Valve RF modulator - UK Vintage Radio Repair and Restoration Discussion Forum

Giulio Maiocco · 6th Feb 2006, 12:16 pm

Hi all,

I decided to try to trace a schematic of an all valves RF modulator to feed my little collection of TVs with a DVD player with the Italian standard.
For now I have "completed" the sound section using a reactance modulator to get the required FM modulation, but I have only traced a part of the video section, as I don't exactly know how to get the required 5MHz bandwidth and how to input the video signal to the video output stage. I have used the frequency multipliers both on the sound and video sections.

Please could some good soul tell me if I'm on the right way and the modifications I should do to have a reasonably good RF modulator?

Best regards

Giulio Maiocco

oldeurope · 6th Feb 2006, 1:00 pm

Hi,
please explain me your sound preemphasis, I can not see it.

In the vision section I can not see your vision input.
You need a DC restorer and a limiter to make sure that the vision
level never goes over 100%.

Oscillators on the output frequency are the better solution because
these multipler stages are noisy.
Your output voltage should be 2mV =66dBuV

Maybe a quasi parallel sound solution is better here because
most sets are intercarrier sound.

Kind regards
Darius

Giulio Maiocco · 6th Feb 2006, 1:19 pm

Hi Darius,

Well, I am about to show my ignorance, but how can I do the sound preemphasis, my design is only a rough idea of the work involved, so is susceptible of wide changes. The same applies to the vision system, the problem is where should I inject the video signal and how? Does it needs some other stages ( sync separator or ..)? I have only restored a couple of TV sets, so I'm still learning, be patient, please. What is a good schematic for a DC restorer I could "copy" into my design. OK for the oscillators at the output frequency, I have read about the noise on a book I have a couple of years ago, but I forgot it until you mentioned. If we don't consider the preemphasis on the audio system, the reactance modulator in my schematic is a good method to have FM modulation in this application? If not, please could you suggest a better one?

Thanks

Giulio Maiocco

Sean Williams · 7th Feb 2006, 8:38 am

Your Ideas of frequency multiplication are interesting, and it does mean that you can build a more stable oscillator, of course a lot more stages! - I think with FM you will need the low starting frequency to get the correct deviation.

I really wonder if you need to use a valve such as a 6CL6 - I am sure that the EF80 would be fine throughout - you are not building a broadcast station

Will watch this thread with interest, and offer any help I can, although my knowledge of Television modulator design is very little!

Cheers
Sean

Giulio Maiocco · 7th Feb 2006, 11:04 am

Hello Sean,

I really appreciate your input, two minds are better than one, especially after you know that my brain is already "burned out"...

. I also am convinced that a lower frequency oscillator in conjunction with a frequency multiplier means a better stability and, in the case of the audio modulator, is the only method for a reactance modulator to work well.. I only need to know how to do a sound preemphasis (50uS) and then I could start to experiment with the audio section. The video section is another "bad beast" to finish and I know I need a lot of help to have it working. A 6CL6 is probably not needed as output stage,I could eventually reduce the screen grid voltage to regulate the output power, but it is needed as reactance modulator, I have a schematic that uses this valve. This RF modulator is a lot of work, but I like challenges. Don't worry about your "little knowledge of Television modulator design", I'm just a beginner, but I like to learn, you bet!!

My English is poor, but I hope you can understand what I wrote

Any input or question is welcome

Best regards

Giulio Maiocco

Ed_Dinning · 7th Feb 2006, 1:38 pm

Hi Giulio, Pre or De emphasis is basically a tone control circuit that gives a lift or cut to part of the frequency spectrum, often to reduce transmission bandwidth. The reverse process is carried out in the receiver to correct the signal. It is typically carried out wit just and R & a C, or possibly 2 sets.
This was also done on records.

HTH Ed

Giulio Maiocco · 7th Feb 2006, 2:40 pm

Hi Ed,

That's interesting, I thought that it was more difficult, so, if I place a parallel RC network in series to the audio input (between the hot terminal of the input and 6CL6's grid leak resistor), with R=10KOhm and C=5000pF, and changing the value of the capacitor across the audio input, I should be set.

Please correct me if I'm wrong

Ed, as always you are very helpful and willing to share your ideas with other people, this is very appreciated!!

Best regards

Giulio Maiocco

oldeurope · 7th Feb 2006, 3:48 pm

Hi Giulio, it is a bit more difficult than you think.
I made a schematic for you that shows a stand alone preemphasis stage.
Often parts of the reactance stage and preveous stages are combinned
with it and makes it easier.
Hi Ed, the preemphasis makes more bandwidth but less noise.
Please note that a level indicator for +-50KHz comes after(!) the
preemphasis.

Kind regards
Darius

Ed_Dinning · 7th Feb 2006, 10:29 pm

Hi Gents, my remarks were more directed to the sound side of things as it is relatively easy there. It is sometimes necessary to boost part of a signal instead of attenuating the "opposite" part of it. This usually calls for an amplifier with frequency selective fedback. The traditionalists prefer passive feedback as it adds much less noise.

Ed

Giulio Maiocco · 7th Feb 2006, 10:55 pm

Hi Darius,

Thank you for your schematic, I owe you a favor

. Preemphasis circuit excluded, has my sound section schematic a minimal chance to work as is? I think that the reactance modulator should be fine as drawn on the schematic, I mean we shouldn't be too far off? It certainly will need some tweaking as I don't think it will be perfect on the first power up, but we can make it work... Now I can prototype the sound section and see (or better...hear) how it acts up. I have still big problems to figure how to correctly do the video section, but this another story for now.

Hi Ed,

Your remarks are readily comprehensive, the audio field is simpler for me because I love vintage Hi-Fi systems, especially the English ones!!

Steve_McVoy · 8th Feb 2006, 12:35 am

Does anyone know the pre-emphasis used in the audio of the 405 line standard? Was it the same before and after the war?

oldeurope · 8th Feb 2006, 8:40 am

Quote:

Originally Posted by Steve_McVoy

Does anyone know the pre-emphasis used in the audio of the 405 line standard? Was it the same before and after the war?

Good morning Steve, preemphasis is ony used with FM.
Because of the changing modulation index with the frequency.
405 AM don't has it.
The biggest disadvantage of preemphasis is you can not transmit
high frequencys with full audio level. With he 75us used stateside this
effect is strong. To get up to 16KHz with a resonable audio level
We are gone to 50us here.
I compared all the advantages and disadvantages of AM and FM.
My conclusion is that AM is the better choise. I think this is why the
french used AM in 819. I am very sureprised about the sound quality
of my 405 sets. Normally you expect medium wafe quality if you hear
AM- sound.
And in the film TV is here again at the beginning they say :
"... the high quality sound for which TV sound is known..."

Kind regards
Darius

6th Feb 2006, 1:00 pm	#2
oldeurope Retired Dormant Member Join Date: Dec 2003 Location: Solingen, Germany Posts: 727	Re: Valve RF modulator Hi, please explain me your sound preemphasis, I can not see it. In the vision section I can not see your vision input. You need a DC restorer and a limiter to make sure that the vision level never goes over 100%. Oscillators on the output frequency are the better solution because these multipler stages are noisy. Your output voltage should be 2mV =66dBuV Maybe a quasi parallel sound solution is better here because most sets are intercarrier sound. Kind regards Darius

6th Feb 2006, 1:19 pm	#3
Giulio Maiocco Tetrode Join Date: Jan 2006 Location: Fossano (CN) Italy Posts: 71	Re: Valve RF modulator Hi Darius, Well, I am about to show my ignorance, but how can I do the sound preemphasis, my design is only a rough idea of the work involved, so is susceptible of wide changes. The same applies to the vision system, the problem is where should I inject the video signal and how? Does it needs some other stages ( sync separator or ..)? I have only restored a couple of TV sets, so I'm still learning, be patient, please. What is a good schematic for a DC restorer I could "copy" into my design. OK for the oscillators at the output frequency, I have read about the noise on a book I have a couple of years ago, but I forgot it until you mentioned. If we don't consider the preemphasis on the audio system, the reactance modulator in my schematic is a good method to have FM modulation in this application? If not, please could you suggest a better one? Thanks Giulio Maiocco

7th Feb 2006, 8:38 am	#4
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,518	Re: Valve RF modulator Your Ideas of frequency multiplication are interesting, and it does mean that you can build a more stable oscillator, of course a lot more stages! - I think with FM you will need the low starting frequency to get the correct deviation. I really wonder if you need to use a valve such as a 6CL6 - I am sure that the EF80 would be fine throughout - you are not building a broadcast station Will watch this thread with interest, and offer any help I can, although my knowledge of Television modulator design is very little! Cheers Sean __________________ Engineers make things work and have spare bits when finished

7th Feb 2006, 11:04 am	#5
Giulio Maiocco Tetrode Join Date: Jan 2006 Location: Fossano (CN) Italy Posts: 71	Re: Valve RF modulator Hello Sean, I really appreciate your input, two minds are better than one, especially after you know that my brain is already "burned out"... . I also am convinced that a lower frequency oscillator in conjunction with a frequency multiplier means a better stability and, in the case of the audio modulator, is the only method for a reactance modulator to work well.. I only need to know how to do a sound preemphasis (50uS) and then I could start to experiment with the audio section. The video section is another "bad beast" to finish and I know I need a lot of help to have it working. A 6CL6 is probably not needed as output stage,I could eventually reduce the screen grid voltage to regulate the output power, but it is needed as reactance modulator, I have a schematic that uses this valve. This RF modulator is a lot of work, but I like challenges. Don't worry about your "little knowledge of Television modulator design", I'm just a beginner, but I like to learn, you bet!! My English is poor, but I hope you can understand what I wrote Any input or question is welcome Best regards Giulio Maiocco

7th Feb 2006, 1:38 pm	#6
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: Valve RF modulator Hi Giulio, Pre or De emphasis is basically a tone control circuit that gives a lift or cut to part of the frequency spectrum, often to reduce transmission bandwidth. The reverse process is carried out in the receiver to correct the signal. It is typically carried out wit just and R & a C, or possibly 2 sets. This was also done on records. HTH Ed

7th Feb 2006, 2:40 pm	#7
Giulio Maiocco Tetrode Join Date: Jan 2006 Location: Fossano (CN) Italy Posts: 71	Re: Valve RF modulator Hi Ed, That's interesting, I thought that it was more difficult, so, if I place a parallel RC network in series to the audio input (between the hot terminal of the input and 6CL6's grid leak resistor), with R=10KOhm and C=5000pF, and changing the value of the capacitor across the audio input, I should be set. Please correct me if I'm wrong Ed, as always you are very helpful and willing to share your ideas with other people, this is very appreciated!! Best regards Giulio Maiocco

7th Feb 2006, 3:48 pm	#8
oldeurope Retired Dormant Member Join Date: Dec 2003 Location: Solingen, Germany Posts: 727	Re: Valve RF modulator Hi Giulio, it is a bit more difficult than you think. I made a schematic for you that shows a stand alone preemphasis stage. Often parts of the reactance stage and preveous stages are combinned with it and makes it easier. Hi Ed, the preemphasis makes more bandwidth but less noise. Please note that a level indicator for +-50KHz comes after(!) the preemphasis. Kind regards Darius Attached Thumbnails

7th Feb 2006, 10:29 pm	#9
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: Valve RF modulator Hi Gents, my remarks were more directed to the sound side of things as it is relatively easy there. It is sometimes necessary to boost part of a signal instead of attenuating the "opposite" part of it. This usually calls for an amplifier with frequency selective fedback. The traditionalists prefer passive feedback as it adds much less noise. Ed

7th Feb 2006, 10:55 pm	#10
Giulio Maiocco Tetrode Join Date: Jan 2006 Location: Fossano (CN) Italy Posts: 71	Re: Valve RF modulator Hi Darius, Thank you for your schematic, I owe you a favor . Preemphasis circuit excluded, has my sound section schematic a minimal chance to work as is? I think that the reactance modulator should be fine as drawn on the schematic, I mean we shouldn't be too far off? It certainly will need some tweaking as I don't think it will be perfect on the first power up, but we can make it work... Now I can prototype the sound section and see (or better...hear) how it acts up. I have still big problems to figure how to correctly do the video section, but this another story for now. Hi Ed, Your remarks are readily comprehensive, the audio field is simpler for me because I love vintage Hi-Fi systems, especially the English ones!!

8th Feb 2006, 12:35 am	#11
Steve_McVoy Pentode Join Date: Feb 2004 Location: Columbus, Ohio, USA Posts: 150	Re: Valve RF modulator Does anyone know the pre-emphasis used in the audio of the 405 line standard? Was it the same before and after the war?