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Old 23rd May 2017, 1:24 pm   #1
SteveCG
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Default Pre-War TRF TV RF stability

Peter Scott's web-site on Pre-War Television,

http://www.nostalgiatech.co.uk/Vintagetech.htm

shows an impressive coaxial-cable feed to the aerial. It also shows the aerial as being situated 160 feet away from the house!

Why so far and why the double screened feeder?

Could the answer be that because of the high output level of the TRF vision amplifier - of order 10 Volts, and diode detected straight to the CRT - meant that RF feedback was a real problem.

Clearly later sets, ie superhet sets and those using a separate video amplifier stage, should not have the same level of stability issues... but how did it work out in practice I wonder?

I could imagine the 160 feet was to get the aerial away from electrical click and pops eminating from the house - but perhaps putting the aerial closer to the neighbouring houses sources of interference? I note that use in terraced housing was not considered !
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Old 23rd May 2017, 2:10 pm   #2
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Default Re: Pre-War TRF TV RF stability

Perhaps cosmetic? "The end of the garden" is often the untidiest place so yet another eye sore (to me aerials a beautiful) stuck there wouldn't matter. And with back to back gardens it would be as far away from any house as it could be, like my loop which is at the end of the garden.
 
Old 23rd May 2017, 7:49 pm   #3
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Default Re: Pre-War TRF TV RF stability

The aerial was put out there from what I have read, to get it away from the road and ignition interference from motor vehicles.
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Old 24th May 2017, 8:00 am   #4
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Default Re: Pre-War TRF TV RF stability

Yes that was always the name of the game, to place receiving aerials as far away as possible from pesky car ignitions.
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Old 24th May 2017, 1:13 pm   #5
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Default Re: Pre-War TRF TV RF stability

Avoiding car ignition interference seems quite a reasonable explanation to me - it's certainly a problem I've read about in the context of cars travelling/crawling through the centre of Oakhampton on their way to Cornwall.

So I wonder then how close to a pre-war televisor could you put the receiving aerial if you were in a weaker signal area?
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Old 24th May 2017, 4:18 pm   #6
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Default Re: Pre-War TRF TV RF stability

There are other drawings of prewar TV aerial installations that just show a feeder leading from chimney stack down to a set in the same house.

The double screened aerial cable on the HMV901 was a sensible measure. If you remove any of the screening cans in the TRF chassis there is a fair probability of instability or at least a change in the response.

Peter
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Old 25th May 2017, 3:52 pm   #7
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Default Re: Pre-War TRF TV RF stability

Thank you Peter. The situation that was going through my mind when I posed the question was for places like Cambridge. As the Ally-Pally field strength map shows this was a fringe place, for which as discussed on other threads here, Pye decided to commission a suitably sensitive receiver using the new EF50 valves. The implication is that an HMV, say, would have required a pre-amplifier. My thoughts were that RF stability would have been more of a consideration in Cambridge (with the sensitivity turned up to max) than for a Televisor in the middle of London (with the sensitivity turned right down and possibly with an aerial attenuator as well!).
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Old 25th May 2017, 8:18 pm   #8
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Default Re: Pre-War TRF TV RF stability

Hi Steve,

With all the cans in place I don't have any instability with the gain at max, it just saturates. That said EMI quoted various staggers offering the trade off of high sensitivity or wide bandwidth. With "Extra Narrow" 10 to 15 uV got you a 5 volt swing at 45 MHz and at the other extreme "Wide" needed 200 to 300 uV for the same 5 volt swing and mine tends to that end of the range.

Peter
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Old 25th May 2017, 10:32 pm   #9
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Default Re: Pre-War TRF TV RF stability

The gain of early TRF TV's wasn't really high enough for instability to be a problem, they needed a pretty strong signal to work at all. the absence of grain on the screen testifies to that, the contrast control often consisted of a variable resistor in the cathode of one of the RF valves so that was in fact a gain control.

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Old 26th May 2017, 12:59 pm   #10
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Default Re: Pre-War TRF TV RF stability

Peter_Scott,

From your experience with the HMV, what level of input signal do you think was the minimum to give an acceptable picture? I presume "Extra-narrow" would have been noticably smeared, whereas "Wide" risked, as Peter N. comments, not enough contrast.

Again I'm thinking of a 'fringe' service area situation.
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Old 26th May 2017, 1:13 pm   #11
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Default Re: Pre-War TRF TV RF stability

On reading this thread I realised that when I used the 'phrase 'RF stability' I was actually thinking about the sitation where stray signals from the Televisor were being picked up at the input (either by the positioning of the aerial, or the shielding failings of the feeder cable) sufficient to causing oscillations etc.

Of course RF stability can also mean as Peter_Scott has discussed, instability because of removed shielding screen and also RF stage detuning because of changes in the stray capacitance. However a related stability question concerns the Sound chain. This is a somewhat narrower frequency response (higher Q) chain and so a small change in the resonance frequency of a stage element could reduce the sound output level. Such a change could come about by temperature and/or component value slowly drifting with ageing. You can say that the Televisors were not in use long enough pre-war to discover any ageing issues. I wonder though if many needed re-tweaking on broadcasting's resumption?
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Old 26th May 2017, 2:48 pm   #12
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Default Re: Pre-War TRF TV RF stability

Although the sound IF was much narrower than vision, I seem to remember it was about 100Khz, so not that narrow.
Having that kind of bandwidth allowed the use of a limiter for impulse interference. The 'relative ' wide bandwidth of the sound IF allowed the pulse to be narrow and be filtered out.
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Old 26th May 2017, 4:46 pm   #13
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Default Re: Pre-War TRF TV RF stability

Hi Steve,

I'm away from home for 10 days so I can't give you an actual sensitivity figure just now. The sound receiver in my set is a superhet with an IF of 1.5MHz. There is a fine tuning control included in the major adjustments to assist with any oscillator drift.

Peter
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