediately cut off the unwound part of the coil wire, but bending it to the side, solder the alternately unwound turns to the “rack” (to a piece of wire from the cut coil sticking out of the board ...).
At the same time, "Degen" allows you to determine the frequency at which the extreme (polar) stations of the range operate. The highest-frequency station is tuned by ear to the maximum signal by rotating the tuning capacitors of the UHF 4C3 circuit and the 4C1 input circuit.
Next, turn on the VHF-2 (press the “IND” button) and picking up (soldering by hanging mounting) a capacitor parallel to the GPA circuit (in the circuit in Fig. 1 it is 8.2 pF, displayed in red, it does not have a “C” designation) achieve that the stations of this range were within the scale of the receiver. The maximum signal of the lowest frequency station is set by rotating the cores of the coils 4L2 and 4L1.
The turns of the unwound coils and their cores, as well as the soldered capacitors of the circuits, are fixed by any known method (wax, paraffin, zapon varnish).
V. Kononenko
The question of "pulling" VHF tube units with inductive tuning, which were used in mass radio receivers manufactured in the 60-70s (such as VHF-I, VHF-IP, VHF-IP2, etc.), to the "upper" VHF range (87.5 ... 108 MHz) rises periodically on various amateur radio forums. I receive many similar letters. I want to say right away that I did not deal with the restructuring of the "regular" VHF units. There are a number of reasons for this, but the main one I would call the complexity of such an operation. These blocks were created according to the "residual" principle and, by and large, in those days they were put in receivers more "for show". Accordingly, during their development they saved on everything. With seemingly simple circuitry, they are very, very complex and capricious to set up. I have neither sufficient experience nor the necessary measuring equipment for such work.
In the photo: domestic VHF tube blocks with inductive tuning.
But I still wanted to try to implement a similar unit based on parts and assemblies from a "regular" VHF unit. The most valuable part in this block is the variometer. Yes, and the case is quite good, and the IF circuits can be used.
So, finally, I decided to build such a device. The conditions were as follows: use the above parts from the "regular" unit, frequency range - 87.5 ... 108 MHz, IF frequency - 6.5 MHz, anode voltage + 150 V.
I took as a basis the already proven circuit on 2 pentodes 6Zh1P:
It took about a week to develop a printed circuit board. There were quite severe restrictions on the placement of some parts, namely: a variometer, one of the lamps, IF circuits, mounting holes for the board and screen. Therefore, the mounting density turned out to be not very uniform, and this also imposes some restrictions on the types of parts used. I tried several options, and the result is this:
In the photo: the printed circuit board of the device. For comparison, in the second picture - the "native" VHF-IP board without tracks.
The "donor" was the VHF-IP block, in which the printed tracks were kept "on parole" and fell off at the slightest touch. As a result, I completely removed all the tracks from the board and used it as a template for adjusting the position of the mounting holes.
It took a few more evenings to make the PCB assembly.
In the photo: the board in the process of assembly and the design of the antenna coil.
Panels PLC-7E, dovetail type, shield flange removed. One of the IF circuits cracked from time to time, I had to put heat shrink on it and gently upset it. I did not install "native" contact petals, but simply slightly increased the removal of part of the board from under the screen. I wound the coil of the antenna circuit with a silver-plated wire with a diameter of 0.84 mm - the central core of the RK-50-xx cable? (I don't know the exact type). I wound a drill with a diameter of 7.8 mm on the shank, and "collected" the coil on it. Then he inserted a piece of the IF frame from a tube TV, the SCR-1 core.
The next "epopee" is the alteration of the variometer coil. The idea was this: try to start using "native" coils, and in case of failure, remove them and install your own. For this, the corresponding holes are provided on the board. In addition, we use capacitive coupling between the circuits, for which you need to have access to the turns of the coil in order to solder a capacitor to them. Therefore, using a soldering iron, tweezers and wire cutters, the polystyrene layer above the top of the coils was carefully removed. It turned out this:
In the photo: converted variometer coils.
I shortened the terminals for the communication coil from above and left them as an additional fastening of the variometer frame. The mechanism went over, cleaned of dirt and old grease and lubricated with thick graphite grease. The cores were cleaned of paint, which fixed their position on the polystyrene rod. The board finally assembled and fixed on the pallet of the case looks like this:
In the photo: a fully assembled VHF unit.
