Since the first thread was "removed" because some issues on the server, i will try again.
I an curious about how to make a rf converter, i have seen in several of them, someone is built with transistors, others with IC´s, but is here somebody who have builded a good working RF converter? in some threads in this forum i have seen some for the 405 line system, but i dont think they are usable for pal 625 system, i mean you´re videosystem for the 405 line is negative AM , pal 625 is positive.
I am interested in the VHF area, so i can use my old telleys 🙂
Alex 🙂
Alex, can you explain exactly what you want to do? Your thread title says 'Modulator' but the post says 'converter', twice.
A modulator takes a video signal and modulates it onto an RF carrier whereas, in this context, a converter would take a modulated RF carrier and convert it to a different frequency - for example, take the UHF output of the modulator in a Video recorder and convert it to VHF.
Also, you've got your modulation polarities mixed up! 405 is positive modulation and 625 lines, with the exception of France, uses negative modulation.
For a simple modulator, this circuit from Elektor published in January 1985 might fit the bill. It produces RF at harmonics of the 27MHx crystal, so the 2nd harmonic will be close to channel E3. If you can find a 27.625MHz crystal, it will be spot on 55.25MHz.
You will also need a 5.5MHz FM modulator for the sound, which can be mixed with the video signal prior to the input of the main modulator.
For an IC solution, something like this Motorala IC would fit the bill but there is the added complexity of providing a data feed for the I²C bus to tell it what to do.
An alternative, as I mentioned earlier, would be to use a UHF modulator removed from a scrap VCR or similar and build a one transistor UHF oscillator. Then combine the two in a simple mixer. Ideally, the oscillator frequency should be lower than the modulator frequency, although with a DSB modulator it isn't so important. The difference between the two frequencies being, of course, the VHF output frequency.
I hope this is of help.
When all else fails, read the instructions
I'm sure Astec must have made VHF modulators in the past - likewise, I can't believe that eBay wouldn't turn up some Kathrein examples.
i have seen some for the 405 line system, but i dont think they are usable for pal 625 system, i mean you´re videosystem for the 405 line is negative AM , pal 625 is positive.
I am interested in the VHF area, so i can use my old telleys 🙂
Alex 🙂
It's still not clear to me whether you want a VHF modulator or a UHF one.
In the UK all 625 line transmissions were on UHF and all 405 line transmissions were VHF.
In Europe some 625 was transmitted on VHF.
If your "old telleys" are from the UK and 625 line then you want a UHF modulator. Not VHF.
Peter
Alex, can you explain exactly what you want to do? Your thread title says 'Modulator' but the post says 'converter', twice.
A modulator takes a video signal and modulates it onto an RF carrier whereas, in this context, a converter would take a modulated RF carrier and convert it to a different frequency - for example, take the UHF output of the modulator in a Video recorder and convert it to VHF.
Also, you've got your modulation polarities mixed up! 405 is positive modulation and 625 lines, with the exception of France, uses negative modulation.
For a simple modulator, this circuit from Elektor published in January 1985 might fit the bill. It produces RF at harmonics of the 27MHx crystal, so the 2nd harmonic will be close to channel E3. If you can find a 27.625MHz crystal, it will be spot on 55.25MHz.
You will also need a 5.5MHz FM modulator for the sound, which can be mixed with the video signal prior to the input of the main modulator.
For an IC solution, something like this Motorala IC would fit the bill but there is the added complexity of providing a data feed for the I²C bus to tell it what to do.
An alternative, as I mentioned earlier, would be to use a UHF modulator removed from a scrap VCR or similar and build a one transistor UHF oscillator. Then combine the two in a simple mixer. Ideally, the oscillator frequency should be lower than the modulator frequency, although with a DSB modulator it isn't so important. The difference between the two frequencies being, of course, the VHF output frequency.
I hope this is of help.
Hi Terry.
It´s a rf modulator i am interested in, actual the circuit, i think it could be interesting to build one day, thank you for your comments, i will take a good look on it, the modulator from elektor is a little bit special, i am not sure i understand how to mix the sound with the the video signal in that circuit.
