How to build an AM-sound TV converter? - UK Vintage Radio Repair and Restoration Discussion Forum

German Dalek · 15th Jun 2022, 3:46 pm

Hi all!

Since a few years I am hanging around with the question, how I could build a converter for the old belgian TV standard.

I talk about the 625 lines television standard for the flemish community.
It is, as I said, AM-sound with positive picture modulation.

It is possible, that I need a converter working on VHF Band I or VHF band III.

There is a nice french converter for 819 lines / channel 8 (VHF band III) on the
internet, easy to build:

http://819lignes.free.fr/Realisation...r_VHF_F8A.html

But I have no idea, how I can change the frequencies to band I or to get closer
to the belgian channel 8!
I hate maths!

Anybody here with a good taste in calculating circuits?

Problem is, that I have a couple of early belgian TV sets which are dual standard/dual channel sets.
These sets are unable to use CCIR or french 819 lines transmissions!

Because of that issue not many of these sets survived. I don´t know
any other TV-collector who owns these kind of rare sets and wants to put new life
into them.

Many thanks in advance,

German Dalek

Mr Hoover · 15th Jun 2022, 4:36 pm

You've got two options for the old Belgian system C, positive video and 5.5 MHz AM audio above
the video, standard CCIR vhf channel spacings.
1 Invert the video into a standard
CCIR modulator, this is as simple as swapping over inner and outer of the video cable to the modulator.
The AM audio stages of the set will slope detect the FM audio of the modulator, maybe giving
"fair" quality. Worth a try, very simple.
2.Get yourself a system L modulator such as this.
https://www.google.com/url?sa=t&sour...ivhDFDP2UJbWxP

This will give +6.5 AM audio on band 3 and maybe you can retune the sound stages of the TV to get audio.
Third option is to build something.
Try option 1,see how it works.

jhalphen · 15th Jun 2022, 7:27 pm

Hi to all
Hi German Dalek,

The Terra MT-47 agile VHF/UHF modulator will operate on the French L' prime system.
L' is a repackaging of VHF Band I & III frequencies after the 819 line E norm was abandonned.

TV collectors on our French RetroTechnique forum have found that some channels will give Vision & Sound on a VHF 819 line receiver thanks to the DSB (Double Side Band) modulation of the modulator & some happy coincidences.
Some optimization of the TV's fine tuning & modulator frequency offset is required.

Links & photos:
- the topic on French forum
- usable channels & offset explanation
- the Terra MT-47 modulator.
- some links to buy it (no connections)

Hope this helps,

Best Regards
jhalphen
Paris/France

topic on Fr forum:
https://forum.retrotechnique.org/t/m...819-625l/82163

MT-47 Modulator
http://terralt.lt/MT47-moduliatorius...&productid=412

https://shopdelta.eu/modulator-tv-mt-47_l2_p3967.html

Note: there are many lookalikes, only these have L/L' modulation
Terra MT-57 also has L/L' + PAL B/G STEREO audio (useless here)

another thread about modulators compatible with Positive Video + AM Audio:
https://forum.retrotechnique.org/t/a...es-secam/81133

Aside from TERRA, read about Fracarro & Ikusi modulators.

15th Jun 2022, 3:46 pm	#1
German Dalek Hexode Join Date: Jan 2011 Location: Düsseldorf, Germany. Posts: 368	How to build an AM-sound TV converter? Hi all! Since a few years I am hanging around with the question, how I could build a converter for the old belgian TV standard. I talk about the 625 lines television standard for the flemish community. It is, as I said, AM-sound with positive picture modulation. It is possible, that I need a converter working on VHF Band I or VHF band III. There is a nice french converter for 819 lines / channel 8 (VHF band III) on the internet, easy to build: http://819lignes.free.fr/Realisation...r_VHF_F8A.html But I have no idea, how I can change the frequencies to band I or to get closer to the belgian channel 8! I hate maths! Anybody here with a good taste in calculating circuits? Problem is, that I have a couple of early belgian TV sets which are dual standard/dual channel sets. These sets are unable to use CCIR or french 819 lines transmissions! Because of that issue not many of these sets survived. I don´t know any other TV-collector who owns these kind of rare sets and wants to put new life into them. Many thanks in advance, German Dalek __________________ And now something completly different: MARC BOLAN, he was/is the real king of Pop Music!

15th Jun 2022, 4:36 pm	#2
Mr Hoover Hexode Join Date: Jan 2007 Location: Faro, Portugal Posts: 269	Re: How to build an AM-sound TV converter? You've got two options for the old Belgian system C, positive video and 5.5 MHz AM audio above the video, standard CCIR vhf channel spacings. 1 Invert the video into a standard CCIR modulator, this is as simple as swapping over inner and outer of the video cable to the modulator. The AM audio stages of the set will slope detect the FM audio of the modulator, maybe giving "fair" quality. Worth a try, very simple. 2.Get yourself a system L modulator such as this. https://www.google.com/url?sa=t&sour...ivhDFDP2UJbWxP This will give +6.5 AM audio on band 3 and maybe you can retune the sound stages of the TV to get audio. Third option is to build something. Try option 1,see how it works.

15th Jun 2022, 7:27 pm	#3
jhalphen Hexode Join Date: Jul 2004 Location: Paris, France Posts: 350	Re: How to build an AM-sound TV converter? Hi to all Hi German Dalek, The Terra MT-47 agile VHF/UHF modulator will operate on the French L' prime system. L' is a repackaging of VHF Band I & III frequencies after the 819 line E norm was abandonned. TV collectors on our French RetroTechnique forum have found that some channels will give Vision & Sound on a VHF 819 line receiver thanks to the DSB (Double Side Band) modulation of the modulator & some happy coincidences. Some optimization of the TV's fine tuning & modulator frequency offset is required. Links & photos: - the topic on French forum - usable channels & offset explanation - the Terra MT-47 modulator. - some links to buy it (no connections) Hope this helps, Best Regards jhalphen Paris/France topic on Fr forum: https://forum.retrotechnique.org/t/m...819-625l/82163 MT-47 Modulator http://terralt.lt/MT47-moduliatorius...&productid=412 https://shopdelta.eu/modulator-tv-mt-47_l2_p3967.html Note: there are many lookalikes, only these have L/L' modulation Terra MT-57 also has L/L' + PAL B/G STEREO audio (useless here) another thread about modulators compatible with Positive Video + AM Audio: https://forum.retrotechnique.org/t/a...es-secam/81133 Aside from TERRA, read about Fracarro & Ikusi modulators. Attached Thumbnails Last edited by jhalphen; 15th Jun 2022 at 7:33 pm.