Baird 8810 School's Television - Thorn 1540 Chassis

[volte-face]

Originally Posted by FERNSEH

The video coupling capacitor C54 destroys the DC component of the video waveform resulting that the video assumes an average value.....

Thanks for that. I should have probably stuck with trying to complete the two piece jigsaw that's been giving me such a headache

However, I still need to find this set a new home, so to that end I have now placed an offered post HERE

[Refugee]

Originally Posted by Nickthedentist

It was a sad day when I spotted out primary school's three Decca mono schools tellies, complete with Unicol stands, dumped outside in the rain awaiting the bin men. All working perfectly. This would have been about 1980-81.

I bet they were replaced with colour and VHS. There would be no way of going back to mono again after that

[Nickthedentist]

Yes, Decca 80 chassis, but no videos fora while.

[Focus Diode]

I remember a Thorn 9000 (I think) housed in a similar cabinet working in conjunction with a piano key VHS VCR in school in 1982.

[mickash]

Originally Posted by Nickthedentist

It was a sad day when I spotted out primary school's three Decca mono schools tellies, complete with Unicol stands, dumped outside in the rain awaiting the bin men. All working perfectly. This would have been about 1980-81.

It's a shame that the stands were dumped as well. The new sets could have been fitted to them. Unicol were even able to supply stand tops to fit specific TV models.

[volte-face]

For anyone interested, there are a couple of links below to videos where you can make your own mind up regarding the DC stability, or not, at the CRT's cathode -

1)
https://www.dropbox.com/s/ca482himcg...id_01.mp4?dl=0

Upper Trace (yellow) = Line Drive | Lower Trace (cyan) = CRT Cathode
There is a brief section a few seconds in where the input signal is removed to show the blank screen condition.

2)
https://www.dropbox.com/s/1hjocfjygn...id_02.mp4?dl=0

Upper Trace (yellow) = Video In | Lower Trace (cyan) = CRT Cathode
Apologies for the occasional trigger slip, it was the best I could do at that time.

Originally Posted by volte-face

Currently, I have offered this set, as a donation, to the Science Museum, or more specifically their Science and Media arm in Bradford.

To which, they said, "Thanks, but no thanks".

Originally Posted by linescan87

A couple of weeks later the home built set turned up on Ebay, it seems the museum were thinning out their collection.

That's exactly what they suggested I might want to do with it...

So, after the little outing, and as no other interested party came forward, I have now put it back into storage.

[GreasedMonkey]

Originally Posted by volte-face

So, after the little outing, and as no other interested party came forward, I have now put it back into storage.

That's a real shame. Its a lovely set. I had assumed it had found a new home as you closed the offered thread.

I would have loved to have had that in the back room, but due to Coronavirus, all of my medical appointments have been cancelled and without my trips to the chiropractor and physiotherapist I am not capable of driving for more than a couple of miles.

Thanks for posting the scope traces, I found them most interesting. I shall refrain from posting my conclusions from seeing them on what appears to be a somewhat controversial topic.

[Accumulator]

I see mention of a TX250 in this thread. I have got a manual for a TX250 and I’ll attempt to upload a few pictures of it. Hopefully it will be of interest.

[jayceebee]

Yes please.

John.

[Accumulator]

Sorry for the delay John but I have tried to upload pictures from my iphone. I must be doing it wrong. I'll try again.

[AndiiT]

Originally Posted by Focus Diode

I remember a Thorn 9000 (I think) housed in a similar cabinet ...

Hi, that would be a Thorn 9700 maybe a 9653(?) the schools version of the 9600 chassis; The smaller screen 9000 was never used as a schools receiver.

Andrew

[Accumulator]

I've put some not very good pictures of TX250 manual in Pictures and albums section.

[jayceebee]

Hi, there is an album under your profile but as yet no pictures in it. Do you have the ability to create a PDF and upload that using the Go Advanced reply?

John.

[Accumulator]

They have now been moderated so they should be there I think.

[Focus Diode]

Originally Posted by AndiiT

Hi, that would be a Thorn 9700 maybe a 9653(?) the schools version of the 9600 chassis; The smaller screen 9000 was never used as a schools receiver.

Andrew

Yes, of course you are right. Thanks for the correction.

[1100 man]

Originally Posted by FERNSEH

And yet despite all the extra circuits to improve picture quality the output from the video output transistor is AC coupled to cathode of the CRT.
IC201 provides video DC restoration and the amplifier comprising of VT202, VT203 and VT204 is direct coupled to the base of VT6 so that the DC component is maintained and then they go and spoil it all by introducing the coupling capacitor C54 into the video signal chain to the CRT.

I've just been studying the circuit for this 'schools 1540' and comparing it to the standard 1500.

They seem to have gone to a huge effort to restore the DC component of the video and then maintain it all the way to the collector of the video amp transistor, using three transistors along the way: all DC coupled.

Why then would they throw all this effort away by coupling the video to the CRT cathode via C54? There is a 270K resistor in parallel with the capacitor, but I don't know what difference that makes from a DC coupling point of view?

All seems very strange to me!

Any suggestions as to why they did this? There must have been some good reason!

All the best
Nick

[dtvmcdonald]

Originally Posted by FERNSEH

The video coupling capacitor C54 destroys the DC component of the video waveform resulting that the video assumes an average value.

DFWB.

Not quite. R55 and R56 form a voltage divider. There is some DC component
at the cathode.

This is in fact done intentionally. People didn't like very very dark pictures when there were no bright spots in the scene. It was one of those cases where a "focus group" run by the marketers determined that technical excellence would not sell as well as otherwise.

My RCA CT-100 1954 color set has DC restorers at the CRT grids. Its grid drive. Its running about 80% restoration. I've tried going to 100% using the circuit in the otherwise very similar TM10 monitor and yep, I like 80% better.

[Heatercathodeshort]

A very valid point. I had numerous customers that simply could not get on with the screen blanking out on dark scenes with the Bush 640 series.
They would turn up the brightness resulting in an over bright picture and then continued on with the remaining controls resulting in a hopeless mess and a service call to sort it out.

I suppose mean level AGC is a bit like audio compression. The result without it can be a pain to listen to with massive changes in volume blowing you out of the room, and a nuisance to your neighbours. Better from a domestic aspect, mean level AGC and sound compression was preferred by 99% of customers.

Set the brightness and contrast then forget it.
The Decca 'Professional 23' with all it's refinements gave a rather poor picture compared with less sophisticated models. John.

[Refugee]

I lot of people had elder relatives with collections of faded photos in the mono days.
Perhaps they expected the TV to fade to a glow as opposed to being blank when there was nothing on.
I can remember the school telly having a lamp that showed through a bezle in the doors so that you could see that it was on.

[Welsh Anorak]

Of course there's no such thing as black!
Nowadays, LED televisions still have the LEDs shining to some extent when the picture should be blacked out. Plasmas are better at this, of course, as are OLED and their variants. Plasma TVs got a bad press for using so much power when in fact it was only when peak whites are displayed that maximum stated power is used. LCD sets use a permanent light source so the consumption quoted is fairly constant.