As an IF at 6.5 MHz, I used a device assembled a couple of years ago. I first checked its performance with the "regular" VHF-IP2 unit, and also made minor changes to the circuit. After that, he rebuilt it using a 6.5 MHz generator built into the Laspi-TT03 device.
Once again I checked the wiring of the VHF unit, washed the board with alcohol from the remnants of rosin, connected it to the UHF unit and made the first switch-on. I immediately checked the modes for direct current - everything is normal. I adjusted the IF contours a little and heard the hiss of the successor. After some "twisting" of the trimmers, I caught the "Echo of Moscow" and started laying the range. It took quite a long time to determine what affects what "and where to turn what" :)
In the photo: the receiver in operation and a view of the VHF unit with a modified screen.
As a result of this, I managed to achieve an overlap of approximately 16 MHz in the range: from Ekho Moskvy (91.5 MHz) to Russian Radio (107.8 MHz) at the extreme positions of the tuning unit. The reception is stable, the gain is about the same ("at the bottom", as usual, a little less than after 100 MHz). Tried to lower the beginning of the range below. Up to 88.0 MHz it turns out fine, and a little lower - the gain drops sharply. In this case, the upper limit, of course, is also "lowered". It was somewhere at this interesting moment that I had to interrupt my experiments. In principle, it has already turned out well: it was possible to achieve overlapping 2 times more than in the "regular" VHF unit. But I think the most interesting is yet to come :)
The sound quality is still unimportant. The reason, I think, is in the PC. Firstly, I most likely made a mistake somewhere with the calculation of capacitors for the output circuits of the IF (in the original, the block was at 8.4 MHz). And there is a suspicion that the first cascade of the UPCH is excited.
Yes, in the photo you can see a block with an already modified case cover. This will be another separate epic, since the aluminum cover will reduce the inductance of all coils and the range will "creep" up. So far, I have made all the settings without this cover.
A little theory: to transfer the modulated signal to another frequency, you only need an oscillator and an analog signal mixer. This conversion is based on the well-known effect of multiplying two radio frequencies F1 and F2. In the mixer, two side radio signals F1+F2 and F1-F2 occur. So this converter received both FM and VHF stations at the same time.
Once, on the contrary, imported receivers with the FM band were rebuilt on the VHF, and this procedure is a little simpler, it was enough to change the number of turns in two coils - the input and the heterodyne, that is, to transfer to VHF, add two turns or rewind with the number of turns by two without changing the inner diameter any more, and then adjust them by compressing or expanding the turns, while laying the range limits and the input circuit according to the best reception. But with our old radios, this cannot be done using simple methods, the design is slightly different there and the circuits are much more complicated, there you need to radically change the inductance and capacitance, both input and heterodyne. Yes, and the FM range is much wider than our VHF, and it is very difficult, and in some cases impossible, to fit it into our range. It is also necessary to select the capacitors of the "stretch marks, couplers" of the ranges.
So if you can’t rebuild the receiver to FM or you don’t have enough skills, then of course it’s better to use a converter. One of the most successful converters that I have met and repeatedly made is a converter on an imported microcircuit LA1185. Converter to K174PS1 an order of magnitude worse than this microcircuit, plus the LA1185 still has UHF, which gives some gain to the input signal, a few decibels, but noticeable.
If you use a 27 MHz resonator, then reception will be possible in the range from 91 to 100 MHz. To receive the rest of the range (100-108 MHz), you need to replace the resonator with 35 MHz, then reception is possible within part of the 99-108 MHz range. Thus, a resonator switch is needed to receive the entire range.
If you need to convert in the opposite direction, then to receive frequencies in the range of 64-73 MHz, one quartz is enough, for any frequency within 27-35 MHz. When using a 27 MHz resonator, reception will be from 61 to 81 MHz, and with a 35 MHz quartz, from 53 to 73 MHz.
The signal from the antenna goes to the input circuit L1-C2, which must be tuned to the middle of the received range. From this circuit, the signal is fed to the input of the URF microcircuit. The L2-C6 circuit is the same as L1-C2, but this is the output circuit, which is loaded with the URF. From it, through C5, the signal goes to the converter. The frequency of the local oscillator is set by the quartz resonator Q1. And the circuit L3-C7 at the output of the frequency converter mixer. From it, the signal is fed to the antenna input of the receiver. This contour must be set to the middle of the working part of the range into which the conversion takes place.
