How to convert a tube receiver to fm. It is used when the original VHF unit is made on variometers
1. DETERMINE HOW WE WILL REBUILD THE RECEIVER.So, with reasonable care, we open the device. We look at what the frequency tuning knob is connected to. It can be a variometer (a metal thing, several centimeters long, usually there are two of them or one double, with longitudinal holes into which a pair of cores are pushed in or out.) This option was often used before. Until I write about it. () And it may be a plastic cube a few centimeters in size (2 ... 3). It contains several capacitors that change their capacitance at our whim. (There is also a varicap tuning method. At the same time, the tuning control is very similar to the volume control. I have not seen such an option).
2. LET'S FIND THE HETERODYNE COIL AND THE CAPACITORS CONNECTED TO IT.
So, you have KPE! We act further. We are looking for copper coils around it (yellow, brown spirals of several turns. Usually they are not even, but crumpled and tumbled awry. And that's right, they are tuned that way.). We can see one, two, three or more coils. Don't be scared. Everything is very simple. We turn on your device in disassembled form (do not forget to connect the antenna more authentically) and tune it to any radio station (better not to the loudest one). After that, we touch it with a metal screwdriver or just with a finger (contact is optional, just swipe something near the coil. The reaction of the receiver will be different. The signal may become louder or interference may appear, but the coil we are looking for will give the strongest effect. It will immediately slip in front of us several stations and the reception will be completely disrupted. So this is what a HETERODYNE coil is. The frequency of the local oscillator is determined by a circuit consisting of this very coil and capacitors connected in parallel to it. There are several of them - one of them is located in the KPI and controls the frequency tuning (we use it to catch different stations), the second is also in the KPI cube, or rather on its surface. Two or four small screws on the back of the KPI (usually facing us) are two or four trimmer capacitors. One of them is used to adjust the local oscillator. Usually these capacitors consist of two plates that run into each other when the screw is rotated. the bottom plate is exactly above the bottom one, then capacity is maximum. Feel these screws with a screwdriver. Move them back and forth a few (as little as possible) degrees. You can mark their initial position with a marker to insure against trouble. Which one affects the setting? Found? We will need it in the near future.
3. ONCE AGAIN, LET'S DECIDE WHERE WE ARE RESTRUCTURING AND ACT.
What range is in your receiver and what is needed. Do we lower the frequency or increase it? To lower the frequency, it is enough to add 1 ... 2 turns to the heterodyne coil. As a rule, it contains 5 ... 10 turns. Take a piece of bare tinned wire (for example, a lead from some long-legged element) and put a small prosthesis. After such a build-up, the coil must be adjusted. We turn on the receiver and catch some station. No stations? Nonsense, let's take a longer antenna and turn the tuning. Here's something caught. What is it. You'll have to wait until they say or take another receiver and catch the same thing. See how this station is located. At the right end of the range. Need to move even lower? Easily. Let's move the turns of the coil tighter. Let's pick up that station again. Now it is OK? It only catches badly (the antenna needs a long one). Correctly. Now let's find the antenna coil. She's around somewhere. Wires from KPE must be suitable for it. Let's try turning on the receiver to insert it into it or simply bring some kind of ferrite core to it (you can take the DM choke by removing the winding from it). Has the reception volume increased? Exactly, it's her. To reduce the frequency, it is necessary to increase the coil by 2 ... 3 turns. A piece of hard copper wire will do. You can simply replace the old coils with new ones containing 20% more turns. The turns of these coils should not lie tightly. By changing the stretch of the coil and bending it, we change the inductance. The denser the coil is wound and the more turns it has, the higher its inductance and below will be the operating range. Keep in mind that the actual inductance of the loop is higher than the inductance of a single coil, as it adds up to the inductance of the conductors that make up the loop.
For the best reception of the radio signal, it is necessary that the difference in the resonant frequencies of the heterodyne and antenna circuits be 10.7 MHz - this is the frequency of the intermediate frequency filter. This is called the correct pairing of the input and heterodyne circuits. How to provide it? Read on.
