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This is an archived website which has not been updated since 2002. Some information may be inaccurate or out of date. |
What is a modulator? Television pictures and sounds originate as raw baseband video and audio signals. At the transmitter they are converted into a combined radio frequency signal which is received by your television receiver (or video recorder). There are no transmitters putting out 405-line television programmes today so instead we must create our own ultra-low power transmitter device, which is called a modulator. To suit an old 405-line television this needs to ‘transmit’ on the range of VHF channels used in the 405-line era, such as channel 1 (45MHz vision, 41.5MHz sound).
You probably possess a modulator already; they are found in most VCRs but these ones have a UHF output, generally in the region of channel 36, which is not suitable for 405-line tellies. What we need is a System A modulator for VHF channels.
Where do you get one? Sony used to make one, the RFM-405, and it turns up occasionally at amateur radio rallies. Incidentally, the two examples I have seen are on channel 3, but they may have made other channels. Anyway, the chances of finding one of these are pretty slim!
What about do-it-yourself? Two designs for modulators have been published in Television magazine and one of them we don’t recommend as it requires you to make your own printed circuit board and wind your own coils very accurately. There's nothing wrong with the design - not at all - but building it and making it work demands more skill than many constructors have.
The other design uses the modulator units found in Rediffusion Teleboxes and these can still be had from Display Electronics (020-8679 4414) for £36.95 - or for much less if you find them at amateur radio rallies or electronics boot fairs. Bear in mind you will need two Teleboxes to make one Band I sound and vision modulator. Some modifications are needed; these are shown below.
READY-MADE MODULATORS
There are potentially two sources of ready-made modulators. The best and most up-to-date product was made by Radiocraft Ltd but has now (October 2000) been discontinued.
One of our German friends, Wilfried Meier (Schepdonksweg 11, D-47625 Kevelaer-Wetten, Germany) is selling modulators, both ready-made and as kits. This is a Mercedes product at Mercedes prices. Details are as follows:
Components:
Payment must be made by Eurocheque or banknote (Deutschmarks, not Pounds Sterling). Delivery is approximately 14 days. Specify channel required, e.g.
We have tested these German modulators and find they work extremely well. The exchange rate, incidentally, was around 2.95 Marks to the Pound at the time of compiling this note.
An alternative modulator is available and comes pretuned for either the London or Birmingham channels (1 or 4). The price is on application to the maker, Andrew Beer: a.beer (at) mdx.ac.uk
These products are not 'given away', nor should they be. To assemble one professionally and align it, then drill a case and make it look presentable is a very time-consuming business. The actual materials cost is not the largest element, which is why of course do-it-yourselfers save so much money - if they have the necessary skills!
MODULATOR KIT
Steve McVoy smcvoyc (at) columbus.rr.com writes:
I designed a simple 405 line modulator, using a couple of ICs from VCRs. I've had several printed circuit boards made. I also have a bunch of MC1374 ICs, which are hard to find in the U.K. All the other parts are common resistors and capacitors.I can ship a PC board, 2 MC1374s, and the documentation in an envelope via air mail, at my cost of £14. If you send me your name and address, I'll send you one. When you get it, send me the money in cash (that's the easiest way to avoid currency conversion costs).
From 405 Alive, issue 42, second quarter 1999.
Although the Dinosaur Band I modulator is no longer produced, this has not reduced the number of enquiries received on buying and building modulators.
Needs must when the devil drives and one viable alternative is to build your own. The task is simplified if ready-to-buy modules are used and the design offered here has the merit of employing parts from an easily obtainable ex-Rediffusion cable television product and requiring relatively few modifications.
To make a sound and vision modulator for Band I 405-line television (i.e. with an output on one of the channels between 1 and 5), you will need to cannibalise two Rediffusion units, one for the sound carrier and one for the vision signal. There is no shortage of these Rediffusion tuners fortunately; you can either buy them from a company called Display Electronics (actually one of the two video-related companies confusingly both called Display Electronics!) or else you can also pick them up at some boot fairs and amateur radio swapmeets (see events calendars in Practical Wireless or Ham Radio Today magazines for details of local happenings). From Display Electronics the unit costs £36.95 each (they call it their TELEBOX) whereas they go for £5 to £20 at swapmeets (you need to know what you are looking for and these may be incomplete or non-working).
The Display Electronics TELEBOX consists of an attractive fully cased mains-powered unit, which, as it stands, converts off-air UHF broadcast channels to a composite video output ready to plug directly into most video monitors and recorders. Push button controls on the front panel allow reception of 8 fully tuneable off-air UHF colour television or video channels. A variant, TELEBOX MB, covers virtually all television frequencies VHF and UHF including the HYPERBAND as used by most cable TV operators. For complete compatibility, even for monitors without sound, an integral 4-watt audio amplifier and low level Hi-Fi audio output are provided on all models.
Power Requirements:
200-240 Volts AC at under 16 Watts.
Mechanical: Fully enclosed in two-tone charcoal ABS moulded shatterproof
case.
Dimensions: 400 mm (15.75") wide, 83 mm (3.25") high, 203 mm
(8") deep.
