Occasionally I covertly re-read back issues of 405 Alive as a means of escapism whilst at my desk! One problem of which I read related to a person who wished to re-play a 405 lines videotape recording through a single standard 625 lines TV. This is sometimes possible and probably requires only horizontal (line) hold adjustment. It is usually possible to increase the value of one of its series resistors if correct locking is out of the control range on older 625 lines monochrome receivers.

I'm a little sceptical since most line output stages used third harmonic tuning and will almost certainly thus operate outside their design limits at 10.125 kHz, perhaps causing excessive EHT, etc.. If an elderly 625 lines set is to be used permanently in this way it might be desirable to check that the boost voltage is within specified limits and adjust the appropriate control accordingly. By displaying a 405 lines test card, adjustments could then be made to ensure correct picture geometry with, perhaps, an increase in the value of the "S" correction coupling capacitor to the line deflection coils.

The Principal Domestic VCR Problem?

The RF output of most video recorders is at UHF (around Ch.36) with negative vision modulation (instead of positive) and FM sound (instead of AM). All 405 lines transmissions in the UK were on the Bands I and III VHF channels. Feeding an RF output from a domestic video to an old 405 lines-only receiver would therefore require either changing the internal video recorder modulator to one designed for VHF, or feeding the audio and video outputs via a suitable external 405 lines standard VHF modulator into the receiver. Various modulators have been mentioned in these pages. 

It is also important that the modulator output be "positive- going" video with 3.5 MHz AM sound. There is another minor problem due to the recorder "drop-out" compensation being inappropriate for 405 lines but this simply manifests in the occasional white "blip".

Direct Video/Audio Input To A 405 Lines Receiver?

On occasions many years ago I fed video and audio experimentally into a receiver. This can be very dangerous since most 405 lines sets, when switched on, have a chassis which is connected to one side of the mains supply. The only fairly safe method to accomplish this task is to operate the receiver via a large, expensive mains isolation transformer. Some video preamplification might still be necessary.

What About RF (UHF) Playback of 405 Tapes on Dual Standard Sets?

The criteria (requirements)

The receiver must:

1. Detect FM sound
2. Demodulate NEGATIVE vision signals
3. Possess a UHF tuner
4. Possess a line timebase capable of operating on the 405-line standard at 10kHz.

Most dual standard receivers meet these requirements and can be adapted often by various link adjustments or simply, in some instances, disconnecting the timebase system switch. Examples are the GEC BT455, the GEC BT2000, most Philips dual standard sets from the sixties, the Bush TV125 and TV161 series. Probably the Thorn "850" could be treated in a similar manner. The IF system switch is left in the "625" position whilst the timebase switch is held in that for "405". [Editor's note: the highly desirable Sony TV9-90UB 9"-screen portable is ideal in this respect. You just push in both the UHF and the 405 buttons and there you are!]

This approach has the advantage that the modification is minor and does not appreciably alter the character of the receiver.

There is a disadvantage in that although the image is composed of 405 lines and accompanied by the audibility of the inevitable line timebase whistle, the image does not have the characteristics of positive modulation which tended to accentuate the highlights.

Most dual standard receivers were fitted with two horizontal hold and two contrast controls. With this mode of operation only the "405" line hold and the "625" contrast control will be effective.

Modify a 405 Recsiver for UHF Negative Modulation?

This is not impossible but purist collectors might not favour the concept. Roger Bunney published several early articles on modifying the Bush TV62 for VHF 625 DX reception (Practical Television). Often reversing the connections to the video detector diode then experimentally decreasing the video output valve cathode resistor should solve the "negative modulation" problem. Of course, the line timebase would be left in its "405" state. You could then fit a UHF tuner or try the following experiment.

To provide a VHF RF feed from the UHF RF output of a video recorder might be possible using a simple make-shift converter. If you have a high gain, transistorised, rotary UHF tuner it could be battery-powered and fitted externally. RF from the video recorder is fed normally to this tuner and, when tuned to the video recorder modulator, the "IF" output from it is fed as an "aerial input" to the receiver since the IF output frequency isn't drastically different from the original VHF Channel 1 vision frequency of 45 MHz. Some internal bending and tweaking to the UHF RF and mixer tuning vanes might be required to maximise its gain, however.

Note: Many years ago I was required to service receivers which received cable reception of BBC2 on channel 5 VHF. I found this concept at least provided a workshop image - that was in the days of valve UHF tuners. Of course the conversion created "sideband" problems but since the output from a video recorder modulator is double-sideband this should not arise. Unfortunately, the 405 lines receiver would not correctly detect the 6MHz FM audio. Sound would need to be tapped from the recorder A/V socket and fed to a separate amplifier/loudspeaker.

I emphasise this is simply an idea and has not been tried in the present context!

Conclusion

I would stress that collecting receivers is a hobby in the context of the magazine. I personally would prefer that vintage TVs be left in their original state but a hobby should be enjoyable so if you wish to modify your receiver, its up to you! Besides, I've seen the occasional "custom" Ford Upright, so....

© 1998 Malcolm Burrell

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