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Old 28th Aug 2011, 11:21 am   #1
Studio263
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Join Date: Apr 2007
Location: Hampshire, UK.
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Default Easy UHF RF modulator

Here’s an idea for a simple UHF RF modulator that you can use to connect a TV set to a DVD player, Freeview receiver etc. The Sony Playstation 1 and 2 models all used to come with a nice little modulator that gave a fixed-tuned output on ch36, some could also be switched from 5.5 to 6MHz sound as well. A scrabble through the boxes at our local car boot has uncovered several at £1 a go but even if you have to buy the whole Playstation it shouldn’t cost more than a fiver.

As well as being very small these are good quality units that provide decent sound as well as picture performance. To convert it for TV use the first thing to do is to cut off the Playstation plug and ferrite ring, three screened leads will then be revealed. The colour of the insulation is pretty meaningless but it isn’t hard to find which is which. Using a meter set to ohms measure across the inner to the screen of each in turn. The one that gives 75R is the video input, either put a phono plug on the end or wire it to a SCART connector, core to pin 19 and screen to pin 17. The other two are more difficult to separate so the easiest thing to do is to apply +5V to the inner of each in turn and see which generates a blank channel on a TV set at around ch36. The other is then the audio, either fit another phono plug or connect it to the SCART connector. As the modulator is mono you will have to mix the two stereo channels if you use the SCART, I use two 2.2k resistors connected to pins 1 and 3 and connect both to the audio input of the modulator.

Providing power is the last hurdle. These units all work on +5V and take about 20mA. Many DVD players and Freeview receivers have a +5V supply inside that you can tap into, although it is wise to add a 100mA fuse or a 10R fusible resistor in series if you decide to use this source. One method of to making the connection is to disconnect pin 8 inside the equipment and connect the +5V supply to it instead. Then all to have to do is to wire the modulator power connection to pin 16 of the SCART plug (and its screen to pin 9). This is a neat setup and works well if you only want to use the modulator with one piece of equipment.

Another method which I prefer is to run the power cable into the SCART plug body and fit a small regulator (78L05) etc inside, along with any necessary capacitors (100nF on the input, 10uF on the output for the 78 series for example). You can then run a wire back out of the plug, put a suitable connector on the end and power the modulator from a universal type AC adaptor. This arrangement means that you can use the modulator with any equipment that has a SCART socket, so it is very versatile.
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