Coils are frameless, with an inner diameter of 4.5 mm. Wound with copper wire with a diameter of about 1 mm. According to the number of turns, there are two types of coils - 6 and 4 turns. And how they are placed according to the scheme depends on the direction of the transformation. The adjustment consists in tuning the circuits by changing the inductance of the coils by compressing - stretching their turns.
Other circuit diagrams of FM converters
The following converter circuit for 2 transistors. KT363 and KT315. The photo says that KT363 can be replaced with KT361. This circuit is connected with an output to the input of the receiver antenna, and the input is connected to the receiving antenna itself.
They asked to drag the Soviet VHF-IP-2A unit to the FM band .... It looks something like this:
After spending one day off, I was looking for a way to extend the rebuilding of this block to ALL FM range, without turning a more powerful brass core and without touching the contour. I will pay tribute to the Soviet Lamp Instrumentation, the local oscillator is made to last .... it was played both with the mode of the heterodyne mixer lamp, and with the ratings of its body kit ...... the frequency changed a little up and down, BUT !!! the tuning range remained uncompromising :)
I came up with the idea to make the hearts of the variometer double by gluing a ferrite choke dumbbell from a computer PSU to the standard brass core. Without hesitation, I dug through the bins of the Motherland and found a pair of chokes of a suitable size, they look like this:
When the glue dries (I glued it with super glue) we screw them in place, but while the glue dries, let's move on to the "mat part", i.e. to the electrical and wiring diagrams of this VHF unit.
Here is the circuit diagram:
The diagrams themselves were given only for clarity, the diagrams are in good quality, as well as the description of the VHF unit itself can be downloaded from the MRB (mass radio library) issue 0788 by clicking on this link: http://www.oldradioclub.ru/radio_book/mrb/ 0701-0800/mrb0788.djvu
So, let's start mocking the board (to give credit, not a single track flew off during the experiments), and re-solder the part values, according to this packing:
After the denominations of the parts are replaced and the glue on the cores dries, we collect all this disgrace in a heap.
Well, then the process of setting up the unit is ahead, for this they connect the power and the local oscillator core (there are two coils in the circuit, it is lower on the wiring diagram) lay the range, I got it with new cores from 46 to 60 MHz, which is doubling (the mixer mixes 2nd harmonic of the local oscillator) covers the entire FM range with a margin, and by moving the second core, they achieve the best sound quality.
I want to warn you right away, since the slots of the cores are sealed with dumbbells, you will have to rotate the cores with your fingers by the threaded plastic shank, and since the unit is regulated when it is on, it is possible to get an electric shock, be careful !!!
Perhaps someone will be interested or useful in the future, I will tell you how I calculated this scheme:
I simply reduced the values of capacitors C1, C2, C3 by 3 times in order to transfer the UHF band to the FM range (there is an article about this in RL 2000 and it can be found on the Internet). With the same Makar, I reduced the values \u200b\u200bof capacitors C6 and C7 .... but I had to tinker with capacitor C8, since this circuit of 3 capacitors balances the UHF Mixer bridge.
So, let's start the calculations: to find out the proportions of the bridge arms, I took the old "native" denominations and remembered the school physics course about connecting series capacitors: C1 * C2 \ C1 + C2.
We are interested in the ratio C6 + C7 to C7 + C8, so we consider 56 * 22 \ 56 + 22 \u003d 1232 \ 78 \u003d 15.7
second diagonal 22 * 3.9 \ 22 + 3.9 \u003d 85.8 \ 25.9 \u003d 3.3
and the shoulder ratio is 15.7 \ 3.3 \u003d 4.75
and since we have reduced the divisor C6 + C7 by 3 times, we will have to recalculate it as well.
18 * 7,5 \ 18 + 7,5 = 135 \ 25,5 = 5,29
well, knowing the ratio of the shoulders, we get the 2nd diagonal of the bridge:
5,29 * 4,75 = 25,12
and since the nearest capacitor is 24 peaks, I installed it.
Successful experiments!!!