ADJUSTMENT (COUPLE) OF THE INPUT AND HETERODYNE CIRCUIT.
FIG.1. The high-frequency part of the VHF-FM radio receiver board. It can be clearly seen that the input circuit trimmer capacitor (CA-P) is set to the minimum capacitance position (unlike the heterodyne trimmer capacitor CG-P). The accuracy of setting the rotors of trimmer capacitors is 10 degrees.
The local oscillator (LG) coil has a large hole in the winding, which reduces its inductance. This gap appeared during the setup process.
Another coil is visible at the top of the photo. This is the input antenna circuit. It is broadband and does not rebuild. The telescopic antenna is connected precisely to this circuit (through a transition capacitor). The purpose of this circuit is to remove gross interference at frequencies much lower than the operating ones.
AND ONE MORE ACTION SINCE WE ARE ALREADY HERE.
Tune in to your favorite station, then shorten the antenna as low as possible when the noise starts to appear and adjust the IF filter, which looks like a metal square with a purple circle (in the middle left of the photo). Fine tuning of this circuit is very important for clear and loud reception. The slot setting accuracy is 10 degrees.
1. The classic way to rebuild the VHF unit:
In this case, the elements of the contours are recalculated for operation at new frequencies.
The next step is to set up the block - laying the range and setting the sensitivity
no worse than it was in the factory version.
This tuning option is used when the VHF unit is tuned by KPI or varicaps.
2. Implantation of the FM 88-108 MHz block.
It is used when the original VHF unit is made on variometers.
Rebuild the variometers to work at a new frequency and retain the sensitivity at the same time,
and laying the range 88-108 MHz is almost impossible. (The cost of such work will be astronomical!)
This is because VHF is 8 MHz long and FM is 20 MHz long.
Adjustment with a converter
not used due to different lengths of the length of the ranges (in this case, only a piece of the 8 MHz range is transferred) and the inability to provide acceptable sensitivity.
In addition to all this, a dead point appears on the range. In addition, the range is clogged with interference.
Of course, it is possible to make a converter free from these shortcomings,
but we are again faced with the high cost of such work.
Separately, it is necessary to mention the installation FM 88-108 MHz in devices that do not have a VHF range at all.
These receivers receive in the MW and LW bands. In this case, everything is removed from the device - only the body and adjustments remain. (volume, tuning knob, presets if available.)
In fact, a new receiver is installed in the case. All control is done by original regulators.
The purpose of the experiment is to try to drag the standard VHF-IP-2 to the FM band. There are several articles on the Internet on the alteration, but the most detailed and best on this issue (in my opinion) is the article by E. Solodovnikov.
You can read the article at this address: http://www.radiolamp.ru/shem1/pages/119/1.djvu. However, with this alteration, it is not possible to completely cover the FM range, since with "native" cylinders in the variometer, the overlap coefficient remains 10-12 MHz. You can increase the overlap coefficient either by rewinding the "native" circuits, or by increasing the size of the cores. Without philosophizing slyly, I went to the turner and ordered new "gadgets". I gave my uncle my native stock (I don’t have a probe - thread gauge) and a drawing of the outer dimensions of the cores. For my reasons, they should have been like this: As it turned out a little later, the internal thread should be M6 x 0.5.
As a result of turning work, these cylinders were obtained (thanks to the turner).
When trying to remove the old nuts, something irreparable happened ..... 
At first I was upset .... but after thinking, I came up with my own version of the stock:
The structure looks like this:
True, because of the head of the screw, I had to slightly drill out the cap of the variometer (the seat of the ball).
And here is the finished stock:
With new nuts, the local oscillator covered 10 MHz, which, in doubling (IP-2 operates on the second harmonic of the local oscillator), managed to cover the entire FM range. Everything would be good fun and great ... BUT !!! signal conversion still takes place at the 2nd harmonic .... and this sharply reduces the parameters of the block. In order to "squeeze all the juice" out of this design, I made an attempt to convert IP-2 into IP. As a result of searching for compromises and facilitating the configuration of the entire structure, the following circuit solution was born: 
Let me explain the color marking of the scheme:
in blue standard elements and their new denomination are indicated.
in red
additional elements are indicated, which are installed by surface mounting.
