VHF modulators

[Sean Williams]

Hi All,

I am looking at building a vhf modulator for the Ferguson (and any others I may find)

I have looked at David Loosers design, And Jeffreys one too, and have been trying to see what Darius has done as well, so I wonder,

Which method would give the better results?

I would prefer a free running oscillator (as you would need four crystals for both London or Birmingham)

David Loosers design appears to be repeatable as the ICs used are still available.

I would envisage producing A PCB for the finished design - this way other people could just assemble a small kit and be able to getr fairly easy results.

I wonder if a wide bandwidth PA device on the output would be a reasonable idea, to bring the output up to 500mw - this would allow more than one set to benefit from the signal......

any comments??

Cheers
Sean

[Duke_Nukem]

The only one I've tried to build is the telebox derived one.

Of the three I tried (for Brum), one works perfect, one has some spurios AM (crystal fundamental) and the third looks like a blocking oscillator. For ch4 you also need to remove a turn or two from the oscillator coil, which is a teenie weenie little thing and I can hardly see the ferrite !

With the second modulator, I think I ran out of adjustment on the coil (as I adjusted it the spurious AM reduced) and I bet if it had been for channel 1 then it would have been as good as the first one. The problem with the third one may be due to a fault, though I couldn't find it.

I ended up scrounging a built modulator (thanks Malcom!) which works fine and I believe uses the same chip as the David Looser design (and is the same as the one in the telebox).

There is a modulator kit available form I think the States - perhaps someone can chip in a link here - and I know of someone who has modified it to add diode switching between ch1 and ch4.

I've not built Darius's design but I definately intend to when I find the time.

I wonder if a wide bandwidth PA device on the output would be a reasonable idea, to bring the output up to 500mw - this would allow more than one set to benefit from the signal......

Not needed ! To run my HMV2805, I feed the set via a fixed 18dB attenuator followed by a variable attenuator set to "lots" so you should be able to knock up a distrbution system from one modulator.

Unless you mean to actually transmit the signal (naughty, naughty )

TTFN,
Jon

[peter_scott]

I built Darius' Ch1 design and found it delightfully stable and free from spurii etc.

Peter.

[ppppenguin]

My design, AKA Telebox, is almost identical to David Looser's. I would not expect significant performance differences between them. The only critical thing I found was that the crystals tended to osciallate on their fundamantal instead of 3rd overtone unless damped a bit.

Darius's design looks good - he has the RF knowledge and test gear - and I would respect Peter Scott's opinion too.

David Robinson's design using a Motorola(?) chip is possibly the most interesting of all as it is inherently programmable to many channels and does not need a special crystal at all.

The direct digital synthesis approach that Darryl (the Aurora man) and I discussed both on and off forum is technically interesting but I would not recommend it unless you really know what you are doing. It wouldn't be cheap for a 1 off.

[Sean Williams]

Hi Jeffrey,

We are playing with DDS at the local radio club, hovever, in this application they would not really offer any benefit - the maximum reliable frequency output would only be 30 megs, so you would need a doubler stage.........

As for the Chips for DDS, Analog Devices will supply sample free of charge - I have a couple of the 9580 chips here.......

Cheers
Sean

[Steve_McVoy]

We supply a tunable modulator kit for 405 line TV. We have sold about 60 of them to users in the UK and the US:

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

[David_Robinson]

Thanks for the mention Jeffrey. The thread about my modulator is still available on this site:

https://www.vintage-radio.net/forum/...ead.php?t=1940

If you can't be bothered to wade through all that, the circuit diagram is here:

http://homepage.ntlworld.com/david.r...0Modulator.pdf

[ppppenguin]

David

Thanks for providing the links. I hope you will also be willing to supply the PIC code if anyone wants to build your modulator. Maybe Paul Stenning would be able to place the schematic and PIC code on his web site for all to use?

[David_Robinson]

Yes, of course I will supply the code. I would prefer if anyone who wants it would send me a PM with their email address, I will then send them the code directly. It's just that I would like to keep track of who is building the project and how they're getting on.

The circuit diagram can be made freely available, no problem.

[Paul Stenning]

Originally Posted by ppppenguin

Maybe Paul Stenning would be able to place the schematic and PIC code on his web site for all to use?

Jon's site at http://www.TheValvePage.com is probably a better place as he has quite a lot of telly stuff there already, whereas I have none.

[Sean Williams]

Well, Motorola ( Freescale? ) Have confirmed my samples of the ICs required are being shipped.....

Best I start trying to get a PCB produced...... Yuck, I hate multilegged fuses at the best of times - these ones do not even have proper pins!!

Will probably build both modulators, David R, and David L.........

Cheers
Sean

[oldeurope]

Good evening Sean,
be quick, I'll sent you the vhs tape next monday.

Please give me your adress via PM.

Kind regards
Darius

[Danny]

Hi ive been using david looser s modulator for a few years and have had no problems on london but i have instability problems on midlands i need to spend more time on it. it does give a large output and this can be split using a passive splitter ive had ten sets running with no problems this way.
i havent tried dariuses modulator yet but shall when im bored with mending teles. Danny

[ccandido]

Oi David Robinson. I am making one prototip with the MC44bs373 demodulator and, I am with difficulty in codigo source for, to configure the frequecias of the channel. the other register I understand but, the logic to change the frequencia of the channel this dificil for me. if yor will be able to emviar me an example or the logic.

example:
channel- 34=

FM—High-order bits = ? <----

FL—Low-order bits = ? <----
or,

channel- 03=

FM—High-order bits = ? <----

FL—Low-order bits = ? <----

of if making the account. I am thankful.

Edited for Carlos Candido of Brazil.

[David_Robinson]

No problem Carlos, I have emailed you directly. You should have received it by now.

[ccandido]

Hi,
my gratefulness the David Robinson for helping I to configure and to function a MODULATOR MC44BS373CA. I, was emcantado with its disposal in helping. alen of I not to dominate the language and, datasheet inguêsa is, very poor of informaçao. now yes, everything is fucionando perfect. my gratefulness to, Forum and the David Robinson.


Edit by Carlos Candido from Brasil.

Originally Posted by ccandido

Oi David Robinson. I am making one prototip with the MC44bs373 demodulator and, I am with difficulty in codigo source for, to configure the frequecias of the channel. the other register I understand but, the logic to change the frequencia of the channel this dificil for me. if yor will be able to emviar me an example or the logic.

example:
channel- 34=

FM—High-order bits = ? <----


FL—Low-order bits = ? <----
or,

channel- 03=
FM—High-order bits = ? <----

FL—Low-order bits = ? <----


of if making the account. I am thankful.




Edited for Carlos Candido of Brazil.

[af024]

Anyone cracked band III channel 8 yet? I'd really love to have a modulator that can do both band I channel 4 and band II channel 8 - to replicate how things used to be round here in the good old days.

Cheers,


Andy

[Sean Williams]

In theory the modulator Darius designed would work with a couple of large modifications......


I will add it to the list of things to play with......

Cheers
Sean

[David_Robinson]

Originally Posted by af024

Anyone cracked band III channel 8 yet?

What's the problem? My modulator does channel 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 and IF!

[oldeurope]

Originally Posted by David_Robinson

What's the problem?

Hi David,
my problem is the PIC and the soft.
If you are able to handle this you don't have to build anything because
there are a lot of ready build modulators using your chip family.

It is a fundamental question, do you want something software based
or a discret analog circuit.


Kind regards
Darius