The modulator, which generates a video + audio signal of about 70 dBmV in the VHF range (45 - 85 MHz) can be directly connected to the antenna input of any TV without SCART (Scart).
It will connect to an old television, still fit for service, a video camera, VCR or any device with a video + audio output.
Figure 1: Photograph of the component side of the TV modulator. The circuit board is installed in the cabinet. 
Figure 2: The ground strip disposed on the perimeter of the circuit board is soldered by a few points on the sides of the metal box. Almost all modern TVs are equipped with a scart (scart). However, you may have an old one but still works perfectly without making that famous.
Without this, you can not connect a VCR or a small CCD video camera except as to have a device with its own module.
If this is not the case, there is still a solution: make a small TV modulator that generates a VHF (Very High Frequency - Very High Frequency, the abbreviation THF is still sometimes used). That is what we propose in this article.
In practice this module behaves like a small transmitter.
Thus, if we allow the TV to its broadcast frequency, we can see the pictures from a VCR or video camera for example. The modulator
we propose to undertake, was designed to operate in the VHF range extending from 45 to 85 MHz. This choice was dictated not only because the oscillator stage operates more stable on these frequencies but also because, on this portion of the band, there is almost no TV transmitters. Thus, our signal will almost never run the risk of being disturbed by external signals.
To achieve this arrangement, we tried various video modulators, and among those we tried, the one who gave us the greatest satisfaction is the LM1889 manufactured by National Semiconductor.
As you can see the drawing of Figure 3, within the integrated circuit we find different floors. We use or the summing stage, or stage of the chroma, because of all the video and all cameras, sor t-composite video signal.
the two oscillators in the circuit (see CSOs. A and CSOs. B) we used the oscillator stage A only.
If the integrated circuit has two floors oscillator is that, normally, one is designed to operate on lower frequencies (45 to 60 MHz) and the other to operate on higher frequencies (60 to 85 MHz). With the adjustable capacitor
we inserted in parallel with the tuning coil, we can set the circuit a minimum of 45 MHz to a maximum of 85 MHz.
Thus, we have no use for the other oscillator between pins 6 and 7.
A third floor, CSOs. AUDIO (audio oscillator, see pin 15), we used to obtain a frequency of 5.5 MHz modules we FM (frequency modulation) using a varactor. The video and audio signals are sampled on both output pins 10 and 11.
Figure 3: Block diagram of the stages contained within the LM1889. In our modulator we do not use the summing stage, neither the chroma oscillator stage, because the recorders and video cameras provide a composite video signal. 
Figure 4: The video and audio available on the outputs of many CCD cameras or more will be applied to the ports for the input of the modulator. The signal available at output OUT RF modulator will be directly applied to the antenna connector on the TV using a coaxial cable of 75 ohms or, failing that, of 52 ohms. Electrical diagram
After describing the block diagram of the LM1889 integrated circuit, we now proceed to complete wiring diagram reproduced in Figure 5.
The video signal applied to the socket on the bottom left of the diagram and marked "INP. VIDEO "joins the input pin 13 of the LM1889 through capacitor C4.
For a VHF signal can cover the range from 45 to 85 MHz, we connected between pins 8 and 9 of the oscillator stage, a small inductor microhenry of 0.27 (see JAF1) and, in parallel with the latter, we put a capacitor of 3.3 pF capacitor and an adjustable 5 / 50 pF (see C9) to be able to vary the frequency.
The signal to be applied to the TV input is taken from pins 10 and 11 that we supply with a voltage of 12 volts through the resistor R19 of 75 ohms.
The low pass filter composed of two capacitors C16 and C17 and inductor JAF2 is located in series with the output line RF OUT (VHF output).
His goal is mitigation of all harmonic frequencies above 120 MHz. Indeed, if these entered into the TV, this could cause interference.
To calculate the cutoff frequency of this filter, we use the following formula:
With the values of scheme we get.
The audio signal is applied to the socket on the top left of the diagram and marked INP. AUDIO joined through capacitor C1, the base of TR1. This transistor, first, a role amplifier but by 4 also provides pre-emphasis signal to meet the high frequencies.
The AF signal (Low Frequency) amplified and pre-emphasized, present at the collector of transistor TR1, is applied through C3, the varactor DV1, which performs frequency modulation (FM) signal generated by the oscillator stage Audio located at pin 15.
