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Television Standards Converters, Modulators etc Standards converters, modulators anything else for providing signals to vintage televisions.

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Old 15th Nov 2005, 1:51 pm   #21
David_Robinson
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Default Re: VHF modulators

Quote:
Originally Posted by oldeurope
... do you want something software based or a discret analog circuit.
I think Andy just wanted something that will do channel 8, system A. What other choices are there for him?

Last edited by Dave Moll; 27th Jun 2007 at 10:13 pm. Reason: correct for username change in quote
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Old 15th Nov 2005, 2:55 pm   #22
oldeurope
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Default Re: VHF modulators

Quote:
Originally Posted by David_Robinson
I think Andy just wanted something that will do channel 8, system A. What other choices are there for him?
Building a ch8 modulator or mixing a ch1 up to ch8 for example.

The interesting question is are there 405 sets that can not receive ch1 or
ch4?

Kind regards
Darius
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Old 16th Nov 2005, 12:37 am   #23
Danny
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Default Re: VHF modulators

Hi yes their are a few i have a pye that has had its coils altered and it doesent want to go back vary well so yes thay are about if you live in that part of the world.
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Old 17th Nov 2005, 9:42 pm   #24
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Smile Re: VHF modulators

Hi folks,

Thanks for your replies. I really wanted to do a pseudo BBC1 on band I channel 4 and a pseudo ITV on band III channel 8 (just like it used to be round here), click the turret tuner round and show people what it used to be like. Obviously I would need two modulators and a combiner (and two simulataneous sources of 405 programmes) to pull it off. I've never seen anything beyond band I channel 4 out there though so my idea was seemingly dead in the water.

I would of course be interested in a solution and if there is something out there that does all of the old channels then great, please tell me more and how I may be able to satisfy my urge.

Regards,


Andy
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Old 18th Nov 2005, 2:02 pm   #25
David_Robinson
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Default Re: VHF modulators

I don't know of a ready made System A all channels modulator. If you are prepared to build your own, my design has already been pointed to further up this thread.
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Old 18th Nov 2005, 7:19 pm   #26
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Smile Re: VHF modulators

Thanks for reminding me about that link David - I'd forgotten that you had been working on a design. I guess I just got out of touch with progress and then lost the old link.

I would be interested in having a go, but I have absolutely no facilities for making PCBs - the only thing I have at my disposal is the old Stripboard.

If I could explore things with you more fully so that I could get an idea of what I would be in for if I had a go (and whether or not a stripboard construction is a none starter) then I'd be greatful.

Do you have finalised plans etc?

Regards,


Andy
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Old 19th Nov 2005, 2:04 pm   #27
David_Robinson
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Default Re: VHF modulators

Hello Andy, I have posted some new closeups of my prototype here

http://homepage.ntlworld.com/david.r...%20buffers.jpg

and here

http://homepage.ntlworld.com/david.r...20RF%20box.jpg

As you can see I haven't got round to putting it into a box yet.

I'm with you on PCBs - I never could be bothered with them. After some searching I have found the perfect prototyping card at Farnell, stock number 4147959. This has square pads on a 0.1" pitch on one side and a colander ground plane on the other. It's cheap too. It can be cut to size with tinsnips. The ground plane not only gives better RF performance, it's also easy to build as you can drop a connection down to ground at any point. Through hole components go in from the ground plane side and surface mount R's and C's can be fitted between 2 adjacent pads. The modulator chips are glued to the board with some thin insulating material underneath. Hookup is done with Kynar wire. As you can see the modulator mostly uses SM components although you could use all through hole parts if you prefer. I use a binocular magnifier worn on the head for SM work, this leaves both hands free.

The project can be tested in stages, you don't have to build it all at once. The vision modulator alone, without the CPU, will default to UHF channel 36, sound 5.5MHz. You can then add the CPU, this will still run OK with the sound chip missing.

If you are combining 2 of these modulators together, you should be aware that the Motorola chip produces a lot of odd harmonics when working on VHF. The third harmonic of channel 4 falls at about channel 7 so I think you will need some sort of filter to prevent interference with channel 8.
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Old 19th Nov 2005, 5:06 pm   #28
oldeurope
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Default Re: VHF modulators

Quote:
Originally Posted by David_Robinson
Hello Andy, I have posted some new closeups of my prototype here

http://homepage.ntlworld.com/david.r...%20buffers.jpg

and here

http://homepage.ntlworld.com/david.r...20RF%20box.jpg

As you can see I haven't got round to putting it into a box yet.

I'm with you on PCBs - I never could be bothered with them. After some searching I have found the perfect prototyping card at Farnell, stock number 4147959. This has square pads on a 0.1" pitch on one side and a colander ground plane on the other. It's cheap too. It can be cut to size with tinsnips. The ground plane not only gives better RF performance, it's also easy to build as you can drop a connection down to ground at any point. Through hole components go in from the ground plane side and surface mount R's and C's can be fitted between 2 adjacent pads. The modulator chips are glued to the board with some thin insulating material underneath. Hookup is done with Kynar wire. As you can see the modulator mostly uses SM components although you could use all through hole parts if you prefer. I use a binocular magnifier worn on the head for SM work, this leaves both hands free..
Hi David,
I like the way you did it with this prototyping card. Thanks for the suggestion.
Isn't it possible to use one clock for both IC's? Maybe this reduces noise.
At the moment I am making a modulator for the OIRT channels one and two.
I just developed a vision input stage that eleminates the videotext etc.

Kind regards
Darius
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Old 19th Nov 2005, 5:37 pm   #29
David_Robinson
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Default Re: VHF modulators

Quote:
Originally Posted by oldeurope
Isn't it possible to use one clock for both IC's?
Yes it might be possible. I guess you could have the crystal returned to ground and half the capacitance (13pF) going to each IC. I haven't tried this.

Last edited by Dave Moll; 27th Jun 2007 at 10:14 pm. Reason: correct for username change in quote
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Old 19th Nov 2005, 8:46 pm   #30
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Default Re: VHF modulators

Thanks David.

You photos have just about given me the courage to have a go. I certainly don't like surface mount stuff, but that's the way things are going so it's probably about time I had a wrestle with some examples.

I'm a bit worried about CPUs too. I've always avoided these like the plague (apart from the PC at home of course), so that's another 'argghhh' for me.

I'll try and chivvy myself up and see if I have what it takes.

Thanks for your help and encouragement.

Best regards,


Andy
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Old 20th Nov 2005, 12:19 pm   #31
David_Robinson
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Default Re: VHF modulators

Andy, you have a PM.

Here are sources for some of the parts I used:

Ceramic resonator 10MHz: Farnell 648164
Prototyping board:Farnell 4147959
Power supply: CPC PW00869
Power connector: CPC CN02178
RF box: Farnell 522181

The 68uH variable inductor (Toko) came from RS but unfortunately seems to have been discontinued. All suggestions welcome...
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