Artem (UA3IRG)
About ten ... twelve years ago, articles on the restructuring of imported receivers with the FM band (88 ... 108 MHz) to the VHF-1 range (65.8 ... 75.0 MHz) were often published in amateur radio magazines. At that time, broadcasting was carried out exclusively in the VHF-1 band.
Now the situation has changed dramatically. The air in the range of 100 ... 108 MHz is filled almost everywhere. On sale there are many imported and domestic radio receivers with a VHF-2 range or with common ones (VHF-1 and VHF-2).
Since the VHF-1 range was actually "orphaned", a giant fleet of old radios and radio tape recorders remained "out of work". You can give them a second life by relatively simple modification of the VHF units of these receivers. In doing so, the following points should be noted. Modification of inexpensive portable receivers ("VEF", "Sport", "Sokol", "Ocean", etc.) should be minimal and provide reception of 3 ... 7 VHF-2 broadcasting stations in the region. For stationary devices of a higher class with an external VHF antenna, it is desirable to keep all its technical parameters (sensitivity, local oscillator stability, wide scale, etc.).
Usually, the VHF radio receiver unit contains an input circuit, 1-2 UHF cascades, a local oscillator, a mixer, and IF cascades. As a rule, these are 4 (less common 5) LC circuits. Having a basic (even better, mounting) diagram of a radio receiver, it is easy to determine all the necessary nodes (inductors, capacitances, etc.). The first circuit of the IF and all subsequent cascades do not need to be altered.
It is clear that for the range of 100 ... 108 MHz, the capacitances and inductances of all LC circuits of the VHF-1 unit must be reduced. Theory and practice state that the capacitance of the circuit changes in proportion to the wavelength, and the number of turns of the inductor - the square root of this value.
When moving from the VHF-1 range to the VHF-2 range and with constant inductances (the number of turns of the inductors does not change) - this is an option for portable receivers for medium frequency ranges (69.0 MHz and 104.0 MHz) - we obtain the following relationship for containers:
With UKV-2 \u003d 0.44 * With VHF-1.
With this in mind, in practice the following ratio of capacities is more suitable:
With UKV-2 \u003d (0.3 ... 0.35) * With VHF-1.
In addition, in VHF units, it is possible to change the inductance of the loop coils within certain limits by rotating the tuning cores. Usually, the local oscillator of the VHF-2 block for the range of 100 ... 108 MHz should be tuned within 110 ... 119 MHz (with a margin) at IF = 10.7 MHz, and within 106 ... 115 MHz at IF = 6, 5 MHz, i.e. above the signal frequency. On the circuit diagram of the VHF-1 block, we mark those capacities that will be completely soldered from the circuit, as well as those capacities that will be replaced by others with a lower rating. Usually these are miniature disk ceramic capacitors.
Capacitors must be selected in advance, cleaned and tinned, shortening them to a minimum. If there is no device for accurately measuring capacitance, the table below will partially help to solve the problem, where the size and color of the capacitor will suggest the limits of the nominal capacitance.
Table 1
For clarity, you can compare the capacitance ratings in the "VEF-221" and "VEF-222" radio receivers, which are built according to the same circuits with the same inductors ("VEF-221" has a range of 87.5 ... 108 MHz, " VEF-222" - 65.8...74.0 MHz). These data are taken from the factory operating manual (Table 2). Capacitance ratings are given in it in picofarads.
table 2
Similar schemes of VHF units are used by the VEF-215 radio receiver and the VEF RMD-287S radio receiver, so the data in Table 2 are also suitable for reworking the VHF units of these devices.
Another example is a removable auto-receiver of the Ural-auto-2 type (input circuit, two UHF stages on GT322A transistors, a local oscillator on a microcircuit of the 224th series with the ZHA1 or XA1 index). In the input circuit in the capacitive divider C1-C2, we change C1 \u003d 22 pF by 5.1 ... 6.8 pF, C2 \u003d 33 pF - by 10 ... 12 pF. Capacitors C5, C7 and C14 of 33 pF each (series capacitances with KPI of the 1st, 2nd stages of UHF and local oscillator) are changed to 12 ... 13 pF. In the local oscillator circuit, the tuning core made of ferrite (0 2.88 mm) is changed to brass with a thread (diameter 3 mm). Another example is the tuner "Radiotechnika T-101-stereo" (VHF unit on transistors KT368A and KT339A, restructuring - varicaps KVS111A). Parallel capacitances SZ = 15 pF (input circuit), C14 = 15 pF (UHF), C18 = 9.1 pF (local oscillator) are dismantled. Serial capacitances C4 = 130 pF, C13 = 130 pF (input circuit and UHF) are changed to 43 ... 47 pF, and C15 = 82 pF (local oscillator) - to 27 ... 33 pF. To stretch the scale, we carefully unsolder the loop coil of the local oscillator and unwind 1.5 turns from the top of the coil, 1 turn from the bottom (the tap from 0.9 ... 1.2 turns as it was). Then carefully solder the coil into place.
