Easy UHF RF modulator

[Studio263]

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.

[paulsherwin]

It's also reasonably easy to convert a modulator from a scrap VCR, especially if you can get hold of a circuit diagram. Many of the cheaper 90s models use generic subassemblies which just need connections for power, audio and composite.

[ianhumph]

Thank you for the information Studio263. Just what I've been looking for!

The instruction you give is so very clear and concise.

I've been thinking of purchasing a modulator for a good few weeks now, but my finances are so tight for the foreseable, that I can't see it happening any time soon. I've been fighting a seemingly losing battle, trying to find how to inject video into one of those cheapy (not vintage.. boo!) 5" b&w tv/radio combis!

I've heard of this method before, and always thought it 'do-able'- just didn't have the 'know-how' till now!

Going down the route of using an ac/dc multi-adaptor or something equally suitable (I'm pretty sure I've something I can put my hand on in one of my 'bits boxes') would be the best option in my case- then I'm all set for something with a little more of a vintage flavour!

All the very best.

Ian

[Studio263]

Originally Posted by paulsherwin

It's also reasonably easy to convert a modulator from a scrap VCR...

This is true, but there are a few drawbacks. Firstly a VTR modulator will be uncased so to you will have to find a box to put it in and drill it neatly if you want to make a tidy job of it.

Secondly many later VTRs have the modulator combined with the tuner / IF unit in a way that is difficult to seperate so you end up with a big lump of useless circuitry to perform what should be a reletively simple function.

Lastly many later VTRs have the RF output channel slection done through the machine's menu system so the modulator won't work without a companion microcontroller to provide (usually) the right I2C codes. Again it's possible but not necessarily easy. Others (e.g. some Philips) are varicap-tuned so you need a stabilised source of a doubtless inconvienient voltage to get them going.

[julie_m]

1980s-era home computers usually contained an Astec UM1286 modulator (capable of sound and vision) even if the speaker was on the motherboard. And some years ago, I obtained very cheaply from a surplus store a tunable modulator box which was meant for use with a home CCTV system (the modulators could be daisy-chained, allowing you to select one of several cameras as easily as changing channel). With the addition of a pair of plugs cut from an old audio lead and an old mobile phone charger as a power supply, it made a nice little self-contained unit.

[Lloyd 1985]

Hi,
I have done a very similar thing, Argos were selling PS1 and PS2 moulators for £4.99 a few years back, so I got a couple and made 1 up as you have done. The other one got used for experiments where I took power from the scart socket of a freeview box (can't remember which pin now..) through a 5v regulator. It worked for a while, but not well, then the sound dissapeared! That ones on the junk pile somewhere now! The idea was to make it so it was just 1 cable between the TV and the Freeview box, no extra sockets used up by power supplies.

regards,
Lloyd.

[Dave Moll]

Here's a page I have showing the pinouts for SCART: