How to build a modulator for the old Belgian 625 line AM-sound standard?

[German Dalek]

Hi all,
New challenge!
I have to build something no one ever built before!

I need a modulator for the Belgian 625 line standard with AM-sound.

It is close to old 819 line French standards (later UHF 625 l, too), but with 625 lines, positive video modulation and AM sound.
Belgium decided for political reasons against the neighbor standards, created something own to try to keep France, Netherlands and Germany out in 1953.
But we all know, that it doesn’t worked.
Because of the fight of two languages, there was a second (819 l - 5,5 MHz video/sound gap) standard, but that is not part of the action.
Most old Belgian TV sets are 4 standard sets to watch the 2 belgian, the french and the European CCIR standard.
Only a small number of sets were only-belgian-standard TV receivers.
They were lower in sales prices, but with the reduced number of programs to watch on, a kind of lemmons.
Only a few sets survived, and until now there is no interest to bring them back to life.
I own a couple of them, so long story short: I have to do something.

I decided to use the French converter, which works well, as a basis design.
My idea is that I can use this set with only a few changes in the frequencies:
http://819lignes.free.fr/Realisation_d_un_modulateur_VHF_F8A.html
They are close-by.
The 2 oscillators of the French design are built in the same way, what I have to do is to adjust the French video osc. from 185,25 MHz to 210,25 MHz (4,5 pF osc. cap), and the other sound osc. 174,10 MHz to 215,75 MHz (6,2 pF osc. cap).
How close I can get to the new frequencies when I use a 3 pF cap for video and 2 pF for sound? Any idea?
(I hate maths….)
What about crystals in both oscillators?
Thanks in advance!
German Dalek

[colourking]

Originally Posted by German Dalek

The 2 oscillators of the French design are built in the same way, what I have to do is to adjust the French video osc. from 185,25 MHz to 210,25 MHz (4,5 pF osc. cap), and the other sound osc. 174,10 MHz to 215,75 MHz (6,2 pF osc. cap).

Why do you need to move the video frequency from 185.25 MHz to 210.25. ? Is it clashing with strong DAB transmissions ?
If video can be left where it is, you only need to move the sound to 190.75

[German Dalek]

Originally Posted by colourking

Why do you need to move the video frequency from 185.25 MHz to 210.25. ? Is it clashing with strong DAB transmissions ?
If video can be left where it is, you only need to move the sound to 190.75

Hi,
I want to stay on the orignal channel with video and sound. I own 7 of these strange TV sets, one has really only 2 channels (819 B & 625 B) 2 others have a channel selection by push buttons like a radio. The others have a turret tuner.
For historical reasons I want to use them with their strange standard.
Changing the frequency of each set down to 185,25 MHz for picture is the same hard way.
Regards,
German Dalek

[Al (astral highway)]

Hi German Dalek,

Originally Posted by German Dalek

How close I can get to the new frequencies when I use a 3 pF cap for video and 2 pF for sound? Any idea?
(I hate maths….)

Not very close! 3pF and 2pF are far too low...

Here's what I'm seeing.

The RF sections around the BF240s look like Clapp oscillators, with feedback coils on the tapped main inductors.

Each oscillator has main frequency-setting capacitor (C9 for video, C15 for sound)
-- a small fixed capacitor (labelled Cf on the drawing)
-- a trimmer in parallel with that fixed capacitor
together, that parallel pair are in series with C9 or C15.

I note what you say about maths, but for capacitors in series, we sum them and then we take the inverse for the total.

The trimmer shunts the fixed capacitor, and that shunted pair returns to the transistors. At the same time C9 C15 are the series element with the inductors

so the trimmer makes a relatively small adjustment to the total with most of the frequency set by C9, 15.

When I ran some numbers, the small series capacitance needed at 185.25 MHz (vision) and 174.1 MHz (sound) is lower than what you get with the original 15 pF parts, as you suggest, but not quite as far as your values.

So we agree we can lower the value of the the main capacitors to help the trimmer lock in

.... and I came up with:

Replace C9 with 4.7 pF for the vision
Replace C15 with 3.9 pF for the sound carrier

I'd use silver-mica or other high-stab types to avoid drift with temp changes.

Disclaimer: I have no knowledge of this set , just taking this as a tuning exercise on paper, so I won't be offended if anyone sees it a different way!

[German Dalek]

Hi Al!

That is great news in the morning!
This will be the way to fix the problem I am fighting with since years.

If I google for silver Micas I get these:
https://www.ebay.de/itm/257148106789?var=0&mkevt=1&mkcid=1&m...id=f48126144e1a120047164f32d7469079

https://www.ebay.de/itm/355811650223?mkevt=1&mkcid=1&mkrid=7...id=1535e8788823968a574cd3e59275b66b

Explanation of silver Micas:
https://www.greentechee.com/advantages-and-disadvantages-of-silver-mica-capacitors_n49
But the max. working frequency is (only?) 1 MHz! (see point 3 of the discription)

According to the typical diy AM/FM transmitters for homebrew projects I wonder, why they use the typical ceramic caps:
https://www.conrad.de/de/p/keramik-scheibenkondensator-radia...m-x-4-mm-1-st-457086.html?insert=VQ

Explanation of ceramic caps:
https://en.wikipedia.org/wiki/Ceramic_capacitor

Question:
If I understand you right, these silver Micas are always the better choice for oscillators?

Many thanks and best regards again,
German Dalek

[German Dalek]

Hi all!

Here are some pages with interesting informations on the western continental TV standards!
See the differences between the french 819 L standard and the belgian 625 L standard.
But the question is now, do I have to do more changes because of some levels?
Just because of some levels....
Regards,
German Dalek

[Al (astral highway)]

Originally Posted by German Dalek

Hi Al!

That is great news in the morning!
This will be the way to fix the problem I am fighting with since years.

Great to hear it, German Dalek!

I'd ignore that 1Mhz entirely! Here's one series with test data from 100MHz to 1 GHz, for example Ignore the 1 MHz reference, it may be a confusing way of saying they tested it at that frequency, nothing more.

Ceramics may be more useful in applications like decoupling- they are certainly cheaper, and there may be new types of ceramic that are advances in materials science that I'm not clued up on.

But to your question, are silver mica always the best choice for oscillators?

I'd say a confident yes. Silver-mica capacitors are the classic choice for high-frequency oscillators (especially RF and tuned circuits). Thy exhibit high Q, high stability, low loss and reliability, all of which qualities which make them ideal

[German Dalek]

Hi Al,
Thanks for your reply.
The cap list you showed up, doesn't list caps with values like 4,7 pF and 3,9 pF.
Maybe a 4 pF hits a 3,9 pF cap, but 5 pF is far away from 4,7 pF.

I prefer to buy in real shops, not online.
So first thing to do is to ask for 3,9 pF and 4,7 pF with a working frequency from about 200 MHz.
There might be other dealers with caps from different sources.

But there are some other specifications which making me scratch my head...

And here is another point to discuss.
From Australia I received that advice:
"For the video it pays to make a sync tip clamp to control the DC axis of the video signal
at the video modulator's input, because in TV's that use direct coupling from the detector
to the CRT, will have a wandering black level if you don't do that in the video modulator.
Of course usual AC coupling is fine for the audio modulator."

How do you construct something like that?

Regards,
German Dalek

[Al (astral highway)]

Originally Posted by German Dalek

Hi Al,
Thanks for your reply.
The cap list you showed up, doesn't list caps with values like 4,7 pF and 3,9 pF.
Maybe a 4 pF hits a 3,9 pF cap, but 5 pF is far away from 4,7 pF.

I prefer to buy in real shops, not online.
So first thing to do is to ask for 3,9 pF and 4,7 pF with a working frequency from about 200 MHz.
There might be other dealers with caps from different sources.

Hi again,

These exact values have existed for years and very likely forum members will have some kicking around, if postage and so on isn't an issue. They will be vintage components but they literally last forever for silver Micah. I'd recommend putting a components sought query in the relevant forum section. If you want to go into a shop instead of asking on the forum (where I can almost bet you'll get the exact ones you need) you don't need to specify working frequency if they are silver mica.

And if you wanted to get the new ones, it's absolutely fine as the trimmer cap will compensate for the differences. So whichever way you go is fine!

I wanted to clarify my assumptions, and also so say that on closer inspection, all the capacitors connect between the same two RF nodes so they are effectively in parallel at RF, where what matters is the AC voltage, not the DC wiring.

Here we can see that the emitter, base, and the power supply rail (which is decoupled), all sit at almost the same AC potential. That's because their impedances are very small at these very high frequencies.

So the bottom side of each capacitor is effectively the same RF node. All their top sides also connect to the tank so they share the same two AC nodes and behave as capacitors in parallel

I thought about a three turn coil at VHF, plugged some figures in and came up with a value of about 25nH from the 3 turns described and the frequencies involved, which is plausible.

On the sound side, to hit 215.75 MHz with L = 24 nH, the required total parallel capacitance is:
Ctotal = 22.7 pF

so now we can substitute the capacitor values we agreed...
New main cap = 3.9 pF
Oscillator cap as before = 6.2 pF
Fixed total = 3.9 + 6.2 = 10.1 pF

So the trimmer must supply the remainder of the capacitance...
Trimmer = C total minus c fixed
Trimmer = 22.7 - 10.1
So trimmer setting required = 12.6 pF

So to recap, assumptions are that a plausible value for L is 24nH, and that the trimmer is set at around 12.6pF.

This gets to the target 215.75 MHz for sound, just by way of example.

I'm not sure I fully understand the second part of your last post? That's to say, it isn't an area of knowledge where I could add anything!

[colourking]

Are any secondhand modulators, capable of Band III operation, for the French analogue System L available?

These may have been used to distribute satellite TV in French hotels etc

These may be easily modifable to move their AM sound carrier 1MHz lower (+5.5 instead of +6.5 MHz) depending on how the sound frequency is generated and you then have a modulator that fully works with that set. Without modification, they will work for vision only (same positive modulation).

If such a modulator is channelised for French Band III. the French Channel "L9" (208.0 MHz vision) is probably near enough to be within the TV's fine-tuning range of the channel you desire. A bit lower than the exact frequency you sought, but that home-brew modulator design isn't going to give you 'exact' consistent frequency anyway (have to adjust fine tuning on TV ) !

[German Dalek]

Good evening to all!

These are interesting add-ons.
I shall ask in a french forum for something like a hotel modulator?
Since many years I entered french motels, but nether saw something like that.

I have 2 frequency counters, but not more my fine expensive inductive tester.
It was stolen with other test equipment, tools and other stuff during the second
burglary in my parents` house.

With Al`s advice I shall be able to construct that modulator.

If I can get more clarity about that "sinc tip clamp", I should be able to do a write-up
to help others in the future.
No software, no special ICs, no SMD-technology!

Maybe another reader of this forum will have some ideas tonight to create
that assy. after a good cup of tea!

Regards,
German Dalek

[German Dalek]

Yep,

Here is a luxurious 405 L RF-converter, done by an australian with a very good knowledge!

https://www.worldphaco.com/uploads/RF_MODULATORS_FOR_VINTAGE_405_LINE_TELEVISION.pdf

Great, if you can do that from scratch.

Question is how to size the signals according the belgian standard.
What about harmonics?

Good ideas are welcome!

Regards,
German Dalek

[FRANK.C]

Any of the Hedghog converters that has a programmable modulator should be able to do those channels.
For example the PSC. Details of what the modulator in it can do can be found on its webpage.

Frank

[German Dalek]

Hello Frank,

If I should be back at the beginning of my personal "world-converter" it will be a good solution.
But I bought 3 Auroras and own a few other modulators.

In that situation I have to go on and will try to reach the goal based on that french design.
I only need some engeneering.

Regards,
German Dalek