It is convenient to divide the process of alteration of blocks of VHF receivers into several stages.
- We provide access to the VHF unit both from the side of the parts and from the side of the printed conductors by removing the covers of the receiver and the VHF unit.
- We determine the LC circuits of the input circuit, UHF, local oscillator, mixer, and the first circuit of the IF (the last alteration does not apply).
- Carefully unsolder the containers to be replaced and dismantled.
- We solder new containers prepared in advance (with cut and tinned leads) for each individual circuit of the VHF unit.
- After making sure that there are no errors, and the circuit is not broken (there are no bad solderings, short circuits of printed tracks, etc.), we turn on the power of the receiver and try to hear at least one powerful (in this place) VHF station. At the same time, we rotate the receiver tuning knob and the local oscillator core. It is very useful to have an industrial receiver with a VHF-2 range nearby. This will help to immediately identify the desired station in the tuned receiver. Having heard at least barely the station, the tuning cores of the coils and the tuning capacitors of the input circuit, the UHF and the mixer achieve a loud reception of this station. At this stage, you can determine whether you need to change the cores from ferrite to brass and vice versa.
- By rotating the core of the local oscillator coil, we set the required place for this station on the receiver scale (focusing on an industrial receiver with a VHF-2 range). Usually, the section of the scale of the tuned receiver, where the stations of the range 100 ... 108 MHz are located, occupies a very small part of the constructive scale of the receiver (about one third).
- We carry out conjugation of the circuits of the input circuit, UHF and the local oscillator of the tuned VHF unit. In the area near 100 MHz, we achieve the highest volume of the stations by rotating the tuning cores of the input circuit, UHF and mixer, and in the area near 108 MHz - by rotating the rotors of the tuning capacitors of the same cascades (in this case, you need to monitor the position of the receiver tuning knobs - the maximum capacity of the KPI or varicaps at the beginning of the range and their minimum capacity at the end). We repeat this operation 2-3 times. In conclusion, it is necessary to reduce the capacitance in the AFC circuit by 2 ... 2.2 times (if its value exceeds 5 ... 6 pF). The last stage must be carried out in the assembled VHF unit through the holes in the covers to adjust the capacitances and inductances with a dielectric screwdriver.
These general rules for reworking VHF units should be followed for various schemes and designs of units. Briefly about receiving antennas. Obviously, directional antennas provide excellent reception quality, but they need to be rotated. The author uses a single square for the rebuilt tuner "T-101-stereo" (in parallel, two copper wires with a diameter of 1.8 mm with a distance between them = 15 mm and with a perimeter of slightly less than 3 m). The wave impedance of the square is about 110 ohms, so it is powered by a PRPPM cable - 2 x 1.2 (wave impedance is about 135 ohms). The height of the mast on the five-story building is approximately 9 m. The plane of the square is perpendicular to the line Chisinau - Bendery - Tiraspol - Odessa. As a result, more than 10 stations from Chisinau and 3-4 powerful stations from Odessa are heard.
Sources
- A brief guide to the REA designer (edited by R.G. Varlamov). -M.: Sov. Radio, 1972, pp. 275,286.
- V.T. Polyakov "Direct Conversion Transceivers". - M.: 1984, p.99.
- P.M. Tereshchuk and others. Handbook of a radio amateur, part 1. Kyiv: Technique, 1971, S.Z0.
- "VEF-221", "VEF-222". Manual.
- Radiotechnika (T-101-stereo tuner). Manual.
- A.N. Maltese, A.G. Podolsky. Broadcast reception in a car.- M.: Radio and communication, 1982, p.72.
- V. Kolesnikov "Antenna for FM reception". - Radiomir, 2001, N11, p.9.