Using 405 Line
By Mike Bennett
One of the main reasons given by vintage radio restorers for not venturing into vintage televisions is that 405 line televisions, unlike old radios, cannot actually be used without the costly converters and modulators needed to convert UHF 625 lines into VHF 405 lines. Well, while expensive converters and modulators can give flexibility to the use of an old set, they are not actually needed to view perfectly good 405 line pictures on your old telly.
Obtaining a 405 line source.
Before you do anything, you need to find a source of pictures to feed into your telly. Here are some suggestions:
Most older VHS* videos can record and playback 405 line pictures just as well as the normal 625 line picture that they were designed for. If you are not too concerned that you can only watch pre-recorded material, then this is the ideal 405 line source. Obtaining a tape full of 405 line video should not be too much of a problem; there are people who advertise 625-405 line conversion services in the magazine "405-Alive", details of which are on the preceding page. As for selecting a video machine to use, there are a few rules to follow to obtain the best results.
- Most modern machines (roughly from 1990 onwards) may complain about playing back a 405 line source tape so unless you already have a machine that you can try, these machines should be avoided.
- Any machine which has a "Crispener" or a "Sharpener" circuit should be avoided (This includes the ancient Philips N1502 / N1512 machines.) The reason for this is that these machines delay the signal by a very accurately set amount, and the different 405 line line-frequency is different and can cause a shadow or ringing on the picture.
- Avoid any machines which feature digital effects or contain a time base corrector such as some late eighties / early nineties Ferguson / JVC models.
- The very best machines to choose are the early second generation VHS machines such as the JVC / Ferguson 3V23/29/30/31/32 or the Hitachi / GEC / McMichael top loading machines. There were rumors that these Hitachi / GEC machines can be tweaked to increase the frequency response to greater than that of the VHS standard, but my advice is not to bother. Even if this is true, it will mean that the machine is then non-standard and it's performance cannot be guaranteed.
There is a slight drawback with using any VHS video machine to playback 405 line material, which is this: If a drop-out (Loss of FM due to a flaw in the tape) occurs on a standard 625 line tape, the video machine's drop out compensator (DOC) will replace the drop out with whatever appeared at the same point on the previous line (64µS previously) so that the drop out is almost unnoticeable. Unfortunately, because the time taken to scan one line on a 405 line signal is longer, 64µS away won't be at the same point on the previous line and so drop outs can be much more obvious. This is one reason that any 405 line source should be recorded on the highest quality tape, especially if the tape is to be used over and over again.
*Betamax, V2000, Umatic or other standard cassette formats may work too, but these have not been tested (at least not by me.)
Occasionally original 405 line source material can turn up on old open-reel video recorders such as the Sony CV2000 or the Philips / Peto Scott EL3400. Although with machines such as the CV2000, the source can be used "as-is", it is much better to copy the material straight onto a VHS tape and then use the copy. This is because the physical tapes used for these recordings will be very old and will usually start to deteriorate even after the first attempt to play them, often clogging the heads on subsequent playbacks. Machines such as the EL3400 may not be any good as a source as the output is provided at I.F. level, and unless you have the correct TV, then playback will be very difficult.
Video Cameras and Optical Standards Converters:
One ideal source of 405 line signals is from a 405 line video camera. These can often be found at amateur radio rallies and swap-meets. Many early 625 line monochrome cameras can be modified for 405 lines quite easily. One particular camera that seems to turn up more than any other is the Pye Lynx. These came in all shapes, sizes and standards, although they are always easily recognisable by the big "Pye" badges on the side. These Pye cameras can be easily converted for 405 lines, although many of them were made as 405 line cameras anyway and need no conversion.
Many optical 625 to 405 line standards converters have been made by enthusiasts by mounting a small 625 line portable television and a 405 line camera in an enclosure and this can work very well if the camera and TV are set up properly. If you are planning on doing this, here's a tip: To remove visible scanning lines from the 625 line television, defocus the picture slightly using the TV's focus control. This will not affect the 405 line picture focus very much, but can improve the conversion quality significantly. If the focus control will not defocus the picture enough, another trick is to rotate the camera tube scanning coils VERY slightly. This can also work well to improve the conversion from 625 to 405 lines although this trick can lead to moiré patterns. If all else fails, try defocusing at the camera end with the focus control on the camera or the lens. For some reason, this seems to affect the picture more than defocusing the TV, but can still be effective at removing visible conversion lines.