Alex 🙂
i have seen some for the 405 line system, but i dont think they are usable for pal 625 system, i mean you´re videosystem for the 405 line is negative AM , pal 625 is positive.
I am interested in the VHF area, so i can use my old telleys 🙂
Alex 🙂
It's still not clear to me whether you want a VHF modulator or a UHF one.
In the UK all 625 line transmissions were on UHF and all 405 line transmissions were VHF.
In Europe some 625 was transmitted on VHF.
If your "old telleys" are from the UK and 625 line then you want a UHF modulator. Not VHF.
Peter
Hi Peter.
It´s a VHF modulator, in Denmark we used the VHF from the Televisions birth to around 1988 where the other national channel starts (yes we only have 1... ONE national tv station here :-/ ) when TV2 (the other national station) starts the UHF band was used, i hope this will give you a little bit better answer
Alex 🙂
I'm sure Astec must have made VHF modulators in the past - likewise, I can't believe that eBay wouldn't turn up some Kathrein examples.
Astec did! But finding them is another matter.
The UM1285 seems to have been made for the US market but there are suggestions that these have easily been retuned from 4.5MHz to 5.5MHz intercarrier sound by adjusting the appropriate core.
Possibly there are other with a different -8 suffix to those I've found a reference to that may have been made for 5.5MHz use.
I've also come across this anonymous UHF/VHF (but Band III only) modulator in Cologne. Postage is high but perhaps Alex, being in Denmark, can negotiate a lower charge!
When all else fails, read the instructions
Hi Alex,
I suspect that your easiest source would be from a discarded video player.
HTH
Peter
Hi Terry.
It´s a rf modulator i am interested in, actual the circuit, i think it could be interesting to build one day ... i am not sure i understand how to mix the sound with the the video signal in that circuit.
Alex 🙂
While I was searching I came across this circuit which shows one way of doing it. I was reluctant to use it at first as it was described as a transmitter but there was the a clear disclaimer: Please Note: This circuit will be illegal to use in many countries, as such, it is displayed here for educational purposes only which is reasonable enough so I have modified it for use only as a modulator before posting it here. The tap on the inductor should be one turn from the top, 12 volt end.
The original notes, which I've modified slightly will help you to understand it:
Notes:
This modulator circuit uses 5.5Mhz, FM modulation for sound, and PAL for video modulation. The transmitter is tunable between 54 and 216Mhz. The transmitter is therefore compatible with TV systems B and G, but check transmitter frequencies used, as they are different for each country..
The audio is applied via input connector, J1. This can be a phono type, note also that audio must be mono, if a stereo input is used, either use a stereo to mono adapter, or one channel only. Transistor Q1 acts as an oscillator and amplifier, the primary of T1 and its internal capacitor oscillating at 5.5Mhz, the transistor itself provides audio gain. Some adjustable of frequency may be obtained by moving the core of T1. T1 should be a 5.5MHz IFT, available from an old TV or FM radio.
A link to a UK company supplying a suitable Toko transformer was also included but I've been through their entire remaining stock but the only 5.5MHz component they have left (and a 6MHz one that should also have been suitable) are simple inductors, not transformers, so you will have to scavenge around for something suitable.
The modulated, audio signal, is now passed to Q2, which acts as a modulator and generates the carrier frequency for the transmitter. C5 and inductor L1 form the tank circuit which oscillate at the carrier frequency. Adjustment of C5, will alter the channel of the transmitted signal.
The video signal is fed via input jack J2. The video signal must be composite, e.g. just video and not contain any audio. Suitable inputs from cameras, or video cameras should be ok. Preset R7 allows adjustment of the video signal input, and hence also controls modulation level. Q2 mixes the video input with the sound input to produce a modulated RF video signal containing both audio and video components.
Apologies for any confusion if my posts have appeared out of sync with yours and Peter's but I've had to stop a few times in the middle of my posts, often while checking possible sources!
When all else fails, read the instructions