The frequency generated by the audio oscillator must be 5.5 MHz, to obtain it, we used a medium frequency transformer of 10.7 MHz with a core of green. Then, we lowered its operating frequency the required value by applying the parallel capacitor C6 150 pF. By turning the core of this coil, we can adjust its frequency to 5.5 MHz exactly. The carrier
audio modulated FM must then be mixed with the video carrier. To do this, we collect in rake to pin 15 of IC1, through resistor R12 of 15 kilohms and capacitor C7 22 pF, and we apply it to pin 12 of our LM1889.
For this pin is fed by a voltage equal to half the supply voltage, we placed the divider bridge formed by R13, R14 and R15 of 2200 ohms.
The maximum signal we can apply decision INP. VIDEO should be around 1 volt peak to peak. In fact it has little importance because this tension is the standard value that delivers all the video equipment (VTRs, cameras).
adjustable potentiometer R9 connected to the video input is used to move the black level. Specifically, by acting on the slider R9, we can vary the contrast of the image.
The maximum signal applied to the INP socket. AUDIO shall not exceed 1.5 volts peak to peak. If the audio signal applied to the input has a lower amplitude, increase the TV volume. By cons, if the amplitude is higher, we can see on the image of diagonal stripes, the result of overloading BF.
The modulator must be supplied with an external voltage of 12 volts and as consumption is only 40 mA, a small model is good enough.
Figure 5: Diagram of Modulator Video / Audio. On the left, we represented the pinout of the transistor BC547 seen below, the side where the pins out of the box. 
Figure 6: Schematic implementation of the modulator video / audio. The adjustable R9 changes the image contrast, the adjustable capacitor C9 is used to vary the frequency of the video signal. In the kit, the taking of inputs and outputs are already fixed on the metal box. List components of the modulator LX.1413
R1: 330 kΩ
R2: 75 Ω
R3: 2.2 kΩ
R4 470 Ω R5
: 47 kilohm
A6: 47 kilohm
R7: 2, 2 k
R8: 1.2 kΩ
R9: 1 kilohm adjustable
R10: 1.5 kΩ
R11: 27 kΩ
R12: 15 kΩ
R13: 2.2 kΩ
R14: 2.2 kΩ R15
: 2 , 2 k
R16: 220 Ω
R17: 220 Ω
R18: 100 Ω
R19: 75 Ω
C1: 10 uF chemical
C2: 150 uF polyester
C3: 470 nF polyester
C4: C5 10 uF chemical
: 100 nF polyester
C6: 150 pF
ceramic C7: 22 pF ceramic
C8: 3.3 pF ceramic
C9: 2 / 50 adjustable
pF C10: 10 nF ceramic
C11: 100 nF polyester
C12: 100 nF ceramic
C13: 100 nF ceramic C14
: 100 nF polyester
C15: 47 uF chemical
C16: 15 pF ceramic
C17: 15 pF ceramic
TR1: NPN Transistor BC547
DV1: BB139 varactor diode
JAF1: Self
0.27 uH JAF2: 0.22 uH Self
MF1: Transfo average frequency 10.7 MHz (green nucleus)
IC1: LM1889 Integrated Circuit
Note: Unless otherwise specified, all resistors are 1 / 4 watt, 5%.
Misc: 2 TV sockets
female solder chassis
taken a male TV chassis for welding a metal housing
Practical realization
The printed circuit board in hand, you can mount all components by placing them as it is shown in Figure 6.
To begin, we recommend welding the support for the integrated circuit IC1. After checking that all the pins are soldered, you can ride all resistance. In this regard, for resistance of 75 ohms, it should be noted that the colors that are purple - green - brown. If you have any doubt, measure it with a multimeter, you read well 75 ohms. After the resistors, you can solder the varactor DV1 directing his ring to R12.
continue with the assembly by welding ceramic capacitors, polyester capacitors and electrolytic.
For the latter, watch out for + / - on their feet. Solder
now potentiometer R9, capacitor C9 and the two adjustable chokes JAF1 - JAF2.
Inductance JAF1 is marked 0.27 and the JAF2 0.22.
It remains to solder the transistor TR1 to be mounted near the outlet INP. VIDEO orienting its flat side to the capacitor C2.