The red crosses are the conductors that need to be broken (in fact, only one track needs to be cut from the anode to the UHF circuit) and the hinged "track" made a piece of the mounting wire. The cross at the input circuit is a jumper on the board that must be removed.
Let me explain the changes in the circuit a little: the resistor in the input circuit is used to reduce the quality factor of the circuit and expand the bandwidth (initially, the input circuit is designed for an 8 MHz band).
In the output circuit of the UHF, the tap of the anode of the lamp is shorted to reduce the inductance of the circuit (with the tap, it was not possible to raise the local oscillator frequency above 105 MHz). Well, actually the cut track of the anode .... in the standard version, the circuit remained "indifferent" according to direct current. The lamp operation mode has also changed: The value of the UHF cathode resistor has been increased, thanks to which it was possible to increase the gain. The mixer grid resistor has also been increased to increase the amplitude of the local oscillator signal.
After changing the denominations and adding new parts, you should get something like this:
After the rod broke, the brass nuts brazenly dangled on the new rod, I had to order new, external dimensions as in the drawing, only with an internal diameter of 5.5 mm.
So, let's start setting up:
We connect the block to the IF, cover it with a casing (if someone uses a digital scale, it can be connected to the connection point of the communication coil and the mixer grid resistor, through a capacitor of 2 - 5 pF).
We turn on and "warm up" the block.
We install nuts approximately in the middle of our seats.
Setting up IF output circuit(on my board it is white), until a characteristic hiss appears in the speakers. If the hiss is too strong, then the block has begun to be excited, this is eliminated by moving one of the cores to the side until this excitation disappears. If the excitation cannot be eliminated by the cores, it is possible to cut the grid tracks of both triodes and solder into the gap along an "anti-excitation" resistor with a nominal value of 50-70 ohms.
Next, we tune in to any powerful radio station (turn the tuning knob), even if the reception is at the noise level. After that, we move the UHF core (which is further from the tuning knob) along the rod to the maximum signal volume. Now we set up IF driving circuit block (on my board it is green) maximum quality signal.
Well, now it's time to make the final adjustment of the block, we are trying to fit the tuning range:
If there is a frequency meter or a digital scale, then we unscrew the variometer until it stops and set the lower frequency of the local oscillator range with the local oscillator core.
If there is no frequency meter, then unscrew the variometer to the stop and move the local oscillator core (which is closer to the tuning knob) in the direction of the variometer knob, so as to tune in to the radio station with the minimum frequency that broadcasts in your area. After receiving, you will have to repeat the adjustment of the first core and the leading IF circuit for the maximum reception quality. The upper edge of the restructuring will climb into the range automatically, with a small margin. With this packing and with new brass nuts, the tuning range was about 25 MHz, which is quite enough.
Although the unit is VERY modest in terms of parameters, but with a fairly precise adjustment, it allows you to receive stations in a fairly good quality.
Successful Experiments!!!
(UA3IRG) Artyom.
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 removal back cover for the purpose of prevention, the receiver pleasantly struck 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 surround wooden case, which gives 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 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 manufactured 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).
AT 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 is required to connect an additional capacitor, provided that appearance the receiver will not be disturbed by the introduction of yet another front panel control (FP).
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 (turning on the 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 place printed circuit board block A2, where, in the area of switch 2S1.1 "IND", according to the diagram (Fig. 1), the conclusions of the switch and conductive tracks are cut and switched.
The setup is simple. First, set the frequency of the GPA. For this, it is convenient to apply 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 of 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, they turn on the VHF-2 (press the “IND” button) and selecting (soldering by surface 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
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. simple methods, there the design is slightly different and the circuit is 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, the 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 mixer output of the frequency converter. 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 copper wire about 1 mm in diameter. 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.
