A variety of VHF modulators

[Paul Stenning]

... referring to this page on the BVWS site:
http://www.bvws.org.uk/405alive/tech/modulator.html

[Paul Stenning]

You'll find some basic info here:
http://www.bvws.org.uk/405alive/faq/workagain.html

[Steve_McVoy]

We have a modulator design that uses 2 MC1374 ICs, one for the video and one for the audio. It is not crystal controlled, but is quite stable. We've provided about 50 of them to collectors in the UK, US and Europe.

A kit consisting of the PC board and the two ICs costs 15 Pounds Sterling including postage to the UK. All other parts are easy to find.

If you are interested we can send a schematic and circuit description.

Steve McVoy
Early Television Foundation
www.earlytelevision.org

[Steve_McVoy]

I've put the information on a web page:

http://www.earlytelevision.org/405_modulator.html

Regards, Steve

[oldeurope]

http://www.your-normwandler-page.de.vu/

Hi all

new updates for the modulator availible now
Q 69 and Q 670 (both BC547C) are added.

Darius

[oldeurope]

Hey, where are the audio and video experts.
http://www.thevalvepage.com/projects...at/modulat.htm
Look here it is David Looser's modulator on Jonz
valve page...


Let us discuss the different designs

Darius

[flyback]

Hi all

The modulator in my Domnio converter uses the MC1496 device but with a Colpitts configuration with 3'rd overtone crystals.
The Colpitts circuit means one less trasformer is needed.
I have used the same circuit for a few CH4 versions the MC1496 gives slightly less output at the higher frequency.

I use ferrite beads for the output transformers they are simple to wind.
I use a single input for the audio modulator and get good linearity, my video clamp uses an OPAMP instead of transistors or a diode, it gives better temperature stability and is adjustment free!.

Also as Steve Mcvoy says, once you've got the boards made its not so easy to change the design, much better to finalise a design and stick with it!
But for home constrction its easy to change the circuit and try alternatives.

Happy modulating which ever circuit you use..

Malcolm

[ppppenguin]

In an ideal world a modulator would have a proper keyed black level clamp. Black level could then be set at the correct carrier level. Sync tips could be allowed to crush below zero carrier and I think this was done on high power transmitters to make best use of the non-linear knee of the output valve. Not necessary on a highly linear low power modulator.

The down side of a keyed clamp is the need for a sync separator. The LM1881 is more than adequate and it has an output that's suitable for clamp pulses.

On balance I would use a DC restorer. My own modulator uses a transistor as the clamp device which is temperature compensated by a diode and it's certainly good enough. The traditional single diode restorer is not very good. There are plenty of circuits using opamps, comparators etc. These include a novel one that I designed a few years ago for a client which is DC coupled throughout. Although it cannot cope with a very wide range of DC offset on the input it can easily be switched on and off under software control. This last requirement was thrown at me by the client without any expectation that it might be possible in a simple design.

Gennum (and maybe others) offer single chip DC restoration solutions but these are hardly worth it for amateur use. I've never used them either as amateur or professional.

Note on terminology: In the UK the word " clamp " normally means a method using a clamp pulse to turn on a switch though this is not 100% true nowadays. The term " DC restorer " is used for the various arrangements of diodes, transistors etc that we are all familiar with. In the USA " keyed clamp " is used for those designs using a clamp pulse. " Clamp " can mean any kind of design and " DC restorer " is almost unknown. I am not familiar with European terminology.

[flyback]

hi

Well lets see...the MC1496 will produce a carrier with an amplitude proportional to the voltage difference between two pins..(1 and 4),
With a standard positive going video signal input, with one set of bias voltages (1 above 4) this difference will give Positive modulation
and the other (1 below 4) Negative, at no difference there will be zero carrier.

So for our positive modulation 405 Line system the sync tips have to be set at the zero carrier point, hence using a sync tip clamp!
referenced to the pin 4 volts in my design..and driven by the sync tips themselves.
So with 1V pp input and the sync tips fixed to pin 4 volts we get positive modulation with the black level set by the video signals proportions,
in Domino these values are derived from the numbers put into the DAC so they follow the 7:3 Video to Sync ratio.

If we wanted negative modulation we would move the bias point to give pin 1 below 4 by moving the reference volts to the clamp
(a change of resistor values.) this applies to the MC1374 as well, if you read the data sheet for the MC1374 you will find its insides
are based on the MC1496 and like its data sheet says can do positive and negative modulation.

I would have used a couple of MC1374's its got the oscillator section in it as well which would have made life easier
but they are not that easy to get over here, Steve McVoy (ETF USA) supplies them with a pcb for his modulator design, I have searched high
and low for a supply here in the UK but had no luck, so I went for the MC1496 and used a reliable sync tip clamp (cos thats what it does!!).

The LM1881 was the source of the clamp circuit I used, have a look on the data sheet, OPAMP and diode, if its good enough for National
it'll be good enough for me.
One problem with using an LM1881 for driving a clamp is that the BP pulses occur in the wrong places during the field sync periods, giving nasty kinks to the signal, not important if you only want the active video regions but for the modulator its not so good.
Dave grant used an active feedback clamp system with the TDA4820 sync seperator IC sampling the sync tips on the modulators he used to supply.

With regard to " Clamp and DC Restorer " , if you want to work from the sync tip upwards clamp the sync tip, if you want the black level (back porch) at a fixed level clamp the back porch, either way you've fixed the DC level or Restored the DC.

Jeff said " I've never used them either as amateur or professional. " I used to repair things designed by " Professionals " ..and what good job they did!!

All the best
Malcolm

[Duke_Nukem]

Originally Posted by Paul Stenning

There is some information about the Dave Looser modulator here http://www.bvws.org.uk/405alive/tech/modulator.html

The design can be found here :-

http://www.thevalvepage.com/projects...at/modulat.htm