Within three holes drilled to accommodate the input jacks and audio output, it is necessary to weld a small piece of stiff copper wire (drop tail component), then the two holes at the bottom of the circuit, solder a black wire into the hole marked in red and a less marked one more.
Having finished, place the LM1889 circuit on its support, taking care to direct its benchmark-keyed U to the capacitor C14. The PCB will
placed in a metal box, but first he must break into the side hole of 5 mm to ensure the passage of feeding son.
mounting in the housing
Assembly must absolutely be completely shielded. For this, we provide in the kit a metal housing on which are already fixed the 3 coax jacks, 2 females for audio and video inputs and 1 male for output.
If you make the box yourself, get inspired pictures. This case can also be manufactured with epoxy single sided on all sides are welded to ensure shielding. For the sides, copper must be inwards. To weld the top and bottom, copper will be turned outwards. Do not solder the bottom before putting in place and soldered the circuit board. Do not weld the lid on all sides but only by points spaced about a centimeter, otherwise you will have the greatest difficulty in the desoldering for a possible intervention on the circuit.
Take the circuit board and slide it into the case, then weld on the side where no components are installed, welded copper track located on the perimeter of PCB to the enclosure.
There is no need to make a continuous line, but to weld on the perimeter with 3 or 4 points.
must also weld the three pieces of stiff wire plug on their respective input and output.
Before asking the cover, you must adjust R9, C9 and medium frequency transformer core MF1.
Setting C9.
C9 used to tune the modulator to the frequency of TV channel that we chose in the VHF band.
If you have a VCR or a camera, you can take the video signal output jack and apply on decision INP. VIDEO. The RF output terminal OUT is connected to the antenna connector on the TV via a coaxial cable of 75 ohms, the model commonly used for this purpose.
Suppose we set the TV to one of the first VHF channels, you must then turn with a small screwdriver, adjustable capacitor C9 until you see an image appear on the TV screen.
There are TVs that automatically explores the whole VHF band and stop when they encounter a signal.
If you have a pattern generator, as LX.1351 our example, you can connect it as shown in Figure 7, then turn C9 to display the image of the pattern on the screen.
After tuning the TV, you can adjust R9 so as to obtain a perfectly mixed.
This being done, there will be touch R9.
Figure 7: If you have no camera, but you have a TV test pattern generator, you can inject the video / audio input, then set to show C9 on the TV screen image of sight. For this, the TV must be tuned to the VHF range. Adjustment with a generator BF
If you have at your disposal or VCR or pattern generator, be aware that you can use any audio oscillator producing a square wave to your settings.
The output of the generator is connected BF on taking INP. VIDEO OUT jack on the RF antenna of the TV.
If you set the generator to 500 Hz, you will see on the screen 5 horizontal lines (see Figure 9).
If you chock on a frequency very close to 31 250 Hz, double the 15 625 Hz which is the horizontal scan rate, a single vertical line appears on the screen.
Now try to lock onto the frequency of 46,875 Hz and you will see on the screen two vertical bars, if you set it to 62 500 Hz, which is four times the scanning frequency, you get 4 vertical bars (see Figure 10).
Figure 8: For the TV set, you can apply to the video input signal sampled at the output of a low frequency generator capable of providing a square wave. 
Figure 9: The LF generator set to 500 Hz, the screen shows five horizontal bars. Set to 31 250 Hz, one vertical bar to appear. 
Figure 10: If LF generator is set to 46 875 Hz on the screen we see two vertical bars. Set to 62 500 Hz, 3 vertical bars are that we see on the screen. 
Figure 11: To adjust the core MF1, you can apply on the input jack, a sinusoidal signal frequency between 400 and 2 000 Hz Audio Setup
After setting the video signal, you must adjust the audio signal. If you sample the audio signal from a VCR, you must set the core of MF1 to obtain the speaker sound free of distortion.
If you do not have a VCR, you can proceed to adjust the audio signal using an audio oscillator producing a sine wave.
For this setting the output of the LF generator is connected to the outlet INP.
AUDIO (see Figure 11). Allow the generator to a frequency between 400 and 2000 Hz, then adjust the amplitude of the signal so as not to exceed 1.5 volts peak to peak.
Slowly turn the core MF1 until the speaker produces a sound free of distortion.
No signal being applied to the INP VIDEO jack, do not be surprised that the screen stays black.
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