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Baird M708 Dual Standard Colour Television

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PYE625
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Posted by: Terry
Posted by: PYE625

Couldn't agree more.... not only that, it was all a new technology at the time and to expect total perfection is asking rather too much me thinks.

New technology?

Yep, I'd say so. 1963 for the PAL system, PAL I decided afterwards for the UK, hardly long for the state of the art to be achieved before the launch here in 1967.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 06/02/2018 2:34 pm
PYE625
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Posted by: Steve Webb

 Now we have settled down to consistent mediocre performance judging by some of the pictures I see in present day retailers stores.

And programme material too.....but that's another story !

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 06/02/2018 2:53 pm
Terrykc
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Posted by: PYE625
Posted by: Terry
 
New technology?

Yep, I'd say so. 1963 for the PAL system, PAL I decided afterwards for the UK, hardly long for the state of the art to be achieved before the launch here in 1967.

Whilst you are right about PAL, don't forget that it is only a modification - an important one, nevertheless - of the established NTSC system - unlike SECAM (System Entirely Contrary to the American Method).

PAL I is not a separate development, merely the identical system in a (slightly) wider bandwidth.

When all else fails, read the instructions

 
Posted : 06/02/2018 7:57 pm
Marconi_MPT4
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Hi,

Interesting thread, particularly as this set is completely unknown to me. With regards to ringing on sharp transition verticals, many manufacturers tried to introduce a certain amount to improve the subjective picture definition on normal programmes. Often they did this around the delay line circuitry. Out of curiosity I decided to (LT)spice things up a bit and reconstructed the circuit to look at time and frequency domains. For the time analysis used a 1V, 1us pulse and frequency plotted over 1Hz to 10MHz .

Baird700 cct
Baird700 freq1png
Baird700 trace1png

Frequency response is shows an increase in gain starting at 200kHz, this is due to network 3k9 and 39pF in series with the input signal. A decent notch at 4.43MHz due to the rejector circuit.

It gets interesting when the amplifier is pulsed. Ringing on the output signal can be seen and is at the same frequency of the sub carrier rejector circuit. Also shown is the oscillation across the inductor.

Checking the frequency of the ring in a real circuit might give further clues. Just a thought.

Cheers

 

 
Posted : 07/02/2018 7:21 pm
PYE625
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Thanks MPT, especially for taking the time to investigate the circuit.  ? 

It certainly is a can of wriggling worms when you factor in L230, R354 and the delay line too. I'm suspicious that L230 is perhaps not operating as it should due to minimal effects when it is adjusted. The other two rejector's  L201 and L233 certainly do operate. (L233 is in the final stage on the CRT base).

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 07/02/2018 8:01 pm
Cathovisor
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Good ol' LTSpice! I remember when such programs were expensive, and now a very good one is freeware! Recently I modelled the input circuitry of a ReVox B77 with one and sure enough, it showed a classic CCIR tape EQ in reply. This may get mated to a Collaro Tape Transcriptor deck to give me a vintage, playback-only machine that is physically incapable of accidentally putting erase current near an archival tape...

 
Posted : 07/02/2018 8:07 pm
PYE625
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It's tempting to shove a bit of spice up this Baird....I'm feeling rather hen-pecked by it.  ? 

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 07/02/2018 10:11 pm
PYE625
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To that end, I have placed the set to one side for a breather. As I said before, it is now working very well indeed aside from the ringing and left striation on 625 only.

To be continued.

IMG 3859 50

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 09/02/2018 6:56 pm
PYE625
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As an antidote to family get-together's on Mothering Sunday, I thought I'd spend some time watching a bit of light entertainment.

Working pretty well, not as good as other set's, but working. On normal programme material, the ringing is not all that noticeable. 

IMG 3972 50
IMG 3973 50
IMG 3974 50
IMG 3975 1 50
IMG 3976 1 50
IMG 3969 50

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 11/03/2018 5:29 pm
Nuvistor
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Still working fine,  you must be pleased with the results.

 

 

 

Frank

 
Posted : 11/03/2018 5:43 pm
PYE625
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Posted by: Nuvistor

Still working fine,  you must be pleased with the results.

I am Frank, especially when you consider the 50 year age of the set and it is quite a bit more complex than a single standard set.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 11/03/2018 5:48 pm
Marc
 Marc
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It's a grand looking set Andrew, It would certainly get pride of place if it was here.  ? 

Marc
BVWS member
RSGB call sign 2E0VTN

 
Posted : 11/03/2018 6:10 pm
PYE625
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Thanks Marc  ?

The only problem is that the screen is hardly big enough to accommodate that fuzzy hair-do !

Oh I must equalise the 405/625 height....a simple adjustment indeed.

IMG 3979 50
IMG 3978 50

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 11/03/2018 6:28 pm
PYE625
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 Posted by: PYE625
Working pretty well, not as good as other set's, but working.

Decided I'm being too critical. The set is good. It is bloody good. I'm not going to bugger about with it trying to get it perfect as sometimes things are best left well alone. Ok, there is slight ringing.... but hey-ho. It is what it is.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 11/03/2018 7:19 pm
PYE625
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The question I would like to pose to you is just how rare is this set? 

I have a good idea from the excellent information already published by Chris in various sections of the site, but it would be wonderful to hear anyone's input and general opinion. 

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 12/03/2018 5:32 pm
Terrykc
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The M708 was introduced well after the rest of the M700 series, not long before Baird moved onto the M710 series, so it may have had a very short sales span. Was the M718 introduced at the same time as the other sets in the series - or later?

Chris has a 718 which has a precise date in October 1968, so they were obviously available by then. Can you find a date on yours for comparison?

If not, date codes on components can be useful. I think valves used to go into sets very quickly after manufacture, so it might be intuitive to check the date codes on any low powered valves that are likely to have never needed replacement.

 

When all else fails, read the instructions

 
Posted : 12/03/2018 6:27 pm
PYE625
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Hi Terry, this set is dated 11/10/68 (see first post) so it was very much alongside the 718.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 12/03/2018 7:16 pm
Terrykc
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 Blimey!

Only 11 days between the two sets!

When all else fails, read the instructions

 
Posted : 12/03/2018 7:44 pm
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