If you are content with a static pattern to demonstrate your TV, then a pattern generator is the ideal thing for you. Old 405 line pattern generators often turn up at amateur radio rallies and swap meets. Modern 405 line test card generators are also occasionally available in the pages of "405-Alive" magazine too.
Analogue Standards Converters:
These are generally huge, unreliable and very unfriendly monsters. They are very rare, and have usually been snapped up by museums. (Gerry Well's radio museum in Dulwich, London has one and it takes up the whole of a seven foot tall rack-mount cabinet.) Normally these beasts started life in transmitter installations and may or may not use standard base-band video inputs and outputs. They use masses and masses of discrete components and fault finding would be a nightmare. Not recommended for use except by the clinically insane.
Digital Standards Converters:
Much friendlier machines than their analogue cousins, these machines are occasionally manufactured by very clever souls such as Dave Grant, maker of the famous "Dinosaur" converter. Unfortunately, they are much prized by their owners and are very rarely seen for sale on the second hand market. They are also extremely expensive due to their complexity. However, if you can obtain one, this is the last word in standards conversion and once used, all other methods of obtaining a 405 line source pale into insignificance.
Feeding The Source Into Your Television.
Once you have a suitable 405 line source at standard 1V P-P level, you need to find a method to feed it into your TV. There are a few different options that can be used for this.
Feed it in at UHF:
If your TV is a dual standard set such as early Sony or Bush / Murphy sets, then you may not need anything else apart from your standard VCR. Certainly with many dual standard Bush / Murphy and Baird sets, the I.F. and Line Timebase sections of the set can be switched separately by disconnecting the linkage between them. This means that you can run the set in 405 lines, but still use the UHF / 625 line section of the I.F. and tuner circuits. The little 9 inch Sony 9-90 and similar sets can be switched to UHF 405 lines by simply pressing both the 625 and 405 line buttons simultaneously. To obtain a perfect picture and sound, simply plug your VCR into the aerial socket of the TV and switch the line timebase to 405 lines. If your source is from a video camera, simply feed the output into the "Line-In" socket of your VCR and switch the VCR to "A.V."
Of course, with some dual standard televisions such as the BRC1400 series, the I.F. and the line timebase sections cannot be switched separately, and another method of feeding a signal into the set will have to be found.
Use a VHF TV Modulator:
The simplest method to get a picture and sound on a 405 line only or a dual-standard telly is by using a pre-built VHF TV modulator. These are available from various sources, many of whom advertise in the magazine "405-Alive." Prices of modulators can vary from around 20 UK pounds to 90 or 100 UK pounds depending on the complexity and features. Don't forget that you usually need to specify what "Channel" you want your modulator on before you purchase it. For instance, single channel sets designed for London will use channel 1, but sets designed for other parts of the country (Birmingham, Wenvoe etc.) will use a different channel. If in doubt, ask the supplier who will usually know.
Make a VHF TV Modulator:
Many circuits have been published in magazines such as "405-Alive" and "Practical Television" (Now simply "Television".) Some of these circuits are fairly straightforward, some more complicated. Unfortunately, one of the most popular designs contains components that are no longer available. The latest design to appear in "405-Alive" magazine uses two Rediffusion converters which can be obtained from surplus component suppliers and can also be found often at amateur radio rallies etc.
Use a Signal Generator:
Although not really a recognised method for feeding a VHF signal into a TV, a VHF signal generator can give reasonable results if you have one to hand. Often many signal generators have an "External modulation" input on them and although not designed for modulation frequencies up to 3MHz or more, very often they will work perfectly well. The one major drawback with using a modulator is that you can either have vision or sound, but not both unless you have two modulators and make a resistive network to join the two together. Also the signal is not "Clamped" so transients and sudden changes of scene in the picture can upset the sync and cause the picture to clip momentarily.
Feed it in at Base-Band:
If you are confident that you can find a suitable location to feed the video and audio signals in at base-band level, then this can work very well. However, there can be many problems and extreme dangers associated with this and for this reason, I cannot recommend this method to anyone unless suitably qualified. If you are not absolutely certain of what you are doing, do not even attempt this. Most old TV's use a live chassis and can easily kill!
As can be seen, there are many ways of getting a picture out of your old 405 line TV, and if you are attempting to do this, then I wish you every success and I hope at least one of these methods is the right one for you. If you have any questions, please feel free to email me at email@example.com I can't promise to answer your question, but I will certainly do my best. Mikey.
Back to the Old TV page