Weight: Unpacked 2.7kg. (5.94lb), Packed 3.15kg. (6.93lb)
Order As: TELEBOX ST
Price: £36.95 (shipping charge on Telebox is £5.50)
MAIL ORDER (Office hours Mon-Fri 9.00 - 5:30) 32 Biggin Way, Upper Norwood,
London SE19 3XF (0181-679 4414, fax 0181-679 1927, e-mail: sales (at) distel.co.uk,
website http://www.distel.co.uk)
SHOP (not for mail order, open Mon-Sat 9.00-17.30) 215 Whitehorse Lane,
South Norwood. On 68A bus route, near Thornton Heath and Selhurst Park Southern
Electric stations.
And now here's an article by Jeffrey Borinsky (405 (at) borinsky.co.uk) on how to turn these devices into a functional Band I modulator. It was first written in 1989, for Television magazine, and has been updated where necessary.
Until now most people who wanted a Band I modulator built David Looser’s excellent design (Television, October 1984). Making use of the Rediffusion units provides a simpler approach with equally high-quality results. You have to buy two of the Rediffusion units to obtain two modulators.
Modulator Conversion
The first step is to extract the h.f. modulator can - it’s the tuner-like can on the right-hand PCB, near the mains transformer. The circuit of this module is shown in Fig. 1(sorry about the poor quality, it's the only copy we could find - website editor).
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Conversion details are as follows.
Replace R2 (vision
modulator only) with a 3K9 resistor.
Replace R8 with a 560R resistor.
Replace C7 with a 220R resistor.
Replace R13 with a 10pF capacitor.
Replace C8 with a 68pF capacitor.
Replace C9 with a 47pF capacitor.
Delete R22, C16 and C12.
In the sound modulator, remove Tr2 and connect a 47K resistor between the base and emitter connections.
Replace X1 with a 41.5MHz crystal in the sound modulator and a 45.0MHz crystal in the vision modulator. Add a 2K2 resistor across the crystal on the back of the PCB to provide damping.
Add a 100pF capacitor from the carrier test point to chassis.
In the vision modulator, add a 1N4148 diode in series with R1 (cathode to R1).
These points are all shown in Fig. 1.
Connect the RF 0V pin to the modulator’s case.
The RF output obtained from the circuit shown in Fig. 1 is about 30mV r.m.s. The arrangement shown in Fig. 2 provides two RF outputs, with the vision at 22mV peak-to-peak and the sound at 10mV peak-to-peak. Clearer diagrams are available to download - see link below.
Each modulator is best aligned with the other one turned off. It’s easy with a wide bandwidth scope looking directly at the output. Otherwise look at the demodulated video using a receiver or diode probe. Set L1 for maximum output. It tunes very broadly and the tuning is not critical. With the vision modulator, feed in a standard video signal (625 lines will be perfectly OK) and set RV1 for positive modulation with zero carrier at the sync tips. If monitoring demodulated video, set RV1 so that sync crushing is just avoided. Without a scope, adjust RV1 until sync fails, then back off a bit. With the sound modulator, use RV1 to set the unmodulated carrier to 10mV peak-to-peak. Alternatively feed in about 100mV of audio tone, with the gain control at maximum, and adjust RV1 until distortion is just heard on the demodulated sound.
If you don’t want to order crystals specially you could try using a series LC circuit, adjusting on test with a counter. Omit the 2k2 damping resistor.
I have not had a chance to look at the modulator’s output on a spectrum analyser - if some lucky person has one, could they please tell me just how revolting the output looks!
These modifications
have been thoroughly proved on Channel 1 but not on the higher Band I channels.
With appropriate crystals they should work throughout Band I, but the MC1496
modulator chip is not really suited to Band III use.
 Figure 2 |
 Figure 3 |
Sources of Parts
When ordering crystals, specify 41.5MHz and 45.0MHz, third overtone, series resonant in HC18/U style. There are no tight tolerances involved. Quartslab Marketing Ltd, P.0. Box 19, Erith, Kent DA8 1LH (01322-330830) can provide both crystals at £7.50 each with a delivery of two-three weeks. There is a minumum order of £10 (although this is no problem if you order both values); postage and VAT are extra.
Editor's Note - Circuit Diagrams and Data Sheets
Figures 2 and 3 have been reduced in size to fit an 800*600 screen, so some details may be difficult to read. To download the clear, full-size versions as a PDF file click here (file size is 19K).
Figure 1 is the circuit diagram provided by the modulator manufacturers. We are trying to locate a better copy, however most of the information is already in Figure 2.
If you want to know more about the MC1496, you can download the manufacturers data sheet together with a couple of application notes by clicking here (file size is 370K).
CONVERSION SERVICES:
N1500 and N1700 tapes converted to VHS or U-Matic free of charge (I can also convert them to 405-line in the process if you require). Please send a blank VHS or U-Matic tape, your original N1500 / N1700 tape and an adequately stamped addressed Jiffy bag to Mike Bennett G7TRF, 3 High View Gardens, Exmouth, Devon. EX8 2JR. Phone: 01395-279732 or e-mail mdb (at) oldtechnology.net
I will convert your 625-line tapes to broadcast-standard 405 lines on my digital line-store standards converter. Free of charge to subscribers of 405 Alive. Please send blank tape (VHS only) for output and return postage. Input tapes can be accepted on Philips 1700, EIAJ, Video2000, Beta or VHS. David Looser, Maristow, Holbrook Road, Harkstead, IPSWICH, Suffolk, IP9 1BP. Phone 01473-328649.
(These offers are most generous and users may care to send a free-will donation towards costs as well. There may be a delay in handling conversions if many people take up these offers.)
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